Safronov, Viktor Sergeyevich (1917-99) Russian planetary astronomer, the foremost theorist of COSMOGONY in the 20th century. His detailed, comprehensive and mathematically rigorous theory of the formation of planets from a circumstellar disk of gas and dust underlies present-day research. He recognized that the rate of collisions among planetesimals, which led to growth by ACCRETION, was controlled by their relative velocities; the velocities were governed by mutual gravitational perturbations, which were in turn controlled by the mass distribution. He also examined the formation of PLANETESIMALS by gravitational instability of a dust layer in the protoplanetary nebula.

Sagan, Carl Edward (1934-96) American astronomer known for his Solar System studies, especially of planetary atmospheres and surfaces, and investigations into life and its origin on Earth. After working at the Smithsonian Astrophysical Observatory he moved to Cornell in 1968, becoming director of its Laboratory for Planetary Studies and professor of astronomy and space science. In the early 1960s he showed that, since a greenhouse effect should be operating on VENUS, its surface temperature could be as high as 800 K. In 1963 Sagan and colleagues at NASA repeated the Miller-Urey experiment (see Harold UREY) using a different set-up and mixture of gases, and showed that not only amino acids but also sugars and ATP - the chemical used by cells to store energy - could have formed in the primordial atmosphere of the Earth.

Sagan's interest in the possibility of LIFE IN THE UNIVERSE involved him in SETI studies and led to his co-authorship of Iosif SHKLOVSKII's Intelligent Life in the Universe (1966). He also helped to compose messages carried on board the VOYAGER and some of the PIONEER probes that could convey information about humans and the Earth to any intelligent entity that might eventually find them. From the 1970s, Sagan was a successful popu-larizer of astronomy, with books on planetary science and other areas, including evolution, as well as the TV series Cosmos. In 1997, the year after his death, the MARS PATHFINDER lander was named the Carl Sagan Memorial Station in his honour.

Sagitta See feature article

Sagittarius See feature article

Sagittarius A The bright point at the centre of this image of Sagittarius A is an X-ray flare from the area near the black hole at the centre of the Milky Way. It brightened dramatically in a few minutes and then declined over a period of three hours.

Sagittarius A (Sgr A) Complex centre of our Galaxy, and the most intense part of the Milky Way's radio emission. The actual centre is marked by a smaller radio source, Sgr A*, less than 4 AU across, pin-pointed using the Very Long Baseline Array of radio telescopes. Sgr A* may mark the presence of a massive BLACK HOLE. Very close to Sgr A* is an intense infrared source (IRS 16), which is a large cluster of newly formed hot B stars. In addition to the radio and infrared emission from the region, X-rays and gamma-rays have also been detected.

Detailed maps of Sgr A in radio emission, mostly made by the VERY LARGE ARRAY, show remarkable structures, loops, arcs and a series of narrow parallel filaments at right angles to the plane of the Milky Way, which radiate by the SYNCHROTRON mechanism. The filaments may indicate the presence of a strong magnetic field, but they could also be created by winds. Within 3 l.y. of the centre is a theta-shaped pattern. The ring of the theta is made of gas that has probably piled up there as a result of some outflow from the centre; alternatively it may be the tattered remnant of one or more nebulae that were shredded by moving in the gravitational pull of the central stars. The bar of the theta, also gaseous, is surprisingly hot and apparently has a different origin. There is a thin streamer of gas that may link the shell of material around the putative black hole with the nearest gas cloud. Sgr A remains a very mysterious object.

Sagittarius Arm Next spiral arm of our GALAXY after the ORION ARM, moving in towards the centre of the Galaxy. At its closest it is about 5000 l.y. from the Sun, and it contains the LAGOON, EAGLE and ETA CARINAE NEBULAE. It extends in the sky from Serpens to Centaurus, and may also continue into Carina to form

SAGITTA (gen. sagittae, abbr. sge) Third-smallest constellation but a distinctive one, representing an arrow, between Vulpecula and Aquila. Its brightest star, 7 Sge, is mag. 3.5. Sge is a triple system with bluish-white and bluish components, mags. 5.6 and 9.0, separation 8".3; the former has a very close companion, mag. 6.0. WZ SAGITTAE is a recurrent nova; FG SAGITTAE is an unusual variable which slowly brightened from about mag. 13.6 in 1890 to mag. 9.4 in 1967. The brightest deep-sky object is M71 (NGC 6838), an 8th-magnitude globular cluster.

SAGITTARIUS (gen. sagittarii, abbr. sgr) Large, conspicuous southern zodiacal constellation, 'the Archer', usually depicted as a centaur aiming an arrow at the heart of neighbouring Scor-pius. It lies between Ophiuchus and Capricornus, in a region rich in star clouds of the Milky Way, in the direction of the centre of the Galaxy. The brightest stars are Sgr (Kaus Australis), mag. 1.8, and a Sgr (NUNKI), mag. 2.1. p Sgr (Arkab) is a wide, naked-eye double with bluish-white and pale yellow components, mags. 4.0 and 4.3; the former has a white companion, mag. 7.2, separation 28".5. RY Sgr is an R CORONAE BOREALIS STAR which is usually around 6th magnitude but from time to time drops unpredictably to 14th magnitude. Eight of the constellation's brighter stars form an asterism known as the TEAPOT.

The brightest part of the Milky Way, just to the north of 7 Sgr (Alnasl), is known as the Great Sagittarius Star Cloud, while another bright part, to the north of u Sgr, is sometimes called the Small Sagittarius Star Cloud. Bright star clusters and nebulae in Sagittarius include: the open clusters M25 (IC 4725), M21 (NGC 6531) and M23 (NGC 6494); the 5th-magnitude globular cluster M22 (NGC 6656), the third brightest in the sky; and the LAGOON NEBULA (M8, NGC 6523), the OMEGA NEBULA (M17, NGC 6618) and the TRIFID NEBULA (M20, NGC 6514), all 6th-magnitude emission nebulae. Also in Sagittarius is the SAGITTARIUS DWARF GALAXY, a dwarf spheroidal galaxy in the Local Group, about 80,000 l.y. away, and the radio source SAGITTARIUS A the carina arm. It may best be traced from its twenty-one centimetre radio emissions from hydrogen and by the presence of young blue stars.

Sagittarius Dwarf Galaxy Disrupted companion to our galaxy, discovered by detailed star counts and distance estimates. It is the closest known galaxy to the centre of our Galaxy, lying mostly behind the central bulge from our point of view so that its sparse stars are lost in the myriad foreground objects. It probably contains the globular cluster M54. The elongated shape revealed by the distribution of stars belonging to the Sagittarius Dwarf suggests that it is being tidally disintegrated by our Galaxy's gravity in the kind of event that may have happened many times during our Galaxy's formation and growth.

Saha, Meghnad (1894-1956) Indian nuclear physicist and astrophysicist who derived the formula (saha'sequa-tion) linking the degree of ionization in a gas to temperature and electron pressure that is vital to the interpretation of stellar spectra. He confirmed his theory of thermal ionization (1920) by studying the solar chromosphere and spectra of stars, especially novae.

Saha equation Equation, formulated by the Indian physicist Meghnad Saha (1893-1956), that determines how the relative number densities of the various levels of ionization (including the neutral atom) of an element change with temperature.

St Andrews University Observatory Teaching and research facility of the University of St Andrews' School of Physics and Astronomy, 50 km (30 mi) north-east of Edinburgh. It is notable for its early work on large schmidt-cassegrain telescopes and the construction of the University's 0.94-m (37-in.) James Gregory Telescope, one of the largest optical telescopes in Britain.

Sakigake Space probe launched to Comet halley in 1985 January by Japan's Institute of Space and Astronautical Science (ISAS). Japan's first deep-space mission, it investigated the interaction between the solar wind and the comet. Sakigake flew past the comet's sunward side on 1986 March 11 at a distance of 6.9 million km (4.3 million mi).

Sakurai's object V4334 in Sagittarius; the central object in a planetary nebula discovered by the Japanese amateur astronomer Yukio Sakurai in 1996 February. He observed a rapid brightening of the 11th-magnitude object, which remained bright for about two years before dimming in mid-1998. The star had probably experienced a late helium flash in 1994. The planetary nebula has since been observed to be expanding at a rate of 31 km/s (19 mi/s). It has an apparent angular diameter of 44".

Sakurai's object is the third late helium flash star to be observed. The first was V605 Aql, which brightened and faded during 1919 to 1923, and the second was fg sagittae, which reached its maximum brightness in 1970 and faded in 1992. It is presumed to have experienced its helium flash at the beginning of the 19th century. See also shell burning

Salpeter process See triple-o process

Sandage, Allan Rex (1926- ) American astronomer who made the first optical identification of a quasar and for many years has worked to establish the distance scale of the Universe and a low value of the hubble constant. Since 1952 Sandage has worked at the Mount Wilson and Palomar Observatories, initially as assistant to Edwin Hubble. In 1960 he and Thomas Arnold Matthews (1927-) used Palomar's 200-inch (5-m) hale telescope to identify a faint, star-like object in the same position as the radio source 3C 48 listed in the third Cambridge catalogue; three years later, Maarten schmidt found the immense redshift in the spectrum of the object, which turned out to be a quasar. In 1965 Sandage found the first radio-quiet quasar.

Sandage also worked on the cosmological distance scale, calibrating the various 'standard candles' in order to establish the distances of galaxies. With Martin schwarzschild, he studied the evolution of globular clusters in order to determine their ages. The ages of their oldest stars were not compatible with a relatively young, small Universe, as favoured by Gerard de vaucouleurs. In 1976 Sandage and Gustav Andreas Tammann (1932- ) gave a value for the Hubble constant of H0 =50 km/s/Mpc, half that given by De Vaucouleurs. In 1979 Sandage proposed that one of the main tasks of the Hubble Space Telescope (HST) should be to strive towards fixing the value of H0. Since its launch, Sandage and others have used the HST to examine cepheid variables and supernovae in distant galaxies, deriving in the late 1990s a value of H0 = 60 km/s/Mpc.

Saiph The star k Orionis, visual mag. 2.07, distance 722 l.y., spectral type B0.5 Ia. The name comes from the Arabic saif, meaning 'sword', but is wrongly applied as this star actually marks the right leg of Orion.

Salpeter, Edwin Ernest (1924- ) Austrian-American astrophysicist, who emigrated first to Australia, then to England and the USA, and specialized in stellar evolution. He spent most of his career at Cornell University, where he worked with Hans bethe applying relativity theory to atomic physics. He was the first to explain how highly evolved stars convert hydrogen to helium to carbon by the triple-a reaction (known also as the Salpeter reaction). By relating the observed abundances of stars of various luminosities to his models of stellar evolution, Salpeter was able to derive the initial mass function (known also as the Salpeter function), which predicts the rate at which these stars will form in the Milky Way based upon their masses.

SALT Abbreviation of southern african large telescope

Salyut space stations Name given to seven Soviet space stations. Two different design bureaux were involved in their development, and each bureau produced its own versions in 1971-76, one series for 'civilian' use (Salyuts 1, 2 and 4) and another 'military' version known as 'Almaz' (Salyuts 3 and 5). Both versions weighed about 18.5 tonnes.

Crews of two or three cosmonauts were ferried up to Salyut by the soyuz spacecraft. The first five Salyuts were equipped with only one docking port. Commencing with Salyut 6, a second docking port was made available. This allowed automatic Progress and heavy Cosmos ferry craft to carry supplies and fuel to the stations, and enabled semi-permanent occupation of the stations by long-duration crews. The last of the series, Salyut 7, re-entered the atmosphere in 1991 February.

Various astronomical experiments were carried on the stations, including instruments to observe the Sun, X-ray and infrared telescopes (Salyut 4) and a submillimetre telescope (Salyut 6).

SAMPEX Abbreviation of solar anomalous and magnetospheric particle explorer

S Andromedae First extragalactic supernova to be detected. Discovered near the centre of the andromeda galaxy (M31) in 1885, the star attained an apparent magnitude of +6.5. Supernovae were then not recognized as a distinct class of objects. The assumption that the star was an ordinary NOVA led to a distance estimate for M31 of about 8000 l.y., well within our own Galaxy. Later it was realized that M31 was a galaxy similar to our own and that S Andromedae, shining temporarily with a sixth of the total light of the galaxy, was vastly more brilliant than a typical nova.


SARA Abbreviation of SOCIETY OF AMATEUR RADIO ASTRONOMERS satellite Smaller body orbiting a larger one. At the most massive end of the scale, this can be taken to refer to small galaxies orbiting a larger primary as, for example, M32 and NGC 205 in relation to the Andromeda Galaxy. Likewise, in this sense, planets are satellites of their star, though they are rarely spoken of in this way. Instead, the term is most commonly applied to a body in orbit about a planet. Every planet in the Solar System, except Mercury and Venus, has at least one natural satellite (as opposed to ARTIFICIAL SATELLITE) in orbit about it. The discovery of planetary satellites was important because measurement of the period and dimensions of a satellite's orbit enables the mass of its planet (or, strictly, the combined mass of planet and satellite) to be determined.

Written without a capital 'M', the term 'moon' is synonymous with 'natural satellite', whereas the Moon is the name of the Earth's only such satellite. The MOON and the Earth have been companions since the final stages of the Earth's formation, 4.5 billion years ago, and the Moon is believed to have formed from the debris of a giant collision (see GIANT IMPACTOR THEORY). A similar process could account for the origin of Pluto's only known satellite, CHARON, which is the most massive satellite relative to its planet. In contrast Mars has two small irregularly shaped satellites, PHOBOS and DEIMOS, which are asteroids captured by Mars probably within the past billion years.

Each of the four GIANT PLANETS has an extensive satellite system. At the outer fringes of the planetary RING system are small, bright, icy moonlets in circular orbits; they may be fragments of larger satellites destroyed by collisions or tidal forces. Slightly nearer the planet come several satellites, such as Jupiter's GALILEAN SATELLITES, that are large enough (greater than about 400 km/250 mi in diameter) for their own gravity to pull them into a spheroidal shape. These too are in virtually circular orbits and are believed to have grown by ACCRETION within circumplanetary disks while the planets were forming. This far from the Sun, the SOLAR NEBULA was cold enough to allow the direct condensation of ice, so the large satellites are mostly icy bodies with deeply buried rocky interiors. At Jupiter, the ice is composed just of water, but with increasing distance from the Sun more volatile species are present, such as methane, ammonia and nitrogen.

Neptune, exceptionally, has only one large satellite, TRITON. It is in a RETROGRADE orbit and is presumed to be a captured EDGEWORTH-KUIPER BELT object. Any family of large satellites previously possessed by Neptune is likely to have been lost during the capture process.

Two of Saturn's large satellites, TETHYS and DIONE, are accompanied in their orbits by small (less than 30 km/20 mi in diameter) irregularly shaped satellites that occupy stable LAGRANGIAN POINTS and bear the same geometric and dynamic relation to them as do the TROJAN ASTEROIDS to Jupiter. Beyond their large satellites, the giant planets have numerous smaller satellites, mostly in inclined, eccentric, and in some cases retrograde, orbits. These satellites are dark irregular objects, possibly coated by carbonaceous material, and are probably captured asteroids or comet nuclei.

TIDAL FRICTION has caused the rotation period of most satellites to become synchronous with their orbital period, so that like our own Moon they always keep the same face towards their planet (see SYNCHRONOUS ROTATION). Slowing down of a large, originally rapidly spinning satellite in this way could cause considerable internal TIDAL HEATING, which would have allowed internal DIFFERENTIATION to take place, with the formation of a rocky CORE (and possibly an iron-rich inner core) beneath an icy MANTLE. Tides are also important in causing satellite orbital ECCENTRICITY to decrease until, in the absence of other perturbations, the orbit would become circular. However, the large satellites of Jupiter, Saturn and Uranus are massive enough to have a significant gravitational influence on one another. The orbits of adjacent satellites have often evolved into a state of orbital RESONANCE, meaning for example that one satellite has twice the orbital period of the next.

When in a state of orbital resonance, the influence of other satellites prevents an orbit from becoming exactly circular. At times the varying forces on the TIDAL BULGES raised on a large satellite by its planet may then be sufficient to act as a significant source of tidal heating, leading to volcanism and/or fracturing of the surface. This activity occurs at the present time within Jupiter's satellites IO and EUROPA, and it clearly affected several other large satellites in the past. Without tidal heating, satellites would simply have ancient surfaces heavily scarred by a 4-billion-year history of impact cratering. One of the greatest revelations of the VOYAGER missions, which were the first to explore the satellites of the outer planets in any detail, was the amazing variety of landscapes they possess.

Some asteroids are now known to have their own tiny satellites. The first, named DACTYL,was discovered on images of the asteroid IDA sent back by the GALILEO spacecraft in 1993. The second, an object 18 km (11 mi) across, was discovered telescopically in orbit around the asteroid EUGENIA (diameter 215 km/134 mi) in 1998.

satellite laser ranger (SLR) Specialized telescope that uses time-of-flight measurements of short pulses of laser light to determine the distances to satellites in Earth orbit. The principle is the same as that of radar, except that light, rather than radio waves, is used. Pulses of laser light are beamed from the telescope at dedicated satellites covered in retro-reflectors. These work like 'cat's eyes' and reflect the light back in the direction from which it came. By timing how long it takes the light to travel to and from the satellite, the distance can be computed to an accuracy of a few centimetres. Because the satellites are in very stable and well-known orbits, the results provide information about the rotation of the Earth, the geometry and deformation of its surface, and variations in its gravitational field.

SLR stations are located all over the globe and the results from each one make it possible to measure very accurately the distance between points on the Earth's surface, making it a good technique for studying plate tectonics. Satellite laser rangers have replaced the PHOTOGRAPHIC ZENITH TUBE (PZT) as the primary means of measuring variations in the Earth's rotation.

Saturn Sixth planet in the Solar System and the second largest; it was the most distant planet known to man before the development of the telescope. The telescopic appearance of the planet is dominated by the majestic system of rings, which were probably first seen by Galileo in 1610, even though he did not recognize their nature (he believed Saturn to be a triple planet). The rings lie in the plane of the planet's equator and are tilted by 27 with respect to its orbit. Consequently, the faces of the rings will be alternately inclined towards the Sun and then the Earth by up to 27. At intervals of approximately 15 years, the rings become edge on to the Earth and are virtually invisible to the observer. This situation occurred in 1995-96. The rings are extremely reflective, so that they can add significantly to the total brightness of the planet. The distance from one edge of the rings to the other is more than 275,000 km (171,000 mi), which is nearly three-quarters the distance of the Earth to the Moon. Saturn is now known to have at least 30 satellites.

Saturn In this high-resolution image of Saturn from the Hubble Space Telescope, the belts in the atmosphere are clearly visible. The open angle of the rings means that some of the finer details can be seen.

Saturn is composed of hydrogen and helium, but they are not in solar proportions. There is a helium depletion in Saturn where the mass fraction is only 11%, compared with the solar abundance of 27%. This significant difference, when compared with JUPITER,may be due to the differing internal structures and current stages of evolution of the two planets. It is thought that the internal temperatures are too low for helium to be uniformly mixed with hydrogen throughout the deep interior. Instead, the helium may be condensing at the top of this region, where the gravitational energy is then turned into heat. This process may have started 2000 million years ago when the temperatures first dropped to the helium condensation point. For Jupiter, this situation can only have been reached recently. The other primary constituents of the atmosphere are ammonia, methane, acetylene, ethane, phosphine and water vapour.

Saturn has a similar visible appearance to Jupiter, with alternating light and dark cloud bands, known as zones and belts, respectively. Saturn's clouds are more subtle and yellowish and, therefore, less colourful than those found on Jupiter. Contrast is also muted by an overlying haze layer. Consequently, the cloud features and associated spots, although varied in nature and colour, are less prominent on Saturn. Several stable ovals of various colours (white, brown and red) have been observed in Saturn's atmosphere. The white clouds are composed of ammonia particles, and the other colours are generated through dynamical and photochemical actions and reactions in the atmosphere. Three brown spots, situated at 42N during the VOYAGER encounters, were seen to behave in a similar fashion to the Jovian white ovals. The largest features seen were a reddish cloud 10,000 X 6000 km (6000 X 4000 mi) at 72N and a red spot 5000 X 3000 km (3000 X 2000 mi) at 55S. These features demonstrate the non-uniqueness of the Jovian Great Red Spot (GRS) and other cloud ovals. A major characteristic of the Saturnian mid-latitude weather systems is a jet stream, which produces alternate high- and low-pressure systems in the same way as the terrestrial phenomenon. Anti-cyclonically rotating cloud systems, like the Jovian GRS and trains of vortices, familiar in the Earth's atmosphere, are also seen. The Saturnian weather systems, like those of Jupiter, are strongly zonal. At the equator the cloud top winds reach 500 m/s (1600 ft/s), which is equivalent to three-quarters of the speed of sound at this level. Although Saturn has a strong internal heat source, the weather systems of Saturn (and of Jupiter and the Earth) are driven by the transport of energy from small-scale features into the main zonal flow.

Saturn The Hubble Space Telescopes STIS (Space Telescope Imaging Spectrograph) instrument imaged Saturns aurorare in 1994. They only glow in ultraviolet light and so are only detectable from above the Earths atmosphere.

Probably the most dramatic cloud features witnessed in the Saturnian atmosphere are the Great White Spots, which break out at roughly 30-year intervals coincident with the planet's northern hemisphere midsummer, as in 1876, 1903, 1933, 1960 and 1990.

The interior of Saturn is thought to consist of an Earth-sized iron-rich core of ammonia, methane and water, which is enclosed by about 21,000 km (13,000 mi) of liquid metallic hydrogen, above which extends the liquid molecular hydrogen and the extensive cloud layers. It is in the metallic hydrogen region that the magnetic field is created by the dynamo action from the rapidly rotating planet. The first detection of the Saturnian magnetic field was made from the PIONEER 11 spacecraft in 1979. At the cloud tops the equatorial field has a strength of 0.21 G, compared with the Earth's value of 0.31 G. The magnetic axis is within 1 of the axis of rotation; it is therefore the least tilted field in the Solar System.

Saturn The rapid changes in Saturns atmosphere can be seen in this pair of Voyager 2 images taken just over ten hours apart. The smallest features visible are more than 50 km (30 mi) across.

The MAGNETOSPHERE of Saturn is intermediate between those of the Earth and Jupiter in terms of both extent and population of the trapped charged particles. The average distance of the BOW SHOCK is 1.8 million km (1.1 million mi), while the MAGNETOPAUSE itself lies much closer to the planet, at 500,000 km (300,000 mi). These distances are, of course, extremely variable, since their precise positions will depend on the temporal behaviour of the SOLAR WIND. The largest satellite, TITAN, is situated near the magnetopause boundary and regularly crosses this division. The magnetosphere is divided into several definite regions. At about 400,000 km (250,000 mi) there is a torus of ionized hydrogen and oxygen atoms; the plasma's ions and electrons spiral up and down the magnetic field lines, contributing to the local field. Beyond the inner torus, there is a region of plasma, extending out to 1 million km (600,000 mi), produced by material coming partly from Saturn's outer atmosphere and partly from Titan. The magnetotail has a diameter of about 80 RS (Saturnian radii). There is a strong interaction between the charged particle environment and the embedded satellites and rings that surround Saturn. All these bodies absorb the charged particles and have the effect of sweeping a clear path through the region where they are located. There is also auroral activity in the polar regions, where the charged particles cascade into the upper atmosphere. The Saturnian aurorae are about two to five times brighter than the equivalent terrestrial phenomena.

Saturn is a powerful radio source, emitting broad band emissions in the ran ge from about 20 KHz to about 1 MHz. The maximum intensity occurs between 100 and 500 KHz with a period of 10h 39m.4, which corresponds to the System III rotation period. The ionosphere, residing above the neutral mesosphere, contains mainly ionized hydrogen, which is strongly controlled by the solar wind.

The interaction between the charged particle environment and the rings is unique at Saturn, where radial features or spokes have been seen in the b ring. These features are confined to the central portion of the B ring and appear to correspond to a location where only small, micrometre-size particles are present. The ring particles become electrically charged and appear as a torus above the plane of the rings. Each spoke, which is generally wedge-shaped and about 6000 km (3700 mi) long, has a lifetime of a few planetary rotations. The detection of electrostatic discharges (lightning) at radio wavelengths suggests that this mechanism is related to the formation of the spokes.

Saturn Most of Saturns satellites orbit the planet directly, but the outer satellites Iapetus and Phoebe have retrograde orbits and are in a different plane from the others, which may imply that they are captured asteroids.

Saturn Nebula (NGC 7009) planetary nebula in western Aquarius (RA 21h 04m.2 dec. -1122'). At an estimated distance of 160 l.y., it is one of the closest to our Solar System. Discovered by William herschel in 1782, the Saturn Nebula takes its popular name - given by Lord Rosse in the 1850s - from the extensions, or ansae, seen to either side, which give it a resemblance to Saturn when its rings are presented close to edge-on. The nebula has overall magnitude +8.0 and covers an area of 0'.4 X 1'.6. The central star is of magnitude +11.5.

Saturn rocket Rocket developed by NASA in the early 1960s for manned spaceflight. The Saturn V, the largest, most powerful rocket ever built, was used for the apollo missions to the Moon. Its thirteenth and last flight was to insert the skylab space station into Earth orbit in 1973.

Saturn rocket Many of NASAs missions have been launched on Saturn rockets, including all of the Apollo trips to the Moon. Here, a Saturn 1B rocket is carrying the third and final crew to Skylab in 1973.

The final Saturn IB launch took place in 1975 during the Apollo-Soyuz Test Programme.

saros Cycle of 18 years 10.3 days after which the Sun, Moon and the nodes of the Moon's orbit return to almost the same relative positions. It is a result of the regression of the nodes. The saros was known to the ancient Babylonian astronomers; it was used to predict eclipses, since any eclipse is usually followed by a similar one 18 years 10.3 days later. Slight variations from cycle to cycle mean that the eclipses are not identical. For example, the total solar eclipse of 1999 August 11 was potentially visible along a track from south-west England to France, Germany, Turkey, Iran and India: the next one in the cycle, on 2017 August 21, will be visible from the United States.

scale height Increase in height over which a physical quantity, for example atmospheric density or pressure, declines by a factor of e (e = 2.71828). For example, at an altitude of one scale height, the density of an atmosphere is 1/e (= 0.37) of the value at its base. In the case of the Earth's troposphere, the scale height is approximately 8.5 km (5.3 mi).

scarp Steep slope of some extent along the margin of a plateau, terrace or bench. Linear scarps on planets and satellites often represent the surface expression of faults; sinuous scarps are usually the result of fluvial (water, lava) erosion. The inner slopes of volcanic craters and calderas are often scarps. In planetary nomenclature, such features are identified by the term Rupes.

scattering Deflection of electromagnetic radiation by particles. Where the particles are very much larger than the wavelength, scattering consists of a mixture of reflection and diffraction, and is largely independent of wavelength. Where the particle sizes are very much smaller than the wavelength, the amount of scattering is inversely proportional to the fourth power of wavelength and so blue light is scattered about ten times more efficiently than red; this is known as rayleigh scattering. The blueness of the sky is due to Rayleigh scattering of sunlight by atoms and molecules in the atmosphere. The setting sun appears red because far more blue than red light is scattered out of the line of sight and thus a greater proportion of red light penetrates to the observer. Scattering is described as elastic if no change in wavelength is produced, and inelastic if the wavelength is increased or decreased by the encounter. Interstellar dust produces both scattering and absorption of starlight; the combined effect is known as extinction. The amount of extinction is inversely proportional to the wavelength.

Scheat The star p Pegasi, distance 199 l.y., one of the four stars that make up the square of pegasus. It is a red giant, spectral type M2 II or III, and varies irregularly between mags. 2.3 and 2.7. Its name comes from the Arabic saq, meaning 'leg'.

Scheiner, Christoph(er) (1575-1650) German Jesuit scholar and astronomer who, independently of galileo, discovered sunspots (1611). He initially tried to explain these spots, not as features on the Sun's surface, which would imply that the Sun was 'imperfect', but as the shadows of small intra-Mercurian planets. Galileo, who claimed that his own sunspot observations predated those of Scheiner, countered by showing that sunspots are features of the solar disk and do not move independently of it. Scheiner was the first to make systematic observations of sunspots (1611-25). Now acknowledging that sunspots move across the solar disk, he demonstrated from plots of their paths that the Sun's axis of rotation is inclined by 7 30' to the ecliptic. He invented the first specialized solar telescope, which he called a heliotropii telio-scopici, or 'helioscope'. This telescope was significant for two reasons: it was the first to have an equatorial mounting, and the first to project the image of the solar disk on to a plane surface beyond the eyepiece.

Schiaparelli, Giovanni Virginio (1835-1910) Italian astronomer whose mapping of Mars gave rise to the idea of Martian canals, and who discovered the relations between comets and meteor showers. Schiaparelli joined Brera Observatory in 1860, becoming director two years later and remained there for the rest of his working life.

His studies of cometary tails in the 1860s, and his researches into previously observed comets, persuaded him that the particles that cause meteors are shed from cometary tails and follow elliptical or parabolic orbits around the Sun, identical with the orbits of comets. Schia-parelli originated the convention of naming showers after the constellation containing their radiant.

In 1877 Brera acquired new equipment in time for the opposition of Mars that year. Schiaparelli prepared a map of the surface of Mars, introducing a nomenclature for the various features that remained standard until the planet was mapped by space probes. His adoption of Angelo SECCHl's term canali (see CANALS, MARTIAN), to describe vague linear markings was the cause of much subsequent controversy; his reporting two years later of the apparent doubling of some canals (he called it 'gemination') further fuelled the debate over their origin.

Schiaparelli prepared maps of Venus and Mercury, believing - mistakenly - that they had SYNCHRONOUS ROTATION. He also made important studies of double stars, and discovered the asteroid (69) Hesperia.

Schickard Very much degraded lunar crater (44S 54W), 202 km (125 mi) in diameter, with rim components reaching 2800 m (9500 ft) above the floor. Because of foreshortening, this crater appears oblong, though its actual shape is more nearly circular. Long after its formation, Schickard was flooded by lunar basalts, giving the floor a dark patchy appearance.

Schlesinger, Frank (1871-1943) American astronomer who directed Allegheny Observatory, Pittsburgh (1905-20) and Yale University Observatory (1920-41). He used astrophotography to determine stellar parallaxes, supplanting the old visual methods. Schlesinger compiled several extensive 'zone catalogues' of precise positions and other data for more than 150,000 stars, and the first edition of the bright star catalogue.

By the late 1960s, enough quasars had been found for Schmidt to use them as a cosmological test for the then rival BIG BANG and STEADY-STATE THEORIES. According to the latter, the large-scale structure of the Universe should be the same at all places and all times. However, the number of quasars was found to increase with distance, supporting the Big Bang theory.

Schmidt camera Type of reflecting TELESCOPE incorporating a spherical primary mirror and a specially shaped glass correcting plate to achieve a wide field of view combined with fast focal ratio.

Schmidt camera This type of reflecting telescope gives a wide field of view. They are used only for astrophotography and are useful in survey work.

The conventional paraboloidal primary mirrors of reflecting telescopes only form a perfect point image as long as the incoming parallel light is exactly aligned with the axis. Departure from this causes steadily increasing degradation of the images due to coma, which is noticeable at 'off-axis' angles as small as a few arcminutes.

The Schmidt camera, which was invented by Bernhard Schmidt in 1930, overcomes this problem by the use of a spherical rather than a paraboloidal, primary mirror. This eliminates the problem of coma but introduces SPHERICAL ABBERATION. To correct this, a thin glass plate, known as a 'Schmidt corrector' and with a very shallow profile worked into one or both of its surfaces, is placed at the centre of curvature. The profile is complex in shape and has to be precisely computed to match and counteract the spherical aberration of the mirror, thus producing perfect images over a wide field of view. Because the image is formed on a curved surface, the photographic film or plate must be deformed by clamping it in a suitably shaped holder.

Schmidt cameras are used photographically to record detailed images of very large areas of sky. This makes them ideal for observing extensive objects such as comets or certain nebula, or for recording large numbers of more compact sources, such as stars or distant galaxies, for statistical studies. Perhaps their most important role is as survey telescopes used for identifying unusual or specific kinds of object, which can then be investigated in more detail with larger, conventional (narrow-field) instruments. Hybrid designs incorporating secondary mirrors (Schmidt-Cassegrains) or additional correctors (BAKER-SCHMIDTS) have also been developed. See also RITCHEY-CHRETIEN TELESCOPE

Schmidt, Bernhard Voldemar (1879-1935) Estonian optician of Swedish-German parentage who invented the schmidt camera. He made lenses and mirrors for several European observatories, including 300-mm (12-in.) and 600-mm (24-in.) mirrors for the University of Prague, and he refigured a 500-mm (20-in.) Steinheil objective later used by Ejnar Hertzsprung to measure very close double stars. In 1930 he made a telescope with a 440-mm (17i-in.) primary mirror and a 360-mm (14-in.) corrector plate which he figured while it was being deformed under vacuum - when the vacuum was released, the plate assumed the proper shape to eliminate coma and other aberrations of the primary mirror.

Schmidt, Maarten (1929- ) Dutch-American astronomer, the first to realize the great distance of quasars when he identified highly redshifted lines in their unusual spectra. Schmidt left Leiden Observatory in 1959 for the california institute of technology, and became director of the Hale Observatories (Mount Wilson and Palomar) in 1978.

Following the discovery of the faint optical counterpart of the radio source 3C 48 in 1960 by Allan sandage and Thomas Matthews (1927- ), it was found to have a curious spectrum. These and similar objects, including 3c 273, were named 'quasi-stellar objects', or quasars. Schmidt showed that broad emission lines in the spectrum of 3C 273 were immensely redshifted, and that if this were

Schmidt-Cassegrain telescope (SCT) Short-focus telescope combining features of the schmidt camera and the cassegrain telescope. The SCT has the Schmidt's spherical primary mirror and specially figured corrector plate, but light is reflected back down the tube by a convex secondary mirror mounted behind the plate and through a hole in the primary to a Cassegrain focus. This makes for a highly compact and portable telescope that has become very popular with amateur astronomers.

Schonberg-Chandrasekhar limit Upper limit on the mass of the helium core of a main-sequence star. A star evolves off the main sequence when it has finished burning hydrogen in its core. This occurs when the star has used about 10% of its total mass in hydrogen burning. The limit can be larger if electron degeneracy becomes important within the core; this can occur in lower-mass stars where the core has a higher density. The limit is named after Mario Schonberg (1916- ) and Subrahmanyan chandrasekhar.

Schramm, David Norman (1945-97) American theoretical astrophysicist who pursued links between particle physics, astrophysics and cosmology. In 1977 he was part of a team that constructed a model using the cosmic abundances of light elements to show that there could be no more than four families of elementary particles, a prediction confirmed in the late 1980s by particle accelerator experiments. Calculations he made suggested that the matter observable by telescopes accounts for only a small fraction of the Universe's total substance - the rest must be some form of dark matter. Schramm developed cos-mological models that incorporated quarks to explain the Universe's large-scale structure.

Schroter, Johann Hieronymus (1745-1816) German amateur astronomer and chief magistrate of Lilienthal, near Bremen, where he erected his observatory. Schroter's 19-inch (480-mm) Newtonian reflector was for a time the largest in continental Europe. He made detailed observations of the topographic features of the Moon and planets.

SCORPIUS (gen. scorpii, abbr. sco) triking southern zodiacal constellation between Ophiuchus and Ara, and one of the few that even remotely resembles the object after which it was named - the mythological scorpion that killed the hunter Orion. Its brightest star, antares (which marks the heart of the scorpion), is a red giant irregular variable (range 0.9-1.2) and close binary; its bluish-white companion, mag. 5.4, separation 2".9, appears pale green in comparison to its primary. Next brightest at mag. 1.6 is X Sco (shaula). p1 Sco (known as Acrab or Graffias) is a double star with bluish-white components, mags. 2.6 and 4.9, separation 13".9, while v Sco consists of two bluish-white stars, mags. 4.1 and 6.8, separation 41", each of which has a fainter, close companion. U Sco is a recurrent nova which is usually around 18th magnitude but which flared up to 9th magnitude in 1863, 1906, 1936 and 1979. Bright star clusters and nebulae in Scorpius include the naked-eye open clusters NGC 6231, M7 (NGC 6475) and the butterfly cluster (M6, NGC 6405); the globular clusters M4 (NGC 6121), 6th magnitude, and M80 (NGC 6093), 7th magnitude; and the Bug Nebula (NGC 6302), a 10th-magnitude planetary nebula. Also in Scorpius is scorpius x-, the brightest X-ray source in the sky.

His two-volume Selenographische Fragments (1791, 1802) contained descriptions, drawings and height measurements for hundreds of lunar craters and mountains. His 1785 October observations of small, dark spots on Jupiter that soon vanished may have been the earliest record of a comet impacting that planet, predating the shoemaker-levy 9 impacts by over two centuries. In 1787 he discovered the solar granulation and also the 'light bridge' features of sunspot umbrae. See also celestial police

Schroter effect See phase anomaly

Schroteri Vallis (Schroter's Valley) Enormous lunar valley in the aristarchus uplift. It appears to be a vast lava flow structure, which emptied out into Mare imbrium. Embedded within the rille is another much smaller sinuous rille. The valley begins in an elongated depression called the Cobra Head, which was probably the source vent for the basalts.

Schwabe, (Samuel) Heinrich (1789-1875) German pharmacist and amateur astronomer who discovered (1843) the approximately 11-year solar cycle. In 1825 he began to search for an intra-Mercurian planet. He realized that he needed to keep a careful record of sunspots, as any planet crossing the solar disk would closely resemble one of these features. Acquiring a Fraunhofer refractor, he continued recording sunspots, and 17 years later he noticed a periodicity in their number. Schwabe's 11-year cycle was later conirmed by the discovery (1852) that the Sun's magnetic ield also varies over the same period. Schwabe made one of the earliest drawings of Jupiter's great red spot (1831).

Schwarzschild, Karl (1873-1916) German astronomer remembered chiefly for his theoretical work, especially for inding the irst solution to the equations of Einstein's general theory of relativity. In the ive years leading up to World War I, Schwarzschild directed both Gottingen and Potsdam Observatories; in 1914 he volunteered for military service, and died two years later of a skin disease contracted on the Eastern Front.0

Schwarzschild designed and improved a number of instruments and techniques for use in spectroscopy and astrophotography. He pioneered photometric methods for measuring stellar magnitudes, demonstrating that there is a difference (the colour index) between a star's magnitude measured from a photographic plate and the visually estimated value.

He was one of the irst, in the mid-1900s, to examine radiative transfer in stars, showing that radiation and gravitation are in equilibrium. His most famous work was done in the inal year of his life. As early as 1900, he had suggested that the geometry of space could be non-Euclidean. In 1916, shortly after the theory of general relativity was published, Schwarzschild worked out the irst exact solution of Einstein's ield equations, from which he developed the concepts of what are now called the schwarzschild radius and the black hole. His son, Martin schwarzschild, became an astrophysicist.

Schwarzschild, Martin (1912-97) German-American astrophysicist, the son of Karl schwarzschild, who made major contributions to stellar structure and evolution. After taking his doctoral degree at Gottingen University (1935), he emigrated to the USA, joining Princeton in 1951 and remaining there for the rest of his life. He made detailed studies of the structure of the Sun and other nearby stars, comparing their masses, temperatures and other properties. Schwarzschild's research placed upper and lower limits on the masses of the stars, demonstrating that objects smaller than 10 Jupiter masses lack suficient mass to initiate hydrogen fusion, while stars of more than 65 solar masses are rare. He calculated ZHe, the total mass density of elements heavier than helium, showing that this quantity varies with the age of a particular star. His work on stellar evolution mainly concerned 'pulsation theory',

SCULPTOR (gen. sculptoris, abbr. scl) which explains the light-curves of variable stars in terms of expansions and contractions of these objects.

Schwarzschild radius Radius that a body must exceed if light from its surface is to reach an outside observer. If an object collapses below this radius, its escape velocity rises to above the speed of light and the object becomes a black hole. For a black hole, the Schwarzschild radius is equal to the radius of the event horizon.

The Schwarzschild radius is proportional to the mass of a body. For a non-rotating body of mass M and no charge, the Schwarzschild radius, Rs, equals 2GM/C2, where G is the gravitational constant and c is the speed of light. For a body the size of the Sun, the Schwarzschild radius is around 3 km (2 mi); for the Earth it is about 9 mm (0.4 in.).

Schwassmann-Wachmann 1, Comet 29P/ One of three periodic comets discovered by Arnold Schwassmann (1870-1964) and Arno Wachmann (1902-90) at Hamburg Observatory. Comet Schwassmann-Wachmann 1 was found photographically on 1927 November 15. The comet has a more or less circular orbit, just beyond that of Jupiter, at a distance of 6AU from the Sun. It can be observed for much of the year. Normally around magnitude +17 to +18, it is prone to unexplained, irregular outbursts, on occasion reaching magnitude + 10.

scintar Source that shows the effects of interplanetary radio scintillation and is therefore point-like rather than extended. This property allows quasars and radio stars to be distinguished from radio galaxies and nebulae. Stars and planets can be distinguished from each other because Earth's atmospheric variations cause the point-like stars to twinkle and the planets (that have a visible disk) to remain steady; a similar twinkling occurs with radio sources but from a different cause. Radio waves are refracted by the plasma clouds that move outwards from the Sun through the Solar System. The point-like radio sources are affected by the clouds, and so scintillate. This means the sources are either intrinsically very small, or very distant (and so appear small). The scintillation method was used to discover quasars, with the 4.5-acre radio telescope in Cambridge. To see the twinkling it was necessary to make the radio receiver respond very quickly to changes in radio intensity. This made it possible for Jocelyn bell to discover pulsars during the course of the scintillation measurements.

scintillation Twinkling of stars and, to a lesser extent, of planets as a result of the uneven refraction of light in areas of different density in the Earth's atmosphere. To the naked eye, scintillation appears as a change in brightness and colour, while in the telescope it may also make the star appear to make rapid slight movements. It is greatest at low altitudes, when the object's light shines through a greater amount of the atmosphere. Most of the effect is caused below 9000 m (30,000 ft).

The planets twinkle less than stars because they present small disks rather than points of light and the whole disk is unlikely to be affected simultaneously. In a telescope, however, the disk will appear blurred; this effect is usually referred to as bad seeing. Scintillation depends on the weather, and is usually greatest at about noon. Modern observatories are sited where the effect occurs least. See also radio scintillation; scintar

Sciama, Dennis William (1926-99) English cosmolo-gist who applied general relativity to problems as far-ranging as black hole thermodynamics and Mach's principle, which holds that the appearance and structure of the 'local' or nearby Universe is affected by matter located at cosmological distances. Originally a proponent of the steady-state theory of cosmology, Sciama re-focused his research towards Einstein-de Sitter models of a big bang universe when observational evidence showed that the former theory was seriously flawed.

Inconspicuous southern constellation between Cetus and Phoenix. It was named Apparatus Sculptoris (the Sculptor's Workshop) by Lacaille in the 18th century. Its brightest star, a Scl, is mag. 4.3; e Scl is a binary with pale yellow and yellow components, mags. 5.4 and 8.8, separation 4".9, period about 1200 years. Deep-sky objects include NGC 288, an 8th-magnitude globular cluster; NGC 253 and NGC 55, 8th-magnitude edge-on spiral galaxies; the Sculptor Dwarf Galaxy, a dwarf spheroidal galaxy in the local group, about 280,000 l.y. away; and the cartwheel galaxy.

Scorpius See feature article

Scorpius Centaurus association Group of about 45 O and B-type stars that are all about 500 l.y. away from us. The stars are found across the two constellations; they form the nearest ob association to the Solar System.

Scorpius X-1 First cosmic X-ray source to be discovered (in 1962), and the brightest known apart from certain x-ray transients. It is a low-mass x-ray binary with an orbital period of 19.2 hours. One member is a neutron star and the other member is unknown, except that it has an ultraviolet excess and optical flaring, and it supplies material to the accretion disk around the neutron star.

Scotch mount Simple camera mount, based in two boards joined by a hinge, that can be driven by hand or a small motor to follow the stars' sidereal motion, allowing moderate-length wide-angle exposures without trailing. The hinge is aligned to the celestial pole. By turning a screw of known pitch at a defined rate, the upper board -to which the camera is attached - can be turned on the hinge away from the lower to mimic the westwards tracking of an equatorial mounting. For exposures up to about 20 minutes, this gives satisfactory non-trailed star images. The mount takes its name from the Scottish roots of George Youngson Haig (1928- ) who designed and popularized it in the early 1970s; it is also sometimes known as the Haig mount, while American amateurs know it as the barn-door mount.

SCT Abbreviation of schmidt-cassegrain telescope

Sculptor See feature article

Scutum See feature article

S Doradus star (SDOR, Hubble-Sandage variable, luminous blue variable) Hot, extremely luminous eruptive variable. S Doradus stars are supergiants of spectral class Op-Fp, with masses that exceed 30 solar masses and luminosities generally at least one million times that of the Sun. They are among the brightest stars in their parent galaxies. Their variations have magnitudes of between 1 and 7, although greater ranges have been noted, and the variations occur over periods of tens of years. Although often irregular, some stars show cyclic activity. Many are associated with expanding envelopes.

SCUTUM (gen. scuti, abbr. sct) Small, inconspicuous southern constellation to the south-west of Aquila. It was named Scutum Sobiescianum (Sobieski's Shield) by Johannes Hevelius in 1684, to honour King Jan Sobieski III of Poland. Its brightest star, a Sct, is mag. 3.9. R Sct is an RV Tauri star, varying between 4.2 and 8.6 with a period of about 147 days; 8 Sct, a pulsating variable (range 4.6-1.8, period 0.19 day), is the prototype delta scuti star. The brightest deep-sky object is the wild duck cluster (M11, NGC 6705).

The mass-loss rate changes with time and the corresponding changes in the envelope contribute to the observed visual brightness changes. The ejection of matter is probably connected with the rapid evolution of a star whose luminosity is near the upper limit for stable stars. ETA CARINAE is generally included in this class, as is P CYGNI. See also GAMMA CASSIOPEIAE STAR


Sea Launch US-led international company that markets launches to equatorial geostationary transfer orbit (GTO) from an offshore platform positioned on the equator in the mid-Pacific Ocean. It uses a modified Ukrainian ZENIT 2 booster with a Russian upper stage. A launch from the equator saves energy, which can be converted into added payload weight. Launches to GTO from higher or lower latitudes require more propellant for 'dog-leg' manoeuvres to reach an equatorial orbit, so payload capability is lost. The Sea Launch Zenit 3SL flies from the Odyssey platform, a converted semi-submersible oil rig positioned close to a launch control ship, Sea Commander. The vessels are based at San Diego, California. Sea Launch offers flights of payloads weighing 6 tonnes to GTO. Sea Launch has flown five times, four times successfully.

Seares, Frederick Hanley (1873-1964) American astronomer who standardized the stellar magnitude system and successfully mapped the Milky Way's structure. After directing the Laws Observatory of the University of Missouri (1901-09), he joined the staff of Mount Wilson Observatory, where he spent the remainder of his career (1909-45). In 1922, the IAU adopted his NORTH POLAR SEQUENCE of fundamental stellar magnitudes as the basis of its photographic and photovisual systems of stellar photometry. Collaborating with Frank E. ROSS, Seares compiled an extremely accurate catalogue of 2271 northern circumpolar stars brighter than 9th magnitude. In his many research projects, he often collaborated with his wife, Mary (Cross) Joyner Seares.

season Part of a cyclical variation in the climatic conditions on the surface of a planetary body over the course of a single revolution around the Sun, brought about by the inclination of the body's axis of rotation to the plane of its orbit.

Because the Earths axis is inclined relative to its plane of orbit, the Earth experiences seasons. The side that is tilted towards the Sun gets more light and has longer days and so experiences summer.

The Earth's axis of rotation is inclined at 2327' so that, during the course of a year, the northern and south-T season Because the ern hemispheres alternately receive longer or shorter peri-

Earth's axis is inclined relative ods of daylight. When the northern hemisphere is tilted to its plane of orbit, the Earth towards the Sun it experiences summer, while the south-experiences seasons. The ern hemisphere experiences winter. Six months later, side that is tilted towards the when the Earth is on the other side of its orbit around the Sun gets more light and has Sun, the situation is reversed; the southern hemisphere longer days and so enjoying summer whilst it is winter in the north. Between experiences summer. the two extremes lie spring and autumn.

The beginning and end of each season is defined by the position of the Sun with respect to the ecliptic. At the time of the VERNAL EQUINOX, on March 21, it is said to be the first day of spring in the northern hemisphere. Spring lasts until the SUMMER SOLSTICE on June 21, which, although marking the longest day, is deemed to be the first day of summer. The end of summer and the start of autumn is marked by the AUTUMNAL EQUINOX on September 23, with the WINTER SOLSTICE on 22 December, the shortest day, defining the end of autumn and the beginning of winter. These seasons are reversed for the southern hemisphere.

Mars, the rotational axis of which is inclined at 2511 ', also displays seasonal variations, particularly to its POLAR CAPS. On Neptune's moon Triton, the polar cap migrates from pole to pole during the course of the planet's 165-year orbital period.

Secchi, (Pietro) Angelo (1818-78) Italian Jesuit astronomer, director of the Collegio Romano Observatory (1849-78) and a pioneer of solar physics and stellar spec-troscopy. Although forced to take refuge at Georgetown (Washington, D.C.) Observatory in 1847, when the Jesuits were driven into exile, Secchi spent most of his career at the Collegio Romano. His observations of 10th-magnitude stars shining through the tail of Biela's Comet (1852) led him to conclude that cometary tails were gaseous, not solid. Secchi was one of the first (1859) to compile a complete photographic lunar atlas. He organized several expeditions to view solar eclipses and another, to Maddapur, India, to observe the 1874 transit of Venus.

Secchi proved that PROMINENCES, which he classified as quiescent or eruptive, were a physical part of the Sun, and he was the first to describe SPICULES. Simultaneously with William HUGGINS, Secchi was the first astronomer to make systematic observations of stellar spectra. From his analysis of the characteristics of the spectra of over 4000 stars, Secchi devised the first spectral classification scheme, which formed the basis of the system developed by Edward C. PICKERING and his staff at Harvard.

secondary (mirror) Small mirror in a REFLECTING TELESCOPE that diverts the converging beam of light from the PRIMARY towards the EYEPIECE.

secondary body Any of the smaller members of a system of celestial bodies that orbit around the largest, the primary body. The planets are secondary bodies in their orbits around the Sun, but each planet is the primary body when considering its own satellite system, in which the satellites are the secondary bodies.

secondary crater CRATER formed by the impact of pieces of EJECTA from a primary impact crater. Secondary craters are typically shallower than primary ones. They are abundant on bodies that lack an atmosphere. In rare cases, however, secondary craters have been observed even on Venus, the very dense atmosphere of which usually brakes the ejecta flight. Secondary craters are typically between one and two orders of magnitude smaller than the primary. They form radial and concentric chains and clusters around the primary crater.

second contact Moment during a SOLAR ECLIPSE when the leading (easterly) limb of the Moon appears to touch the easterly limb of the Sun. During a TOTAL ECLIPSE, this marks the arrival of TOTALITY. Second contact in a LUNAR ECLIPSE defines the moment when the Moon's trailing (westerly) limb enters Earth's UMBRA; in a total lunar eclipse, this is the time when totality begins.

second of arc (symbol ") Unit of angular measure also known as an arcsecond; it is equivalent to 1/60 of a MINUTE OF ARC and 1/360 of a degree. The unit is widely used in astronomy, particularly as a measure of angular separation or diameter of celestial bodies. The RESOLVING POWER of a telescope is also usually expressed in arcseconds.

secular acceleration Continuous, non-periodic rate of change of the orbital motion of a body. For example, in the Earth-Moon system, frictional effects in the TIDES raised on the Earth by the Moon cause a slight lag in the Earth's response to the tidal force, with the result that the TIDAL BULGE does not lie precisely along the Earth-Moon line. The resulting slight asymmetry in the gravitational attraction between the Earth and Moon causes a transfer of angular momentum and energy from the Earth's rotation into the Moon's orbit. This slows the Earth's rotation, increasing the length of the day by 0.000014 seconds a year, and causes the Moon to recede at about 3.8 cm (1.5 in.) a year. A similar effect occurs for the Martian satellite Phobos, but because it is below SYNCHRONOUS ORBIT height, it is spiralling into the planet instead of receding. The radius of Phobos' orbit is decreasing by about 1.9 cm/yr (0.7 in./yr) and it is expected that it will enter the ROCHE LIMIT and break up in about 38 million years time. See also TIDAL EVOLUTION

secular parallax Angular displacement of a celestial body over time caused by the Sun's motion through space relative to the LOCAL STANDARD OF REST. Secular parallax provides us with a method of measuring the distance to nearby groups of stars. Relative to nearby stars, the Sun is travelling at around 20 km/s (12 mi/s) in the direction of the constellation Hercules. The apparent displacement of a star arising from this solar motion is called its parallactic motion and by measuring the parallactic motion of a homogeneous group of stars, it is possible to obtain the secular parallax of the group. This method has been applied to estimate the average distances of groups of variable stars, such as RR LYRAE and CEPHEIDS, which are beyond the range of TRIGONOMETRIC PARALLAX.

secular variable Star that is suspected to have increased or decreased markedly and presumably permanently since ancient times. Thus PTOLEMY and other early observers ranked MEGREZ (8 Ursae Majoris) as the equal of the other stars in the Plough pattern, whereas it is now obviously fainter; DENEBOLA, in Leo, was ranked of the first magnitude, but is now below the second, and so on. On the other hand RASALHAGUE (a Ophiuchi) was given as magnitude 3 and is now 2. On the whole it seems very unlikely that any of these changes are real: more probably they are errors of observation or - even more plausibly -translation or interpretation.

segmented mirror Very large telescope mirror made up of a number of smaller mirrors that fit together to form one continuous optical surface. A segmented mirror offers the advantages of being much thinner and lighter in weight than a solid mirror of the same size. The 10-metre primary mirrors of the twin Keck telescopes on Hawaii are each made up of 36 hexagonal pieces, each about two metres in diameter. During observing, a computer-controlled system of sensors and actuators adjusts the position of each segment relative to its neighbours to an accuracy of four nanometres. See also ACTIVE OPTICS; SPIN CASTING

segmented mirror The mirror of the Gran Telescopio Canarias is made up of 36 hexagonal segments measuring 936 mm (36.85 in) on each side. The thin segments, one of which is seen here being polished, combine to form a large, flexible surface that would not be achievable with a single mirror.

seeing Effect of the Earth's atmosphere on the quality of images produced by optical telescopes. The term is also used to express the quality of observing conditions and is usually expressed in arcseconds ("). It corresponds to the angular diameter of a star image produced at the focal plane of a telescope; the smaller that image is the better. Seeing refers to the spatial effect on the image and should not be confused with transparency, which is a measure of how much (or how little) the atmosphere attenuates the light. Ironically, clear nights when the stars are bright can suffer from poor seeing, and the seeing can be very good on nights when the stars are dimmed by slightly hazy conditions.

Meaningful comparisons of seeing measurements can only be made when consistent methods of defining and measuring the diameter of the star image are used. Seeing also varies with time, so the timescale over which measurements are made is also significant.

Where important decisions rely on consistent seeing measurements, such as choosing a site for an observatory, differential image motion monitoring is often used. Two or more images of the same star are formed by parallel paths through a single telescope, and the relative motion of these images is converted into an assessment of the seeing. Prior to quantitative measurements of seeing the ANTONIADI SCALE was used.

The effect of seeing is caused by light passing through air of differing temperature and therefore differing refractive index. These differences are mostly due to air masses mixing in the upper TROPOSPHERE, but the local effect of heat sources in the telescope building and heat rising from the surrounding ground can be significant. Astronomers go to great lengths to choose sites with good inherent seeing, where the airflow is smooth with little turbulent mixing above the site. Increasingly, they pay great attention to the design of telescope buildings to minimize their effect on seeing. On world-class sites 'good seeing' would be better than half an arcsecond, whereas one-arcsecond seeing would be considered very good on a site in the United Kingdom.

Avoiding atmospheric seeing is one of the main reasons for placing optical telescopes such the HUBBLE SPACE TELESCOPE into orbit. The detail revealed in Hubble pictures clearly illustrates what can be achieved when seeing is removed and the quality of the image depends only on the size and quality of the optics.

seismology Study of the shockwaves produced by earthquakes and other disturbances, such as meteoritic impacts and explosions, that propagate through planetary bodies. The strength and characteristics of the waves are measured by instruments called seismographs. The principal types are P (pressure) and S (shear) waves, which travel through rock at different speeds, depending on its density and mechanical properties. Measurements of such waves provide information on the internal structure of a planet, such as its division into CRUST, MANTLE and CORE. Seismographs left by the APOLLO astronauts provided valuable information about the Moon's interior.

selected areas Set of 262 small, uniformly distributed regions of sky in which the magnitudes, spectral and luminosity classes of stars have been accurately measured to provide standard comparison data and statistics on the distribution of stars.

Selene Japan's Selenological and Engineering Explorer, which is scheduled to be launched in about 2003-2004 to orbit the Moon and demonstrate a lunar landing. Selene's primary objective is to investigate lunar origins and evolution and to develop technology for future missions. The spacecraft, comprising a mission module and a propulsion module with a small data relay satellite attached, will eventually be manoeuvred from its initial elliptical orbit into a circular polar orbit of 100 km (60 mi). During the transition, the data relay satellite will be deployed. It will relay the Doppler ranging signal between the orbiter and ground station to measure the far-side gravitational field. The orbiter will map the Moon for a year, after which the propulsion module will separate and will be deorbited. It will make a landing and send radio signals from the landing site for differential very large baseline interferometry observation.

SRPENS (gen. serpentis, abbr. ser)

Rather inconspicuous constellation, representing a huge snake coiled around the body of Ophiuchus, the serpent-bearer. The constellation is unique in consisting of two separate parts: Serpens Caput (the Serpent's Head), between Bootes and Hercules, and Serpens Cauda (the Serpent's Tail), between Ophiuchus and Scutum. The brightest star in Serpens, Unukalhai, is mag. 2.6. Alya (0 Ser) is a wide double with white components, mags. 4.6 and 5.0, separation 22"; 8 Ser is a close binary, also with white components, mags. 4.2 and 5.3, separation 4".0. Deep-sky objects include M5 (NGC 5904), a 6th-magnitude globular cluster, and the eagle nebula (IC 4703), an emission nebula containing M16 (NGC 6611), an open cluster of more than 60 stars fainter than 8th magnitude.

semidetached binary binary star system in which only one star fills its roche lobe. Material from the lobe-filling star escapes through the inner lagrangian point and accretes on to the companion, usually via an accretion disk. See also close binary; detached binary

semimajor axis (symbol a) Half of the longest axis of an ellipse. For an object in an elliptical orbit, it is the mean distance from the primary body (that is half way between the nearest distance and the farthest distance). It is one of the orbital elements, and is related to the mean motion n (the average rate of angular motion around the primary) by the relation n2a3 = G(M + m), where G is the gravitational constant, and M and m are the masses of the primary and the object.

semiregular variable (SR) Pulsating giant or supergiant variable star, generally of late spectral type (M, C, S or Me, Ce, Se), with period 20 to 2000 days or more, and amplitude from a few hundredths to several magnitudes. There are various subtypes. Some (designated SRA) differ little from mira stars except in having amplitudes less than 2.5 mag., but the shapes of their light-curves vary. Other stars (subtype SRB) show a definite periodicity, interrupted at times by irregularities, or intervals of constant light. A small subtype (SRC) comprises supergiants of late spectral type with amplitudes of no more than 1 magnitude and periods from 30 to several thousand days. mu cephei is an interesting example of this subtype. There are also stars classified as semiregulars but with earlier spectral types (F, G and K), sometimes with emission lines. These have been assigned to subtype SRD. See also rv tauri star

separation Angular distance, measured in seconds of arc, between two celestial bodies, particularly the members of a visual binary or multiple star system. Separation is one measure of the relative positions of the components of a binary system, the other being position angle.

sensitivity Ability of a detector or instrument to measure a faint signal from an astronomical object, making the detector many times more sensitive than the naked eye. The term can also be used to describe the region of the electromagnetic spectrum where the instrument or telescope works best, for example the Jodrell Bank telescope is sensitive to radio waves, whereas the Hubble Space Telescope is not sensitive to radio waves.

Serenitatis, Mare (Sea of Serenity) Lunar lava plain, roughly 700 km (430 mi) in diameter, located in the north-east quadrant of the Moon. This area was struck by an enormous object, producing a multi-ring impact basin, the inner region of which was later flooded by lava. The inner ring is marked by a series of circular mare (wrinkle) ridges. Apollo 17 landed in the Taurus-Littrow Valley, in the outer ring of Serenitatis, to sample a dark-mantling deposit.

Serpens See feature article

Service Module See apollo programme

Setebos One of the several small outer satellites of uranus; it was discovered in 1999 by J.J. Kavelaars and others. Setebos is about 20 km (12 mi) in size. It takes 2273 days to circuit the planet, at an average distance of 17.88 million km (11.11 million mi). It has a retrograde orbit (inclination near 158) with a substantial eccentricity (0.551). See also caliban

SETI Acronym for Search for Extraterrestrial Intelligence, the umbrella term for all endeavours, especially those using radio telescopes, to find signs of intelligent life elsewhere in the Universe. SETI assumes that life exists on some extrasolar planets; a small percentage of that life is sufficiently advanced to have developed civilizations and technological capabilities; such civilizations are either unintentionally emitting or deliberately transmitting signals that are detectable across intervening space; and such signals if received will be identifiably of artificial origin. Almost 50 years of SETI research has produced no evidence for extraterrestrial intelligence (ETI).

SETI Shown here is the SETI@home project screensaver. More than 890,000 years of computerprocessing has been performed by the 3.5 million participants in the project.

A vast number of stars must first be checked for the presence of suitable planetary systems, very few of which will contain planets potentially suitable for complex life, and very few of those may be expected to host technologically advanced civilizations. Natural cosmic radio sources produce 'noise' spread across a wide frequency range. Radio SETI experiments look for narrowband signals, with a frequency spread of just a few hertz, characteristic of a purpose-built transmitter - these are the easiest signals to detect as their energy is concentrated in a small region of the radio spectrum.

The earliest serious attempt to detect ETI signals was Project Ozma (the name is from Frank Baum's The Wizard of Oz). In 1960 Frank drake and others used a 26-m (85-ft) antenna at Green Bank to search at the 21-cm hydrogen line. Ozma was unsuccessful and was abandoned after a few months. Since then a number of other projects, mostly privately funded, have scanned the heavens in various regions of the radio spectrum.

Project Phoenix targets 1000 carefully selected stars within 200 l.y. of the Sun. A 28 million channel receiver provides high sensitivity to weak signals. Observations began at Parkes, NSW, Australia in 1995 February, and at Green Bank in 1996 September; observations are also made for two 3-week sessions each year at Arecibo. By mid-1999, Phoenix had examined about half its targeted stars.

SEXTANS (gen. sextantis, abbr. sex)

In the Serendip project, spectrum analysers developed and operated by the University of California (UC), Berkeley, have been used on various radio telescopes. Serendip I, a 100 channel per second analyser, operated at Hat Creek Observatory in 1979. The most recent instrument, Serendip IV, was installed at Arecibo in 1997 June and examines 168 million channels every 1.7 seconds.

A 1971 report from NASA's Ames Research Center proposed an array of between 1000 and 2500 100-m (330-ft) antennae to synthesize a single 10-km (6-mi) diameter dish to search for ETI signals from up to 1000 l.y. away. Project Cyclops, as this was called, would have cost about $20 billion at 2001 prices and the project was stillborn, but the idea of an integrated multi-dish array has resurfaced in the allen telescope array (ATA). Its ability to scan many areas of the sky at once, with more channels than in previous searches and for 24 hours a day, will give the ATA a much greater capability than Project Phoenix.

Analysis of the vast amounts of data generated in SETI research has low priority on large astronomical computers. The UC Berkeley SETI team realized if a large number of small computers - PCs - could work simultaneously on different parts of the analysis, the job could be done in a relatively short time. By mid-2001, the SETI@home project had 3.5 million computers in homes, offices, schools and colleges linked to the Berkeley through the Internet and processing data during idle time.

Until recently, SETI was restricted to radio astronomy techniques. However, optical SETI (OSETI) aims to detect pulsed lasers, infrared messages or other artificial optical signals. Theoretical studies began in the early 1960s, since when developments in optical technology have made OSETI capable of detecting ET signals. Charles Townes (1915- ) suggested in 1962 that extraterrestrial civilizations might use lasers for interstellar communication, for which the directionality of laser beams makes them ideal. He calculated that a 10-kW laser directed through a 5-m (200-in.) space telescope would appear brighter than the Sun, and would therefore be detectable by an ETI on a target planet, using a similar telescope, 100 l.y. away. Attempts to search for such signals using small telescopes and fast photon counters are already under way, and more sensitive searches are planned.

Seven Sisters See Pleiades

Sextans See feature article

sextant Astronomical instrument containing a 60 arc (from Latin sextans, meaning one-sixth of a circle). In the 1570s Tycho brahe developed large wooden sextants of 1.8 m (6 ft) radius mounted on a ball joint, whereby two astronomers working together could measure horizontal or right ascension angles between pairs of stars, for celestial map-making. The term 'sextant', however, is more commonly used nowadays to describe a precision hand-held instrument used by navigators for measuring celestial angles. The modern navigator's sextant is based on the original 1731 design by John Hadley (1682-1743) in which the observation of a stellar image reflected from a small built-in mirror doubles the instrument's amplitude from 60 to 120; it replaced the mariner's astrolabe. Eighteenth-century precision engineering, as embodied in the dividing engine made in 1775 by Jesse Ramsden (1735-1800), soon brought the sextant to technical maturity, enabling it to be used to find the longitude at sea from the position of the Moon.

Seyfert, Carl Keenan (1911-60) American astronomer best known for his investigations of what are now known as seyfert galaxies. He spent his career at McDonald Observatory (1936-10), Mount Wilson Observatory (1940-16) and Vanderbilt University (1946-60). He studied the spectra and distribution of stars in the Milky Way and developed new equipment and techniques for photoelectric photometry and wide-field astronomical photography. His most famous research, published in 1943, concerned galaxies with inconspicuous spiral arms and very bright nuclei, the class of active galaxy now named after him.

Small, insignificant equatorial constellation, representing a sextant, between Leo and Hydra. It was named Sextans Uraniae (Urania's Sextant) by Johannes Hevelius in 1687 to commemorate the astronomical instrument. Its brightest star, a Sex, is mag. 4.5. The brightest deep-sky object in Sextans is the Spindle Galaxy (NGC 3115), a 9th-magnitude lenticular galaxy.

Seyfert galaxy Class of active galactic nucleus, defined as showing a starlike nucleus and emission lines that are of high ionization and large velocity width. The class was first recognized by Carl seyfert. When examined carefully, about 5% of bright galaxies have Seyfert nuclei, which are found mostly in spirals of early type (Hubble classes S0, Sa, Sb). There are two subtypes: type 1, in which the broad balmer lines of hydrogen are much broader than other emission lines, spanning thousands of km/s; and type 2 in which all emission-line widths are comparable (usually a thousand km/s or less). Type 1 Seyferts have spectra very much like quasars. As in the case of quasars and radio galaxies, there is a unified model in which type 2 Seyfert nuclei are type I objects seen through an obscuring torus. This model is supported in many cases by polarization measurements and by the detection of cones of highly ionized material to each side of the (sometimes invisible) nucleus itself. These cones are not generally well aligned with the galaxy's overall rotation axis. Some Seyferts show small-scale radio structure resembling distorted jets, although their interstellar gas seems to prevent the jets from propagating very far.

Like quasars, Seyfert nuclei are variable, and the response of the surrounding gas to changes in the core brightness has been used to map the size and structure of the gas seen in emission lines. Much of the gas seen in the broad emission lines must be situated only a light-day or so from the core. This small size combined with the large linewidths, indicating that a very deep gravitational field is needed to keep the gas from escaping, are points giving rise to the popular picture of a massive black hole powering Seyfert galaxies.

The best-known Seyfert galaxies are M77 (NGC 1068), the prototype for type 2, and NGC 4151, the standard type 1. Such galaxies as M81 also show weak Seyfert activity when examined closely.

shadow bands Phenomenon seen immediately before and after totality at a total solar eclipse, consisting of rapidly moving ripples of light and dark passing over the ground. Attempts to photograph them have failed, but these shadow bands have been recorded on video when seen crossing a white screen. It is believed that they result from small-scale changes in the refractive index of the air along the eclipse track, with the brighter bands being images of the tiny remaining visible sliver of the solar disk exposed within a few moments of totality. Observers who witnessed a strong display of shadow bands at the 1998 February total solar eclipse in the Caribbean likened the effect to the play of sunlight on the bottom of a water-filled swimming pool.

Shakerley, Jeremy (1626-C.1655) English astronomer. By 1649 he had discovered the writings of the recently deceased Jeremiah horrocks, William gascoigne and William crabtree, and was a fervent supporter of the Copernican system. Between 1649 and 1653 he published three books attacking astrology. He clearly considered Horrocks his scientific role model, and his Anatomy of Urania Practica (1649) was the first occasion that Horrocks' work was acknowledged in print. Shakerley was making telescopic observations of comets and eclipses after 1650, and of the 1651 transit of Mercury.

Shapley, Harlow (1885-1972) American astronomer who first defined the nature and extent of the Milky Way galaxy. His early years were spent at Mount Wilson Observatory (1914-21), where he became the first (1918) accurately to measure the distances to globular clusters, by calibrating Henrietta leavitt's period-luminosity law for Cepheid and RR Lyrae variable stars in the clusters. The globular clusters are concentrated near the central bulge of the Milky Way, so these studies enabled Shapley to identify a region in Sagittarius at galactic longitude 325 as the galactic centre. He calculated its distance as 50,000 l.y., which, though about 60% greater than the distance obtained if the effects of interstellar reddening are taken into account, still implied that the Galaxy was much larger than previously thought. Shapley defended his idea of a large 'metagalaxy' in the famous 1920 great debate with Heber Curtis.

From 1921 to 1951 Shapley directed harvard college observatory, modernizing the facilities and equipment and establishing the boyden observatory in South Africa. Shapley catalogued many thousands of galaxies beyond the Milky Way, publishing, with Adelaide Ames (1900-1932), the first comprehensive catalogue of these objects - A Survey of the External Galaxies Brighter Than the Thirteenth Magnitude (1932). He was one of the first to recognize that galaxies are not distributed uniformly in the Universe, but arranged in groups and clusters. In 1938 Shapley discovered the Fornax and Sculptor dwarf galaxies.

Shaula The star X Scorpii, visual mag. 1.62, distance 703 l.y., spectral type B1.5 IV. It lies in the tail of the scorpion, and its name is the Arabic word for 'stinger'.

Shedir The star a Cassiopeiae, visual mag. 2.24, distance 229 l.y., spectral type K0 III. Small telescopes show a wide, unrelated companion of mag. 8.9. Its name, which is also spelled Schedar or Schedar, comes from the Arabic sadr, meaning 'breast'.

Sheliak Name of the star beta lyrae.

shell burning Nuclear burning of elements in thin spherical shells in the atmosphere of stars. Shell burning occurs when nuclear fusion has been exhausted in the core, whereupon the core collapses producing thermal energy, which raises the temperature of the material surrounding the core sufficiently for nuclear fusion to occur there. The shell burning gradually moves outwards, causing the star to expand, until all the nuclear fuel has been exhausted in the envelope.

Hydrogen shell burning occurs in stars as they evolve off the main sequence to become red giants. More massive stars will burn other elements in their cores and have other episodes of shell burning. See also stellar evolution

shell galaxy elliptical galaxy that shows faint, thin shells of stars on either side of the nucleus. Attention was first called to these galaxies in 1979, when the photographic image-enhancement techniques of David Malin (1941- ) showed these subtle features. They appear to result when a disk galaxy merges with an elliptical, and the coherent stellar motions from the disk lead to its stars wrapping across the centre of the elliptical, producing narrow shells where they reverse direction. As many as 28 concentric shells have been seen in a single galaxy. Related shell-like features have also been found in S0 and a few Sa galaxies, and their occurrence has been used as evidence of advanced mergers or galactic cannibalism.

shell star main-sequence star that is surrounded by a shell of gas, generally of spectral type Be. Examples are y Cassiopeiae, BU Tauri and 48 Librae.

The spectrum of a shell star typically shows a rapidly rotating underlying B star, with broad absorption lines superposed with emission lines and additional sharp absorption features. The star is rotating rapidly, losing mass to an expanding shell that is pulled into a disk around the equator. The sharp absorption features are caused by the disk. Be-shell stars are believed to be the same as 'classical' be stars viewed along the equator.

Other Be stars are in close binary systems, with the material surrounding them being accreted from their companion stars. Herbig Be objects are believed to be a more massive version of ttauri stars.

Shen Gua (or Guo or Kua) (c.1031-c.1095) Chinese civil and military administrator, mathematician and astronomer. He devised a model of the Solar System using new mathematical techniques to explain the planets' retrograde movements, and developed a theory of the Moon's complex orbit; his ability to predict celestial events ingratiated him with the emperor's government. Shen designed and built astronomical instruments, including an improved gnomon and a highly accurate armillary sphere. His other major achievement was to reform the calendar.

Shen Zhou Chinese 8-tonne spacecraft, which, in about 2003-2004, should carry the first Chinese astronauts into orbit. The craft made two unmanned long march 2F-boosted orbital flights in 1999 and 2001, the descent capsule being safely returned to Earth. Shen Zhou is based on the design of the Russian Soyuz U spacecraft but its forward docking module is cylindrical rather than spherical and can be left in orbit as an unmanned orbital facility after the crew capsule, which can carry two 'taikonauts', has returned to Earth. The rear service module of Shen Zhou is equipped with a manoeuvring engine and solar panels. After some manned test flights, China plans to dock two Shen Zhou spacecraft together in orbit, forming an interim space station.

Shepard, Alan Bartlett, Jr (1923-98) American astronaut who made the first suborbital Mercury space flight in 1961. He was selected in 1959 as one of the original Mercury astronauts, and on 1961 May 5 he piloted the Freedom 7 spacecraft to an altitude of 188 km (117 mi) before returning to Earth. Although grounded for medical reasons (1964-69), Shepard later commanded the Apollo 14 mission (1971), becoming the fifth man to walk on the Moon.

Shepard, Alan Bartlett, Jr In 1961 Alan Shepard piloted Freedom 7 to the edge of space. Here he is seen in his space suit during preflight testing

shepherd moon Minor moon, the gravitational influence of which holds the particles of a planetary ring in place, preventing them from dispersing. Often found in pairs, the shepherd moons share the orbit of the particles in the ring system, an example being prometheus and pandora, which stabilize Saturn's f ring. The moon closer to Saturn moves slightly faster, having the effect of speeding up any particles which stray, while the outer moon moves more slowly and drags any stray particles back into the ring. Two shepherd moons, cordelia and ophelia, are found in the Uranian system, situated on either side of the planet's Epsilon ring.

Shergotty meteorite that fell in Bihar, India, in 1865 August; a single stone of approximately 5 kg was recovered. Shergotty is the type specimen for the Shergottites, one of the main subdivisions of martian meteorites (SNCs). Shergottites are subdivided into the basaltic and lherzolitic shergottites. The former are finegrained pyroxene-plagioclase rocks, which formed as flows at or close to the Martian surface. The latter are more coarse-grained olivine-pyroxene plutonic rocks.

Shklovskii, losif Samuilovich (1916-85) Ukrainian astrophysicist who in 1953 explained that the continuum radio and X-ray emission from the Crab Nebula was synchrotron radiation and correctly predicted that OH would radiate at microwave frequencies. He discovered many X-ray binary systems, and improved the calculation of distances to the planetary nebulae associated with white dwarfs. Shklovskii also showed that the temperature of the solar corona is about 1 million K, and how magnetic fields outline its structure. He also was an early proponent of seti studies, and his Intelligent Life in the Universe (1966), co-written with Carl sagan, is a classic text.

shock Important process in many branches of astronomy and space science. Though the simple shock produced by a supersonic aircraft is familiar, the processes involved in astrophysical shocks are less well understood. The general description of a shock is that there is some boundary, the shock surface, and that as matter crosses the boundary there is a rather sudden change in the state of the matter. This change is an irreversible change in thermodynamic terms. The usual form of shock is that there is conversion of kinetic or flow energy into random thermal energy. The temperature is higher on one side of the shock.

Examples of shocks are found in the solar wind, including the bow shock caused by planetary magnetospheres. Shocks are also probably involved in solar flares. In the interstellar medium, for the clouds of gas and dust that form the region between the stars as well as the more spectacular nebulae, shocks form important boundaries. They are thought to be involved in various early stages of star formation from these clouds and are certainly involved in the late stages of stellar evolution such as supernovae. On a larger scale it is thought that shock processes occur in radio galaxies.

Shoemaker, Eugene Merle (1928-97) American geologist and astronomer who extended geological principles to the Moon and other bodies, advanced the impact theory of cratering, and pioneered the study of near-earth objects. In 1937 he joined the united states geological survey (USGS), with which he remained associated until his death. In 1951 he married future collaborator, Carolyn (Jean) Spellmann Shoemaker (1929- ). The Shoemakers' visit to meteor crater the next year convinced Gene that terrestrial and lunar craters resulted from asteroidal impacts, which he followed up with a study of craters formed in the deserts of Nevada during US nuclear bomb tests.

ShoemakerLevy 9, Comet The scars left in Jupiters atmosphere by the impacts of Comet ShoemakerLevy 9 were probably gases brought up from deeper within the planets atmosphere by the explosions. They appear dark red because they absorb light at different wavelengths from the highlevel clouds.

In the 1960s, Shoemaker organized the geological experiments on NASA's manned and unmanned lunar missions, and in 1965 was appointed chief scientist of the USGS's new Center of Astrogeology. In 1969 his interest turned from impact craters to the objects that produced them. Initially with Eleanor Helin and later with Carolyn Shoemaker, he began a search for near-Earth objects, using palomar observatory's 0.46-m (18-in.) Schmidt telescope. The programme resulted in the discovery, from 1973, of many near-earth asteroids and, from 1983, the first of many comets now bearing the Shoemaker name, the most famous of which was Comet shoemaker-levy 9. It established Palomar as the leading discovery site for asteroids of all kinds.

Shoemaker-Levy 9, Comet Comet discovered on 1993 March 25 by Eugene and Carolyn Shoemaker and David Levy (1948- ) on a photographic patrol plate exposed at Mount Palomar, San Diego, USA. The discovery image appeared unusually elongated, and further investigation revealed that the comet had, in fact, been broken into a number of fragments. Analysis of the comet's motion showed it to be in a two-year orbit around Jupiter, into which it had probably been captured as long ago as 1929. At a close (21,000 km/13,000 mi) perijove in 1992 July, tidal stress had disrupted the comet's fragile nucleus into at least 21 sub-kilometre-sized fragments.

Calculations showed that these fragments would impact on to Jupiter over the course of the week of 1994 July 16-22. The fragments were named alphabetically in order of anticipated impact, and as they continued along their terminal orbit they became spread out into a 'string of pearls' imaged from ground-based observatories and the Hubble Space Telescope. Ahead of 1994 July, there was much speculation as to what effect, if any, the impacts would have on the giant planet.

Intensive observing programmes were established worldwide. The impacts themselves occurred just beyond the jovian limb, out of sight from Earth (but visible from the Galileo spacecraft at that time en route to Jupiter). The planet's rapid rotation would carry the impact sites into view from Earth within about an hour of each event.

Observers were surprised by the scale and violence of the impacts. Cameras on Galileo revealed the entry fireballs as bright flashes, and infrared observations from the terrestrial viewpoint showed huge plumes of material thrown high above the jovian cloud-decks following the impacts. The energies involved were most graphically shown by the easily visible Earth-sized dark spots that marked each impact site on Jupiter. Over the course of the week of the comet's demise a series of dark impact scars peppered Jupiter's clouds at 44S latitude, with impacts coming, on average, about seven hours apart. The impact scars remained visible for some weeks, eventually merging into a new dark belt on Jupiter, persisting for the next 18 months.

Searches through historical observations of Jupiter have produced only flimsy candidates for previous similar impacts during the telescopic era since the early 17th century. It has been suggested that such events may occur once in a thousand years.

The Shoemaker-Levy 9 impacts again emphasized the fragility of comet nuclei (many of which appear to have been captured as distant, small satellites by both Jupiter and Saturn), and the catastrophic energy scales involved in cosmic collisions.

shooting star Popular, non-scientific description of a METEOR.

Short, James (1710-68) Scottish optician who worked in London, the first to give telescopic mirrors a true parabolic figure. Short realized that a mirror having a parabolic, instead of a spherical, surface would be freer from spherical aberration. He also made the first successful telescope based on the design of Laurent Cassegrain (1629-93) (see CASSEGRAIN TELESCOPE). This instrument, completed around 1740 and nicknamed 'Dumpy', had a diameter of 6 inches (150 mm) and a focal length of 24 inches (600 mm).

short-period comet COMET for which the interval between successive perihelion returns is less than 200 years. Abut 150 such comets are currently known. Many have become faint due to continued depletion of their volatile materials. Comet 1P/HALLEY is perhaps the best-known, bright example.

short-period comet Most short-period comets are thought to have originated in the EdgeworthKuiper belt and to have been perturbed into the inner Solar System by the gravitational influence of the gas giants. Encke has an orbital period of only 3.3 years, GriggSkjellerup 5.1 years and GiacobiniZinner 6.5 years. Bielas Comet suffered the probable fate of all short-period comets: on one orbit, its nucleus was seen to have broken into at least two pieces and it never reappeared.

Sickle (of Leo) Distinctive pattern (asterism) of stars forming the head of LEO the lion, shaped like a sickle or reversed question mark. REGULUS is at the southern end of the Sickle.

sidereal Pertaining to the stars or measured relative to the stars; the term is used particularly to denote astronomical measurements of time with respect to the stellar background. The SIDEREAL DAY, which lasts 23h 56m 4s.091 of MEAN SOLAR TIME, is the time taken for the Earth to complete a single revolution on its axis, relative to the stars rather than to the Sun, which is the basis for a civil day. Astronomers use sidereal clocks, which run at a slightly different rate from conventional timepieces, to determine whether a celestial object can be observed at any given time.

sidereal day Time interval between two successive transits across an observer's meridian of the FIRST POINT OF ARIES or of any given star. The sidereal DAY is a measure of the period of rotation of the Earth, relative to the background stars, and is equivalent to 23h 56m 4s.091 of MEAN SOLAR TIME.

sidereal month Time taken for the Moon to complete a single revolution around the Earth, measured relative to a fixed star; it is equivalent to 27.32166 days of MEAN SOLAR TIME. See also ANOMALISTIC MONTH; DRACONIC MONTH; MONTH; SYNODIC MONTH; TROPICAL MONTH

sidereal period Orbital period of a planet or other celestial body around its primary, as measured relative to the stellar background. The sidereal period of a planet provides a true measure of its year as opposed to the SYNODIC PERIOD, which is a measure of the time taken for the body to complete a single orbit as observed from the Earth.

sidereal time Time measured by the rotation of the Earth relative to the stars, rather than to the Sun. The Earth rotates on its axis once a DAY, at the same time moving in its orbit around the Sun, which it takes a YEAR to complete. This means that, relative to the stars, it completes one extra rotation during the course of a year, resulting in a SIDEREAL DAY being approximately 3m 56s shorter than a mean solar day (see MEAN SOLAR TIME).

Sidereal time provides an indication as to whether a celestial object is observable at any given time, since objects cross the local meridian at a local sidereal time equal to their RIGHT ASCENSION. Astronomers therefore use clocks that show sidereal time and that run at a slightly different rate from ordinary clocks.

The zero point for measuring sidereal time is the FIRST POINT OF ARIES, the position on the celestial sphere where the ecliptic crosses the celestial equator. When this transits the observer's meridian, it is 0h sidereal time. Sidereal and solar times coincide once a year near the VERNAL equinox, on or about March 21, when the Sun lies in the direction of the First Point of Aries.

sidereal year Time taken for the Earth to complete a single revolution of the Sun, measured relative to the fixed stars; it is equivalent to 365.25636 mean solar days. Because of the effects of precession, a sidereal year is 20 minutes longer than a tropical year. See also anomalistic year

siderite Obsolete name for iron meteorite

siderolite Obsolete name for STONY-iron meteorite

siderostat Flat, altazimuth-mounted mirror driven to counteract the rotation of the Earth and continuously to direct the light from the Sun on to a fixed, focusing mirror. From here it can be directed towards instruments such as a spectrohelioscope. Unlike the similar heliostat, which is equatorially mounted, the mirror of the siderostat has to be driven about two axes simultaneously in order to follow the Sun across the sky. The benefit of this arrangement is that the beam is directed horizontally, rather than in a direction parallel to the Earth's rotation axis, allowing greater flexibility in the positioning of the fixed mirror. As in the heliostat, the final image slowly rotates. See also coelostat

Siding Spring Observatory Australia's major optical observatory, 20 km (12 mi) west of Coonabarabran in New South Wales. The observatory is at an elevation of 1150m (3770 ft) on Siding Spring Mountain, one of the higher ridges of the Warrumbungle Range. It was developed in the early 1960s as an outstation of mount stromlo observatory and remains the property of the Australian National University (ANU). The first group of telescopes had apertures of 0.41, 0.61 and 1.02 m (16, 24 and 40 in.). In 1984 the ANU added a 2.3-m (90-in.) telescope of advanced design known as the Advanced Technology Telescope (ATT), which for the first time incorporated in a single instrument a thin mirror, an altazimuth mounting and a rotating building. The ANU also operates the 0.5-m (20-in.) Uppsala Schmidt Telescope on behalf of NASA for near-Earth object searches.

Siding Spring also hosts facilities belonging to other organisations, including the two telescopes of the anglo-australian observatory (the anglo-australian telescope and the united kingdom schmidt telescope, respectively the largest of their kind in Australia), the Automated Patrol Telescope of the University of New South Wales and the southern faulkes telescope. Siding Spring's modest elevation and easterly location do not provide the best atmospheric stability, but it does enjoy particularly dark skies and it remains Australia's premier site for optical astronomy.

Sigma Octantis Closest naked-eye star to the south celestial pole, visual mag. 5.45, distance 270 l.y., spectral type F0 III. Currently it lies about 1 from the pole, and the distance is increasing due to precession.

signal-to-noise ratio Measure of signal clarity. All measurements (M) are the sum of the signal (S) and noise (N), so when the signal-to-noise ratio (R) is calculated the noise must be removed from the measurement first (to give the actual signal) before dividing by the noise: R = S/N =(M - N)/N. If the background noise is very high, the signal is overwhelmed by it, and the signal-to-noise ratio is very low (much less than 1). Technology continually strives to lower the noise in the detector in order to raise the signal-to-noise ratio.

Simms, William See cooke, troughton & simms

singularity Point in space or spacetime at which the current laws of physics make non-real predictions for the values of some quantities. Thus at the centre of a black hole, the density, the force of gravity and the curvature of spacetime are all predicted to be infinite. It is likely that radical changes to the laws of physics or new laws will be required to deal correctly with singularities. This may however not be necessary if the idea of cosmic censorship holds. Cosmic censorship requires that all singularities be hidden from the rest of the Universe by event horizons. If correct, then the difficulties presented by a singularity will never have to be dealt with in the real world (see also naked singularity). It is possible that the Universe as a whole originated in a singularity at the start of the big bang.

Sinope One of jupiter's outer moons, discovered in 1914 by Seth Nicholson (1891-1963). It is about 36 km (22 mi) in size. Sinope takes 758 days to orbit Jupiter, at an average distance of 23.94 million km (14.88 million mi), in an orbit of eccentricity 0.250. It has a path (inclination 158) in common with other members of its group. See also ananke

sinuous rille Winding channel that typically emanates from distinct, circular or elongated regions of collapse. Sinuous rilles are known on the the Moon, Mars and Venus. They are usually a few hundred metres to 1 km deep, 1 to 3 km (0.6-2 mi) wide and up to 100 km (60 mi) long. Their morphology and association with volcanic plains and constructs suggests that they originated from thermal erosion by flowing lava. apollo 15 landed close to Hadley Rille, one of the largest sinuous rilles of the Moon. Astronauts took detailed photographs of the rille's inner slopes and collected samples of basalts from its edge.

Sirius The star a Canis Majoris, the brightest star in the sky, at visual mag. -1.44. At a distance of only 8.6 l.y., it is also one of the closest stars to us. Sirius is popularly known as the Dog Star, since it lies in the constellation of the greater dog. It is a main-sequence star of spectral type A0 with a luminosity 22 times that of the Sun. The hipparcos satellite detected signs of variability, but the range and cause are unknown. Sirius has a spectrum that shows enhanced absorption lines due to heavy metals, and is classified as a metallic-line A star, or am star. Accompanying it is a white dwarf known as Sirius B, discovered in 1862 by the American astronomer Alvan G. clark. Sirius B is of visual mag. 8.44 and has just 0.005% of the Sun's luminosity. It orbits Sirius every 50 years but is too close to be seen separately with small telescopes. Sirius B was originally the more massive of the two stars and evolved more quickly, transferring some of its gas to Sirius A and hence giving rise to the unusual spectrum. The name Sirius comes from the Greek word seirios, meaning 'scorching'.

Sirius Shown here are Sirius A and B viewed at X-ray wavelengths. Although Sirius A outshines it in visible light, Sirius B is far brighter at these low-energy wavelengths. This is because it is far hotter its surface temperature is some 25,000 K. The spike pattern is an optical effect of the detector. Sirius A is not in fact hot enough to be seen at X-ray wavelengths and it is thought that some ultraviolet radiation from it leaked on to the detector to give the fainter component.

Sirrah Alternative name for the star a Andromedae. See alpheratz

SIRTF See space infrared telescope facility

Sisyphus apollo asteroid discovered in 1972; number 1866. It is the largest known Apollo, being about 10 km (6 mi) in size. See table at near-earth asteroid

Sitterly, Charlotte (Emma) Moore (1898-1990) American astronomer, an expert on the solar spectrum and standard spectral identifications. Working with Henry Norris russell at Princeton University, she studied binary star systems and the masses of their components. At Mount Wilson Observatory she made a detailed study of the solar spectrum. Later (1945-90), at the National Bureau of Standards and Naval Research Laboratory, Sit-terly did much to standardize the nomenclature of solar spectral lines and general spectral line multiplets, including data for the near-ultraviolet obtained by rocket-launched UV instruments.

Sixty-one Cygni (61 Cygni) First star to have its parallax measured, by the German astronomer Friedrich Wilhelm bessel in 1838. It also has the greatest annual proper motion of any naked-eye star: 5".2. 61 Cygni is a binary star, consisting of two orange dwarfs divisible in small telescopes or even binoculars, mags. 5.20 and 6.05 (4.79 combined), spectral types K5 V and K7 V, with an orbital period of 659 years. The stars are so widely spaced that they have measurably different distances from Earth -11.36 and 11.43 l.y. respectively.

Skalnate Pleso Observatory Observatory of the Astronomical Institute of the Slovak Academy of Sciences, located at an elevation of 1780 m (5850 ft) at Tatranska Lomnica in the eastern Tatras Mountains. The main instrument is a modern 0.6-m (24-in.) reflector with equipment for photoelectric stellar photometry. The observatory was founded by Antonin Becvar

Skjellerup-Maristany, Comet (C/1927 X1) Bright long-period comet discovered by John Francis Skjellerup (1875-1952) at Melbourne, Australia, on 1927 December 3, and independently by Maristany, at La Plata, Argentina, on December 6. At perihelion (0.18 AU) on December 18, the comet was visible in daylight only 5 from the Sun; peak brightness was estimated at magnitude 6. At the end of 1927 December, the comet was visible in dark skies, with a tail approaching 40 in length. The orbit is elliptical with a period of 36,500 years.

Skylab First US space station. Skylab was developed from the third stage of the saturn V rocket. The 75-tonne station was launched into a near circular orbit 433 km (270 mi) above the Earth's surface on 1973 May 14.

Skylab Three American crews spent a total of more than 170 days in Skylab during 1973 and 1974. Despite technical setbacks, the science achieved exceeded expectations.

Skylab consisted of four sections, the largest being the orbital workshop, 14.7 m (48 ft) long and 6.6 m (22 ft) in diameter. This section also contained the living quarters. An airlock module contained equipment for the control of the station and also the hatch for space walks. A multiple docking facility contained an Apollo docking port at one end and a second, rescue port. Six telescopes for monitoring the Sun were powered by a windmill-shaped array of four solar panels.

Just after launch, the station's micrometeorite shield deployed prematurely and broke away, destroying one solar panel and damaging the other. After being briefed on the damage, the first crew was launched on May 25 and, after some difficulties, managed to erect a sunshield and cut free the jammed solar panel. The astronauts Charles Conrad (1930-99), Joseph Kerwin (1932- ) and Paul Weitz (1932- ) returned to Earth on June 22, after a record stay of just over 28 days. In spite of the early technical problems, the Apollo Telescope Mount was operated for 88% of the planned time, making almost 30,000 exposures. Nearly 10,000 images of the Earth were also obtained.

On July 28, a second crew, comprising Alan Bean (1932- ), Owen Garriott (1930- ) and Jack Lousma (1936- ), was launched to the space station. The astronauts installed a new and more efficient sunshield and also installed a micrometeorite detector on the telescope mount truss. During the mission they took 25,000 photographs of the Sun and studied in detail more than 100 flares, including a major disturbance on August 21. The crew also returned with 16,800 pictures for use in crop surveys, land-use planning and searches for natural resources. They returned to Earth on September 25 after 59.5 days in space, then a record.

Gerald Carr (1932- ), Edward Gibson (1936- ) and William Pogue (1930-) entered Skylab on November 16. They spent much of their time repairing and replacing failed equipment. Nevertheless, they spent nearly two and a half times longer than planned observing the Sun and Comet kohoutek, and more than three times the allotted time on materials-processing experiments. They returned to Earth after a record 84 days on 1974 February 4.

During the three missions over 120,000 photographs of the Sun were taken and 72 km (45 mi) of magnetic tape were used to record data from on-board experiments. Five years later, Skylab re-entered the atmosphere over the Indian Ocean, scattering some debris over Australia. Fortunately no one was injured.

Slipher, Earl Carl (1883-1964) American planetary astronomer (younger brother of Vesto slipher) who spent virtually his entire career at lowell observatory (1906-64). Slipher was noted for the huge number of high-quality photographs, especially of Mars, that he took over a 50-year period. He confirmed (1937) the blue clearing phenomenon, when the Martian atmosphere, normally opaque to blue and violet light, suddenly becomes transparent at those wavelengths, allowing surface features to be seen clearly. In 1954 Slipher discovered Mars' 'W-clouds', which formed over the Tharsis region. Slipher was among the last of the leading planetary astronomers to hold to the belief that the canals of Mars were real physical features.

Slipher, Vesto Melvin (1875-1969) American astronomer (elder brother of Earl slipher) who discovered the recession of the galaxies. He spent his entire career at lowell observatory, which he directed from 1916 to 1952, and where he supervised Percival Lowell's search for Pluto, discovered by Clyde tombaugh in 1930. He used a large Brashear spectrograph attached to Lowell's 24-inch (0.61-m) Clark refractor accurately to measure the rotation periods of Venus, Mars, Jupiter, Saturn and Uranus. His spectrograms of Jupiter showed bands of methane and ammonia that were later confirmed by Rupert wildt and others.

Slipher became the first (1912) to recognize the red-shifts of lines in the spectrum of the Andromeda Galaxy (M31) and 36 other galaxies, a discovery later used by Edwin hubble to determine the scale of the Universe. Obtaining radial velocities for galaxies required very long exposures with the Lowell spectrograph - as long as 60 hours - and meticulous calibration of the equipment. He announced a radial velocity of 300 km/s for M31, a result that was initially met with disbelief, as it implied that this was an 'island universe' far beyond the Milky Way (see great debate). Slipher discovered that other spiral galaxies were receding at high speeds and that they rotated, too; in 1916 he announced a rotation rate for M31 and later determined similar rates for other galaxies.

He also investigated the nature of the interstellar medium, being the first to confirm the existence of interstellar calcium and sodium atoms. Slipher discovered reflection nebulae, explaining their luminescence. Expecting that the bluish nebulosity around Merope and other stars of the Pleiades would show a spectrum similar to the Orion Nebula and other bright diffuse nebulae, Slipher instead found that the Pleiades' nebulosity had spectra corresponding to the hot, young stars in the cluster, and correctly surmised that the Merope Nebula must shine by reflected starlight.

Sloan Digital Sky Survey (SDSS) Large-scale survey, the first to be made with purely electronic detectors, thus giving unprecedented photometric quality in five colours. It began in 1998, aimed at covering one-quarter of the sky. Redshifts are also being measured, and it is hoped that the resulting redshift survey will plot the distribution of galaxies in a volume of space over 30 times as large as in any previous survey. The SDSS has a dedicated 2.5-m (100-in.) reflector at apache point observatory. It is named after Alfred Pritchard Sloan, Jr (1875-1966), whose foundation financed the project.

slow motions Manual controls that allow movement of a telescope around the axes of its mounting and allow a celestial target to be followed. Adjustments are normally effected by turning a hand-wheel or fine screw. For a telescope on an equatorial mounting, only the right ascension axis requires a slow motion control. An instrument on an altazimuth mounting will require controls for both altitude and azimuth.

slow nova (NB) nova that takes at least 150 days to decline 3 magnitudes below maximum light. Such a decline excludes any dip and recovery such as that exhibited by the slow nova DQ Herculis.

SLR See satellite laser ranger

Small Astronomy Satellites (SAS) Name given to three NASA spacecraft launched in the 1970s for X-ray and gamma-ray observations. SAS-1 was also known as uhuru. SAS-2, launched in 1972, carried a gamma-ray telescope. This was followed in 1975 by SAS-3, which discovered a type of X-ray star known as a 'rapid burster'. See also gamma-ray astronomy

Small Magellanic Cloud (SMC, Nubecula Minor) Small galaxy that is the second nearest external galaxy to us. It is about 190,000 l.y. away and has about 2% of the mass of our galaxy. It extends over about 3 of the sky within the constellation of Tucana (RA 01h 05m dec. 72), where it is easily visible to the naked eye, appearing like a detached portion of the milky way. The SMC, along with the large magellanic cloud (LMC), is named after Ferdinand Magellan, who observed them during his voyage around the world in 1519, but they were known before that date. It is an irregular galaxy with a hubble classification of Irr I. The SMC's physical width is about 15,000 l.y., but it is highly elongated with the long axis along the line of sight. Its depth is therefore some 60,000 l.y. The curious, long, twisted structure that has been found for the galaxy (and that may in fact divide the cloud into two distinct portions) suggests that strong tidal forces have distorted the SMC. This may have occurred during a close passage some 200 million years ago between the SMC and our Galaxy. This same close passage may have produced the Magellanic Stream, one of the largest high-velocity clouds known. Although the SMC contains less gas and dust than the LMC, like the latter it also contains many HII and star-forming regions. The SMC orbits our Galaxy roughly at right angles to the plane of the Milky Way, and it may at some time in the future be disrupted and captured by our Galaxy.

SMC See small Magellanic cloud

Smithsonian Astrophysical Observatory (SAO) Major US research institute, founded in 1890 as a bureau of the Smithsonian Institution primarily for studies of the Sun. It moved from Washington, D.C. to Cambridge, Massachusetts in 1955 and established itself as a pioneering centre for astrophysics and space science. In 1973 its close ties with the harvard college observatory were cemented by the creation of the harvardsmithsonian center for astrophysics (CfA), and the SAO became part of one of the largest and most diverse astrophysical institutions in the world. Today, in collaboration with CfA and a number of other institutions, the SAO runs several major facilities including the fred l. whipple observatory, the mmt observatory and the submillimeter array. The SAO retains its headquarters in Cambridge, Mass.

Smithsonian Astrophysical Observatory Star Catalog (SAO) Catalogue giving the positions and proper motions for 258,997 stars to 9th magnitude, published in 1966. It was the first large 'synthetic' catalogue, created on a computer by combining data from several large astrometric catalogues.

Smoot, George Fitzgerald III (1945 ) American observational cosmologist who led the cosmic background explorer (COBE) team that in 1992 discovered minute variations in the cosmic microwave background radiation. Working at Lawrence Berkeley National Laboratory since 1970, Smoot has designed and led several missions to map the afterglow of the Big Bang, using instruments known as Differential Microwave Radiometers (DMRs) designed to measure minute fluctuations in the cosmic microwave background. Three ultra-sensitive DMRs launched on COBE detected temperature variations of just 30 millionths of a degree. These findings, announced by Smoot's team on 1992 April 23, also supported the theories of inflation and cold dark matter.

Smyth, Charles Piazzi (18191900) Scottish astronomer, the second Astronomer Royal for Scotland (184688), whose 1856 expedition to Tenerife, in the Canary Islands, proved the value of high-altitude observatories. He was born in Naples, Italy, the son of noted British amateur astronomer William Henry smyth, and his middle name honours his father's friend Giuseppe piazzi. Because the Scottish observatory at Calton Hill was so poorly funded, he did much of his research abroad.

His 1856 voyage to Tenerife was undertaken to compare observations of the planets and stars made at sea level from England with similar observations made from a temporary observatory located near the summit of a volcano, at 3718 m (12,200 ft). From this altitude, bad weather was less of a problem than at sea level; and the thinner air resulted in better seeing, which enabled him to detect fine detail on the Moon and planets and to resolve very close double stars. This expedition also marked the beginnings of infrared astronomy, as Smyth was able to make delicate measurements of the Moon's heat radiation that would normally have been absorbed by the lower levels of Earth's atmosphere, which are opaque to the infrared.

Smyth, William Henry (1788-1865) English naval officer, hydrographer and amateur astronomer, famous for his Cycle of Celestial Objects (1844). Inspired by his work assisting Giuseppe Piazzi in preparing a star catalogue, Smyth built a well-equipped private observatory at Bedford, near London, in 1830. Its main instrument was a 5.6-inch (142-mm) Tulley-Dollond refractor equipped with the first weighted clock drive designed by Richard Sheepshanks (1794-1855). Smyth published a decade of observations as the Cycle, which included as its second volume The Bedford Catalogue, a listing of 850 double stars, star clusters and nebulae. This work, aimed principally at amateur astronomers, was the first comprehensive guidebook to observing celestial objects beyond the Solar System.

Smythii, Mare (Smyth's Sea) Lunar lava plain located in the far eastern limb of the Moon. It formed from a multi-ring impact basin, the centre of which was later flooded with lava. Quite irregular in shape, Mare Smythii is best observed during a favourable libration.

SNC meteorite See martian meteorite

Snow Telescope Horizontal solar telescope of mount wilson observatory, and the oldest telescope there. It was moved to Mount Wilson in 1904 on loan from yerkes observatory, to which it never returned. Named after its benefactor, Helen Snow, it consists of a coelo-stat feeding a concave mirror of 0.61 m (24 in.) aperture and 18 m (59 ft) focal length. By 1910 the Snow Telescope had been superseded by the 60-ft (18-m) and 150-ft (45-m) Solar Towers, which are still used for solar research. The Snow Telescope itself is now used for educational programmes.

SNR See supernova remnant

SOAR Abbreviation of southern astrophysical research telescope

Society of Amateur Radio Astronomers (SARA) Organization that was formed in the early 1980s to coordinate what is probably the least practised aspect of amateur astronomy - radio observations of Solar System and cosmic radio sources. SARA encourages the continual surveillance of wide areas of the sky in search of new or unusual radio emissions. It publishes a monthly Journal; the membership also includes professional radio astronomers.

SOFIA Abbreviation of stratospheric observatory for infrared astronomy

software, astronomical Modern professional astronomy relies extensively on software - the programmed sequences of instructions that control modern digital computers. Astronomers use computers networked through the Internet and World Wide Web to plan observations. Some observatories use intelligent scheduling programs to determine when observations will be made. During an observation, a telescope is controlled by programs that correct for telescope deformation and keep the telescope tracking the target object (see active optics, adaptive optics). The instrumentation generates electronic data (perhaps passing it first through a processing sequence that removes any instrument-specific effects) in a standard format that can be handled by one of the standard astronomical data reduction systems. If the observation forms part of a large-scale survey, the data may be automatically processed to determine basic characteristics such as object type and redshift, and stored in an Internet-accessible database.

The astronomer may wish to examine the data in detail, displaying it in different ways, experimenting with image processing techniques, probing for the secrets it contains. The Flexible Image Transport System (FITS), developed by astronomers to encode definitions of image data and the data themselves, is a platform-independent system now widely used for interchanging data between observatories that has also become widely used outside astronomy.

Astronomical data reduction is a specialist process requiring specialist software, usually written by programmers employed at observatories or other astronomical institutions such as universities. Telescope and instrument control software is generally even more specialized, since each instrument has its own particular characteristics, so the software is usually written by those building the instrument or telescope. The software required for a new instrument now often represents a significant fraction of the total cost.

The main astronomical data reduction systems are NOAO's Image Reduction and Analysis Facility (IRAF) and ESO's Munich Image Data Analysis System (MIDAS) in the optical region, and NRAO's Astronomical Image Processing System (AIPS, and its successor, AIPS++) in the radio region. These can be seen as software component frameworks, with a structure that allows new components to be added easily and provides most of the basic services (for example data file access, user interaction) that the new component needs. Data acquisition systems can be structured in the same way, although, being real-time systems, they are significantly more complex. Most major observatories have developed their own data acquisition frameworks, but there are only a few such systems, notably the royal greenwich observatory's ADAM (Astronomical Data Acquisition Monitor) and its descendant, DRAMA, developed at the Anglo-Australian Observatory and first used on the Two-degree Field (2dF) project (see anglo-australian telescope).

Astronomical software is not restricted to large research institutions. Commercial packages are available for amateur telescope control (see drive; go to telescope). Image-processing techniques previously used only by professionals are now available to amateur observers wishing to use their personal computers to enhance images obtained with CCD cameras or to eliminate the effects of light pollution. 'Virtual planetarium' programs can bring the Universe to one's home.

SOHO Abbreviation of SOLAR AND HELIOSPHERIC OBSERVATORY C/1996 B2 HYAKUTAKE was visible in the SOHO coronagraph field at its 1996 May 1 perihelion, and 2P/ENCKE has also been observed close to the Sun using this spacecraft. The SOHO comet discoveries follow on from several made using the SOLWIND coronagraph aboard the US military P78-1 satellite in the late 1970s and early 1980s.

SOHO, Comets Numerous comets, making close perihelion approaches to the Sun, discovered by the LASCO C2 and C3 coronagraphs aboard the SOHO spacecraft. As of late 2001, almost 380 had been found, most of them very small objects on KREUTZ SUNGRAZER orbits. Nearly all of these comets were tiny, with nuclei perhaps only a few tens of metres in diameter. Very few SOHO comets have been observed to survive perihelion passage. One notable exception was C/1998 J1 (SOHO), discovered on 1998 May 3 as a magnitude 0 object in the coronagraphs' field of view. The comet, which was not a Kreutz sungrazer, faded rapidly after perihelion, but was reasonably well seen in small telescopes from southern hemisphere locations.

Solar and Heliospheric Observatory (SOHO) Part of the European Space Agency's INTERNATIONAL SOLAR TERRESTRIAL PHYSICS programme, together with the CLUSTER programmes, contributing to an international effort involving many spacecraft from Europe, the USA, Japan and other countries. The Solar and Heliospheric Observatory is a 1.8-tonne spacecraft; it was launched by an Atlas booster in 1995 December. Its mission is to make continuous observations of the solar photosphere, corona and solar wind to investigate the processes that form and heat the CORONA, maintain it and give rise to the expanding SOLAR WIND. It is also investigating the internal structure of the Sun. It carries 11 instruments, including a range of spectrometers and particle analysers. SOHO is situated at the L1 Lagrangian point between the Sun and the Earth (1.5 million km/0.9 million mi from the Earth), where it points to the Sun continuously, with the Earth behind the spacecraft.

Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX) NASA satellite launched in 1992 July to observe the activity of the Sun and to investigate the origin and transport of galactic cosmic rays. SAMPEX studies the changes in the effect of different geomagnetic influences on the spacecraft, allowing studies to be made of the ionization and composition of irregular, or anomalous, components of cosmic rays having the same properties irrespective of the direction from which they come. The craft also observes precipitating magnetospheric electrons that interact with the atmosphere and investigates the isotropic composition of particles originating in solar flares.

Solar B Japanese-led international spacecraft, involving the USA and the UK, to be launched in 2005 to make detailed observations of the Sun. It will study the way in which magnetic fields emerge through different layers of the Sun's atmosphere, creating the violent disturbances that affect the Earth. Solar B will be placed into a Sun-synchronous polar orbit around the Earth, in which the craft's optical telescope, X-ray telescope and ultraviolet imaging spectrometer will remain in continuous sunlight for nine months each year.

solar constant Total amount of radiant solar energy reaching the top of the Earth's atmosphere at a mean Sun-Earth distance of one ASTRONOMICAL UNIT. The mean value of the solar constant from 1978 to 1998 was 1366.2 1.0 watts per square metre. Variations in the solar constant were first reliably determined in the 1980s when suitable instruments were placed aboard satellites such as the SOLAR MAXIMUM MISSION. The solar constant rises and falls in step with the SOLAR CYCLE, but with a total change of just 0.1% from 1978 to 1998. During the two cycle minima of this period, the mean value was 1365.6 watts/m2. SUNSPOTS crossing the visible solar disk briefly decrease the solar constant by a few tenths of one percent for a few days: such decreases are outweighed by the brightness increase due to FACULAE and PLAGES at times of high solar activity. Observations of sunlike stars suggest that larger-scale variations can occur. A 0.25% reduction in the value of the solar constant is capable of explaining the estimated drop of about 0.5 K in global mean temperature during the MAUNDER MINIMUM.

solar cycle Cyclical variation in solar activity with a period of about 11 years between maxima (or minima). The solar cycle is characterized by waxing and waning of various forms of solar activity, such as ACTIVE REGIONS, CORONAL MASS EJECTIONS, FLARES, the SOLAR CONSTANT and SUNSPOTS. The rise to maximum activity is usually much more rapid than the subsequent decline.

solar cycle The number of S sunspots observed peaks roughly every 11 years, although the peak intensity can vary widely. During the second half of the 17th century there were very few spots and this period is known as the Maunder Minimum.

Over the course of an approximate 11-year cycle, sunspots vary both in number and latitude. The conventional onset for the start of a solar cycle is the time when the smoothed number of sunspots (the 12-month moving average) has decreased to its minimum value. At the commencement of a new cycle sunspots erupt around latitudes of 40 degrees north and south. Sunspots and their associated active regions are found in belts or zones at both sides of the equator with latitudes that move closer to the equator over the course of the cycle, finishing at around 5 degrees north and south. This pattern can be demonstrated graphically as a BUTTERFLY DIAGRAM and is known as SPORER'SLAW. The magnetic polarity of sunspots varies over a 22-year cycle, described by Hale's law. The leading (westernmost) spots of sunspot groups in the northern hemisphere generally share the same polarity, while the following (easternmost) spots have the opposite polarity. In the southern hemisphere, the leading and following spots also exhibit opposite polarities, but their magnetic configuration is the reverse of that in the northern hemisphere. All of the spots' magnetic polarities reverse each roughly 11-year solar activity cycle, so the complete cycle of the solar magnetic field is around 22 years. Reversal of field polarity can occasionally happen earlier for one hemisphere than the other, meaning that there may be intervals of up to a year when northern and southern sunspots actually have the same magnetic configuration.

The solar cycle may be maintained by a DYNAMO EFFECT, driven by differential rotation and convection. Longer-term variations, with periods of the order of a thousand years may also occur, and there have also been episodes such as the MAUNDER MINIMUM when the normal workings of the cycle appear to have been suspended. Observations of chromospheric activity in other stars suggests that similar activity cycles are typical of stars with convective zones.

solar day Mean time interval between two successive noons. See also DAY; EQUATION OF TIME; MEAN SUN solar eclipse Eclipse of the Sun. It occurs when the Moon's shadow falls on Earth; for this to happen the Moon must be in line between Sun and Earth. In fact, it is an OCCULTATION of the Sun by the Moon and the purist will call it an eclipse of the Earth, since an eclipse is by definition the passage of a celestial body through the shadow of another.

A solar eclipse can occur only at new moon (when the Moon is in conjunction with the Sun), but not at every new moon because of the 5.15 inclination of the Moon's orbit to the plane of the ecliptic. An eclipse will result only when the new moon roughly coincides with a NODE (the intersection between the orbit of the Moon and the Earth). This coincidence need not be exact, an eclipse can occur up to 18.75 days before or after the alignment, thus creating the 'eclipse season'. Two eclipses can occur in every eclipse season because the synodic month (29.5 days) is less than the eclipse season (37.5 days).

solar eclipse In a total solar eclipse (left) the Suns photosphere is completely obscured by the Moons disk in the area covered by the cone of shadow. In an annular eclipse (right) the Moon is slightly farther away and so the cone of shadow does not reach the Earth.

The nodes shift gradually westwards along the ecliptic, so the Moon reaches the opposite node less than six months later and realignment with the original node takes place after 346.62 days, representing the 'ECLIPSE YEAR'.

Four solar eclipses can occur in one year, but since the calendar year is greater than the eclipse year a fifth eclipse is possible in one calendar year on rare occasions, and then only in January or December. The maximum of five solar eclipses will next happen again in 2206. At least two solar eclipses of some kind must occur every year. A LUNAR ECLIPSE precedes or follows a solar eclipse by about two weeks because the same conditions prevail for the Moon before or after that interval.

It has been known since Babylonian times that the nodes regain their original positions after 18 years 10.3 days, the SAROS period. It lasts 223 synodic months (29.5306 X 223 = 6585.32 days). This cycle closely corresponds with 19 eclipse years (346.62 X 19 = 6585.78 days), hence eclipses recur after such cycles and form series. The added 0.32 of a day of the Saros is responsible for the westwards shift of subsequent eclipses by one-third of the Earth's circumference (120 longitude). They also shift 2 to 3 north or south due to the 0.46 day difference between 19 eclipse years and the saros. This eventually causes the series to end by passing one or the other pole. Each series comprises some 70 eclipses over a period of about 1262 years.

The apparent sizes of Sun and Moon as seen from Earth are very similar, but subject to variation due to the elliptical orbits of the Earth and Moon. A total eclipse occurs when the Moon appears larger than the Sun and the shadow cone reaches the Earth. When the Moon is at its largest (at perigee) and the Sun at its smallest (Earth at aphelion) a long total eclipse occurs; the maximum duration is 7 minutes 31 seconds, and this can happen only if the shadow cone reaches the Earth near the equator around local noon.

An ANNULAR ECLIPSE results in the opposite situation, when the shadow cone fails to reach the Earth. A ring of Sun will surround the Moon at mid-eclipse. An annular-total eclipse occurs if the apparent size of Sun and Moon are the same; it will be annular along most of the path, but in the middle of the eclipse path the shadow cone will just reach the Earth and in this location a very short totality is seen. A PARTIAL ECLIPSE results if only part of the Sun is occulted by the Moon. A partial eclipse is seen over a wide area on Earth where the Moon's penumbra reaches the Earth's surface. A total eclipse can be seen only on the narrow path caused by the shadow-cone sweeping over the Earth's surface from west to east with a velocity of some 3200 km/hr (2000 mph), the maximum width of this path is 270 km (170 mi). An observer situated outside this path will see a partial eclipse.

Various phenomena can be seen at a total eclipse. The mystery of a total eclipse is enhanced because the uninitiated have no warning of the impending spectacle, since the Moon cannot be seen approaching the Sun: the partial phase passes unnoticed unless the observer knows to look for it. It is dangerous to look at the Sun at any time, especially with optical aids. The only safe way to observe the partial phase is to project the image of the Sun on to a white surface. Only during totality is it perfectly safe to look directly at the occulted Sun and the CORONA.

The eclipse begins when the east limb of the Moon appears in the same line of sight as the opposite limb of the Sun and seems to encroach upon the Sun. This is the FIRST CONTACT. This moment goes by quite unnoticed. The projected image of the Sun will show a small notch some 10 seconds after first contact, this increasing in size as the Moon travels across the face of the Sun during the next hour or so. The light reduction is imperceptible at first and the temperature drops very little until the last five minutes of the partial phase. Then the real drama begins: the sky becomes darker, often with an eerie greenish tinge, quite unlike the darkening caused by clouds. Far on the western horizon an ominous cloud-like darkening appears to be increasing in size; this is the approaching shadow of the Moon. At the same time curious moving ripples of dark and light bands appear on any white smooth surface - a strange atmospheric phenomenon known as SHADOW BANDS. During the last few seconds of the partial phase light fails rapidly, it becomes noticeably cooler, birds settle down to roost, some flower petals close, and the wind tends to drop. As the last rays of sunlight fade, a dramatic change of the scene occurs: darkness descends on the countryside. The last sliver of the Sun is broken up by the Moon's irregular limb, forming BAILY'S BEADS, and as the last bead disappears SECOND CONTACT has occurred: totality has begun. The observing site on the path of totality is engulfed in darkness, illuminated only by the beautiful pearly white corona surrounding the pitch black Moon. The resulting brightness on the Earth's surface varies from eclipse to eclipse; it is comparable to that of the full moon. In contrast to the slow progress of the partial phase, events around second contact progress with incredible rapidity.

A few seconds after the disappearance of the last Baily's bead the pink CHROMOSPHERE (shining in the light of HYDROGEN-ALPHA emission) becomes visible on the eastern edge of the Moon's dark disk, only to be covered quickly as the Moon advances. PROMINENCES of various shapes and sizes are seen during totality as pink flame-like projections. Large prominences remain visible throughout totality, while smaller ones appear and disappear as the advancing Moon uncovers or covers them; initially, those on the eastern limb are best seen, while towards the end of totality, prominences on the western limb become uncovered. The corona is the most striking feature of the total eclipse. The bright inner corona contains elegantly shaped arches, loops and helmet-like structures tapering off into the fainter streamers of the outer corona for a distance of several solar diameters. These various forms are created by the solar magnetic field. The shape of the corona varies with the 11-year solar cycle. At solar minimum, the corona is drawn out into long equatorial streamers, extending to enormous distances east and west, whilst the poles are studded with shorter plume-like jets. At solar maximum, the corona surrounds the Sun more evenly, the whole circumference of the Sun being surrounded by medium-sized streamers of intricate structure.

It pays to take the eye off the features surrounding the Moon and look at the sky, where planets and bright stars can be seen with the now dark-adapted eyes. The surrounding landscape shows a 360 orange glow - similar to sunset - bordering the shadow of the Moon.

Brightening at the Moon's western limb heralds the end of totality. As THIRD CONTACT occurs, the first rays of sunlight from the brilliant photosphere, shining through a lunar valley, gives rise to the famous DIAMOND-RING EFFECT. The corona and the brightest planets may be discernible to the still dark-adapted eyes for some 10-20 seconds, but the main spectacle is almost over. Events now happen in reverse: as the sky brightens one can see the shadow of the Moon receding towards the eastern horizon, and shadow bands reappear; the temperature gradually rises, cocks crow as in the early morning, and day-time activity resumes again after the short interruption. The projected image of the Sun shows the gradual uncovering of the solar disk by the advancing Moon. The partial phase lasts another hour or so until the last notch on the solar disk dwindles and finally disappears: the Moon has parted from the Sun: fourth contact has occurred and the eclipse is over.

Much information can be gained at a total eclipse. The corona, the outer atmosphere of the Sun, can be studied both visually and spectroscopically only during a total or annular eclipse. However, Bernard LYOT's CORONAGRAPH creates an artificial eclipse and allows limited study of the inner corona from Earth. Spacecraft-borne coronagraphs have proven particularly useful. Prominences, also first seen during an eclipse, can now be studied in more detail with the spectroscope or the interference filter.

solar interior Not only does the Sun rotate faster at the equator than at the poles but also belts of plasma have been observed moving at different speeds. These belts drift down towards the equator during the solar cycle and, as sunspots appear to form on the boundaries, may be a major factor in the changes in the Sun's magnetic field. There are also deeper currents within the convective zone, which travel from the equator towards the pole.

solar interior Not only does the Sun rotate faster at the equator than at the poles but also belts of plasma have been observed moving at different speeds. These belts drift down towards the equator during the solar cycle and, as sunspots appear to form on the boundaries, may be a major factor in the changes in the Suns magnetic field. There are also deeper currents within the convective zone, which travel from the equator towards the pole.

Timing the four contacts is still important today. The results may show a slight discrepancy between observed and predicted times, due principally to perturbations of the lunar orbit and irregularities of Earth's rotation.

The FLASH SPECTRUM can be photographed at second and third contacts, when the dark absorption bands of the photosphere change to the bright EMISSION LINES of the lower chromosphere, thus giving information on the intensities of the various spectral lines. The shadow bands occur as a result of effects in Earth's atmosphere, and have been successfully recorded using video equipment.

Einstein's theory of GENERAL RELATIVITY was first tested during the total solar eclipse of 1919 May 29. Starlight was proved to be deflected by the Sun's gravitational field. Ionospheric studies found that the ultraviolet radiation cut-off during totality resulted in an alteration of the electrical conductivity of the upper atmosphere.

solar interior Sun's internal structure. It comprises three principal regions: a central CORE, above which is found the radiative zone, and finally the convective zone, from which material rises to the visible surface of the PHOTOSPHERE.

All of the Sun's energy is produced by NUCLEAR REACTIONS fusing hydrogen into helium in the dense high temperature core, which extends for about one quarter of the solar radius (174,000 km/108,000 mi) from the centre. The core accounts for only 1.6% of the Sun's volume, but about half its mass. At its centre, the core has a temperature of 15.6 million K and a density of 148,000 kg/m3.

The radiative zone surrounds the core out to 71.3% of the Sun's radius, 496,000 km (308,000 mi) from the solar centre. Core radiation, initially in the form of gamma rays, is continuously absorbed and re-emitted at lower temperatures (and longer wavelengths) as it travels out through the radiative zone. Recent computations indicate that it takes about 170,000 years, on average, for the radiation to work its way out from the Sun's core through the radiative zone to the convective zone.

At the bottom of the convective zone, the temperature has become cool enough, at about one million K, to allow some heavy nuclei to capture electrons. Their light-absorbing ability (opacity) obstructs the outflowing radiation and causes the PLASMA to become hotter than it would otherwise be. Because of its low density, the hot plasma rises, carrying energy through the convective zone from bottom to top in about 10 days. On reaching the visible solar disk, the hot material cools by radiating sunlight into space and then sinks back down to become reheated and rise again. These churning convection cells create a GRANULATION pattern in white-light images of the PHOTOSPHERE, which marks the top of the convective zone.

Turbulent motions in the convective zone excite sound waves that echo and resonate through the Sun. When these sound waves strike and rebound from the photosphere, they cause the gas there to rise and fall with a period of about five minutes. Observations of these oscillations with instruments on the solar and heliospheric observatory (soho), and from the global oscillation network group (gong), have been used to examine sound waves with different paths inside the Sun, determining its internal structure and dynamics with the techniques of helioseismology.

The dominant factor affecting each sound is its speed, which in turn depends on the temperature and composition of the solar regions through which it passes. The sound waves move faster through higher-temperature gas. Helioseismologists determine the difference between the observed sound speed and that calculated from the standard model of solar structure. Relatively small differences between the theoretical calculations and the observed sound speed are used to fine-tune the model and establish the Sun's radial variation in temperature, density and composition.

A small but definite change in sound speed has been detected at the lower boundary of the convective zone, pinpointing its radius. After suitable refinements to the Standard Solar Model, the measured and predicted sound velocities do not differ from each other by more than 0.2%, from 0.95 solar radii down to 0.05 radii from the centre. This places the central temperature of the Sun very close to the 15.6 million K of the model, confirming predictions of the expected amounts of solar neutrinos.

Measurements of sound wave frequencies to infer rotational and other motions inside the Sun have shown that the differential rotation, discovered by observations of sunspots, persists to just below the convective zone. The equatorial regions in this zone spin rapidly, while the regions near the poles rotate with slower speed. At deeper levels the rotation rate remains independent of latitude, becoming uniform from pole to equator to pole. The Sun's magnetism is probably generated at the interface between the deep interior, which rotates at one speed, and the overlying gas, which spins faster in the equatorial middle. Relative motions between neighbouring layers of electrified gas at this deep level probably help to amplify and generate the Sun's magnetic field by the dynamo effect.

Material in the Sun's interior flows in ways other than rotation. Broad zonal bands sweep around the equatorial regions at different speeds. The velocity of the faster zonal flows is about 5 m/s (16 ft/s) higher than gases to either side, but this difference is about 400 times slower than the mean velocity of rotation. A single zonal band is more than 65,000 km (40,000 mi) wide and 20,000 km (12,000 mi) deep. These zonal bands gradually drift from high latitudes towards the equator during the 11-year solar cycle, moving in step with a similar motion of sunspots.

Both the sunspots and the zonal bands are moving against another steady flow from the equator to the poles, which has a speed of about 20 m/s (66 ft/s). This flow penetrates to a depth of at least 25,000 km (15,500 mi). Researchers suspect that a return flow towards the equator exists at deeper levels, but detailed motions have not yet been observed at this depth.

solar mass (symbol MJ Mass of the Sun, used as a benchmark against which the masses of other stars are compared. One solar mass is equivalent to 1.99 X 1030 kg and accounts for over 99% of the total mass of the Solar System. Other stellar masses range from less than 0.2 M0 up to 100 Mo. or more.

Solar Maximum Mission (SMM) NASA satellite launched in 1980 February to study the Sun during a period of maximum activity at the peak of the solar cycle. It failed after nine months, but repairs were successfully done by a Space Shuttle crew in 1984. The Solar Maximum Mission satellite re-entered Earth's atmosphere in 1989. One of its important results was the discovery of the variability of the solar constant.

solar nebula protoplanetary disk that surrounded the early Sun and produced the planets of our Solar System. It had the same composition as the Sun, mostly hydrogen and helium, with about 1% of heavier elements. In order to contain enough heavy elements to produce the planets, the total mass of the solar nebula must have been at least a few per cent of the Sun's mass; it may have exceeded a tenth of a solar mass if planetary formation was inefficient. Most of that mass, in the form of gas and dust, dissipated after the planets had formed. See also cosmogony

solar neutrinos Electron neutrinos (see neutrino astronomy) produced in vast quantities by nuclear reactions in the Sun's energy-generating core. Every second, an estimated 3 X 1015 solar electron neutrinos enter each square metre of the Earth's Sun-facing side and pass out through the other side unimpeded. The neutrino reaction rate with matter is so small that a special Solar Neutrino Unit (SNU) is used to specify the flux. One SNU is equal to one neutrino interaction per second for every 1036 atoms. To measure this flux, neutrino detectors must contain large amounts of material and be placed deep underground to filter out confusing energetic particles generated by cosmic rays.

The observed flux of electron neutrinos is compared with the number predicted by the standard model of the Sun. Typically, only one-third to one-half the predicted number were found in the pioneering experiments, a discrepancy known as the solar neutrino problem. For example, the homestake mine experiment obtained an average flux of 2.550.25 SNU compared with the predicted 8.0 1.0 SNU. Other detectors, including Kamiokande, Sage and Gallex, gave similar findings.

The solar neutrino problem might have been solved if there was a mistake in our models of the solar interior. However, helioseismology observations accurately confirm the predicted core temperature of 15.6 million K, indicating that the neutrino problem could not be solved with plausible variations in the standard model.

In one alternative explanation, the electron neutrinos produced by nuclear reactions in the Sun's core can change into muon or tau neutrinos on their way out of the

Sun, thereby becoming invisible to these detectors, which responded only to electron neutrinos. This metamorphosis is known as neutrino oscillation, since a neutrino oscillates back and forth between types as it travels through space and time. The neutrino transformation that occurs when travelling through matter, such as the Sun or Earth, is named the MSW effect after the surname initials of the scientists who developed the theory in the 1970s and 80s.

Experiments at the underground Sudbury Neutrino Observatory in Canada confirm the previously observed deficit in solar neutrinos to be a result of neutrino oscillation. Containing a 1000-tonne reservoir of 'heavy water' (D2O) in which the hydrogen has been replaced with a heavy isotope, deuterium, the Sudbury detector is, uniquely, capable of distinguishing between neutrino types. Results published in 2001 showed the expected numbers of neutrinos, as predicted by the standard model of the solar interior.

solar neutrino unit (SNU) Measurement of the rate of flow of neutrinos per unit area (neutrino flux). One SNU equals one neutrino-induced event per 1036 target atoms in a neutrino detector. See also SOLAR NEUTRINOS

solar parallax Apparent shift in position of the Sun as measured using the diameter of the Earth as a baseline. PARALLAX is an important astronomical tool when determining distances to celestial objects. The apparent change in position of an object, when viewed from opposite ends of a chosen baseline of known length, allows its distance to be computed using simple trigonometry. For objects within the Solar System, the diameter of the Earth is used, which gives the DIURNAL PARALLAX (or geocentric parallax). For more distant objects, the Earth's orbit around the Sun provides a more appropriate baseline. Solar parallax is defined as being the angular size of the Earth's equatorial radius as measured from a distance of one astronomical unit (1 AU). This method of determining the distance to the Sun has been very important in establishing the scale of the Solar System.

solar particles Energetic particles that are expelled by the Sun into interplanetary space. Electrons, protons and other ions are being continuously blown in all directions into interplanetary space by the perpetual SOLAR WIND. CORONAL MASS EJECTIONS and FLARES hurl more energetic charged particles into space, producing powerful gusts in the solar wind that are important in solar-terrestrial interaction. Nuclear reactions in the SOLAR INTERIOR produce SOLAR NEUTRINOS, which are emitted from the Sun's energy-generating core in all directions at nearly the speed of light.

solar particles Charged particles from a solar flare were detected by SOHOs LASCO camera just 3 minutes after the flare erupted from the Suns surface. The resulting full-halo coronal mass ejection can be seen spreading out from the cameras occulting disc.

Solar Radiation and Climate Explorer (SORCE) NASA satellite to be launched in 2002 to provide total irradiance measurements and the full spectral irradiance measurements in ultraviolet, visible and near-infrared wavelengths required for climate studies. The spacecraft is a combination of two original spacecraft merged into one.

Solar System Collective name given to the Sun and all the celestial bodies within its gravitational influence. This includes the nine planets and their accompanying satellites and ring systems, asteroids, comets, meteoroids and tenuous interplanetary dust and gas. The point at which the Solar System ends is called the heliopause, estimated to lie at a distance of 100 AU from the Sun. See also HELIOSPHERE

solar telescope Fixed telescope, usually located within a tower, and dedicated to making observations of the Sun. Observing the Sun is not as straightforward as making observations of the stars or other celestial objects because of the extreme heating effect of its light. The Earth absorbs solar radiation, producing a layer of hot, turbulent air near ground-level, causing images formed by mirrors at this height to be blurred and unsteady. To overcome this, the light-gathering equipment is placed high above ground level at the top of a tall tower, sometimes known as a solar tower.

solar telescope The New Swedish Solar Telescope on La Palma is operated by the Institute for Solar Physics of the Royal Swedish Academy of Sciences. As well as studying the Sun, it will be used for highresolution observations of Mercury and other objects in the Solar System.

Light rays from the Sun are collected by an instrument such as a COELOSTAT or HELIOSTAT. These track the apparent motion of the Sun across the sky and use plain mirrors to direct its light down into the tower towards a curved, focusing mirror located at the bottom. From here, the light beam is reflected back up the tower and, via one or more flat mirrors, into a room where large instruments such as high-dispersion SPECTROHELIOGRAPHS may be located.

The use of a tower allows long focal lengths to be obtained, which are necessary in order to form an image of the Sun with easily distinguishable detail. The tower, which is usually painted white in order to reduce the amount of solar radiation it absorbs, also protects the internal mirrors from wind and vibration. Its interior is often cooled or evacuated in order to further reduce distortion of the image by moving currents of warm air.

solar wind Solar material flowing into interplanetary space. The Sun's atmosphere is expanding radially outwards in all directions at supersonic speeds of hundreds of kilometres per second, filling interplanetary space with charged particles and MAGNETIC FIELDS. This solar wind is made up of an equal number of electrons and protons, with lesser amounts of heavier ions. It carries solar material out into interstellar space at a rate of almost a million tonnes each second, flowing past the planets, which essentially orbit within the Sun's outer atmosphere. The solar wind carves out a huge bubble in space with the Sun at its centre, known as the HELIOSPHERE, extending out to about 100 AU from the Sun.

The existence of the solar wind was suggested from observations of COMET ion tails by the German astronomer Ludwig Biermann (1907-86) in the 1950s. A comet's ion tail always points away from the Sun. While the RADIATION PRESSURE of sunlight is sufficient to push comets' curved dust tails away from the Sun, it is not enough to create their ion tails. Biermann proposed that electrically charged particles pour out from the Sun at all times and in all directions, accelerating the ions to high speeds and pushing them radially away from the Sun in straight ion tails.

In 1958 Eugene Parker (1927- ) of the University of Chicago showed how a flow might work, dubbing it the solar wind. It would naturally result from expansion of the Sun's CORONA. At a critical distance of a few solar radii, the corona's thermal energy overcomes the gravitational attraction of the Sun, allowing coronal plasma to expand supersonically into interplanetary space. Parker also demonstrated how the Sun's magnetic fields would be pulled into interplanetary space, acquiring a spiral shape as a result of the combined effects of radial solar wind flow and the Sun's rotation.

Spacecraft have been making in situ measurements of the solar wind since 1959. The average solar wind density near the Earth was shown in 1962-63 by Mariner 2 to be 5 x 106 particles per cubic metre. Such a low density close to Earth's orbit is a natural consequence of the wind's expansion into an ever-greater volume. The Mariner 2 data also indicated that the solar wind has a slow and a fast component. The slow component travels at a mean speed of about 400 km/s (250 mi/s) and emanates from coronal streamers close to the solar equator; the fast component travels at a mean speed of about 7500 km/s (4700 mi/s) and originates from the coronal holes. Subsequent spacecraft showed that the interplanetary magnetic field has a strength of approximately 0.00006 Gauss (6 x 10-9 Tesla) at Earth's orbit distance from the Sun.

When fast solar wind streams interact with slow streams, they produce Co-rotating Interaction Regions (CIRs), in which forward and reverse shocks are generated. Intense magnetic fields are also produced, and solar particles can be accelerated to high energies by CIRs. In 1994-95, near a minimum in the solar cycle, the ulysses spacecraft made measurements of the solar wind over the full range of heliographic latitudes and at a distance comparable to that of the Earth. Ulysses' velocity data conclusively prove that a uniform, fast, low-density wind pours out at high latitudes near the solar poles, and that a gusty, higher-density, slow wind emanates from the Sun's equatorial regions at solar minimum.

Simultaneous observations with soho and yohkoh pinpointed the sources of the solar wind on the Sun. The slow wind is associated with the narrow stalks of coronal streamers, at least during minimum in the solar activity cycle. Much of the high-speed wind escapes from polar coronal holes. Instruments on these spacecraft also showed that the high-speed wind is accelerated very close to the Sun (within just a few solar radii), and that the slow component attains full speed about ten times farther away.

The SOHO observations also indicated that the fast wind in polar coronal holes emanates from the boundaries of the magnetic network seen in the chromosphere, and that heavier particles move faster than light particles in polar coronal holes. Magnetic waves might provide this preferential acceleration. For example, oxygen ions have agitation speeds 60 times greater than those of protons in coronal holes. Ulysses has detected magnetic fluctuations, attributed to alfven waves, far above the Sun's poles; they may block cosmic rays.

The solar wind plays an important role in shaping Earth's magnetosphere and the magnetospheres of the other planets. Particles and magnetic fields carried by the solar wind drive magnetic storms.

Solis Lacus See solis planum

Solis Planum (formerly Solis Lacus) Dark oval feature on mars (25.5S 86.5W). Set against the bright neighbouring Syria and Thaumasia deserts, Solis Planum is nicknamed the 'Eye of Mars'. It is highly variable in albedo and extent because of the frequent incidence of dust storms, which excavate and redistribute the lighter dusty surface deposits.

solstice Time when the Sun is at its greatest declination, 23.5N or 23.5S, marking the northern and southern limits of its annual path along the ecliptic. In the northern hemisphere the summer solstice occurs around June 21, when the Sun reaches its highest altitude in the sky and is overhead at the tropic of cancer. This marks the longest day of the year, the period of maximum daylight. The winter solstice occurs around December 22, when the Sun reaches its lowest altitude, being overhead at the tropic of capricorn. This is the point of the shortest day, when daylight hours are at a minimum. The solstices are reversed in the southern hemisphere.

solstitial colure great circle, or hour angle, on the celestial sphere that passes through both the north and south celestial poles, and intersects the celestial equator at the right ascension of the summer and winter solstices. See also equinoctial colure

Sombrero Galaxy (M104, NGC 4594) Spiral galaxy in the constellation Virgo (RA 12h 40m.0 dec. -1137'), seen almost edge-on. It has a prominent lane of dark material, obscuring part of the nuclear bulge. The galaxy was discovered by Pierre mechain in 1781. The Sombrero Galaxy is a member of the Virgo-Coma supercluster, at a distance of 65 million l.y. It has apparent dimensions of 7'.1 x 4'.4, and magnitude +8.0. The actual diameter is probably in excess of 135,000 l.y.

SORCE See solar radiation and climate explorer

source count Number of sources, such as stars or galaxies, per unit area and/or per unit flux/brightness interval. The source count is often used to determine the rate of change. It can demonstrate, for example, that there is a dark cloud obscuring faint stars like the Coalsack Nebula or the Horsehead Nebula, or that there are more galaxies at large distances than nearby.

South African Astronomical Observatory (SAAO) Major South African optical astronomy facility founded with UK participation in 1972. Its headquarters are at the old royal observatory, cape of good hope, but the telescopes are located near Sutherland in the semi-desert of the Karoo, about 320 km (200 mi) north-east of Cape Town, at an elevation of 1760 m (5775 ft). The main instruments are a 1.88-m (74-in.) reflector, moved from the Radcliffe Observatory, Pretoria, in 1976, and a 1.02-m (40-in.) reflector, originally erected at the Cape Observatory in 1964 and known as the Queen Elizabeth Telescope. By 2004 the observatory will be home to the multi-national 9.2-m (30-ft) southern african large telescope (SALT).

South Atlantic anomaly Region located off the east coast of South America in which the magnetic field on the Earth's surface is at its weakest. Since inner radiation belt particles circling the Earth follow contours of constant magnetic field strength, they move to lower altitudes at this point above the South Atlantic. Within this region, energetic particles, and COSMIC RAYS in general, have the greatest chance of precipitating into the atmosphere or adversely affecting low-Earth-orbiting satellites. A major fraction of particle loss from the inner radiation belt to the atmosphere occurs within the South Atlantic anomaly.

Southern African Large Telescope (SALT) Major optical telescope under construction at the SOUTH AFRICAN ASTRONOMICAL OBSERVATORY at Sutherland, South Africa, expected to become operational in 2004. The telescope is being built in partnership with Poland, New Zealand, and universities in Germany and the United States. It is a close copy of the HOBBY-EBERLY TELESCOPE (HET), with a similar 11-m (36-ft) hexagonally segmented mirror fixed in a 'tilted ARECIBO' configuration. However, SALT will use an improved version of the HET's Spherical Aberration Corrector to allow more of the mirror's surface to be used at any one time, giving it a slightly larger effective aperture than the HET's 9.2 m (30 ft). Like the HET, it will have both imaging and spec-troscopic capabilities. SALT will be the largest single-mirror telescope in the southern hemisphere.

Southern Astrophysical Research Telescope (SOAR) Optical telescope of 4.2-m (165-in.) aperture located at Cerro Pachon, close to the southern telescope of the GEMINI OBSERVATORY and on the same ridge system as the CERRO TOLOLO INTER-AMERICAN OBSERVATORY. The telescope was built by the Southern Observatory for Astrophysical Research (SOAR), a consortium consisting of Brazil, the US NATIONAL OPTICAL ASTRONOMY OBSERVATORY and two US universities. It became operational in 2002.

Southern Cross Popular name for the constellation CRUX.

southern lights (aurora australis) See AURORA

South, James (1785-1867) English surgeon, amateur astronomer and observer of double stars, remembered for his 'telescope war' with the telescope-maker Edward Troughton (see COOKE, TROUGHTON & SIMMS). With John HERSCHEL he co-authored a standard catalogue of 380 double stars, later discovering another 160 pairs. Realizing that further discoveries required a grander telescope, in 1829 South bought an 11f-inch (300-mm) objective lens - then the world's largest - by the French optician Robert Cauchoix (1776-1845). To build an equatorial mount he contracted with his then-friend Troughton. But Troughton, who by 1832 was ageing and had never built such a large mount, did not complete the job to South's satisfaction, as the mount suffered from excessive vibration. South refused to pay Troughton, and years of costly litigation followed. When South was ordered to pay for the work he destroyed the mount and sold it as scrap metal.

South Pole-Aitken Basin Lunar basin (55S 180W). It is the largest (2500 km/1600 mi diameter) and deepest (12 km/7 mi) impact structure on the Moon. Most of this basin is on the farside of the Moon, but the outer ring does come across the south pole region on to the nearside. The South Pole-Aitken Basin formed when an enormous object struck the Moon. This basin is from the oldest period of the Moon, and so is deeply degraded. Scattered mare regions of limited area occur inside the basin.

South Tropical Disturbance Major feature on JUPITER. It was in the same latitude as the GREAT RED SPOT, which it periodically passed. Its mean rotation period was 9h 55m 27s.6. Seen from 1901 to 1940, it has not subsequently been observed and has presumably disappeared permanently.

Soyuz TM In addition to ferrying crews to and from Mir and the International Space Station, Soyuz modules have been used to transport equipment and satellites. The unmanned version is called Progress and it was one of these that collided with Mir.

Soyuz TM Russian-crewed spacecraft that is used as a ferry to and from SPACE STATIONS. It is based on the original Soyuz craft, which was first flown on a manned spaceflight in 1967, upgrades to which have been made during the craft's distinguished career. The Soyuz TM first flew in 1986 and there were 37 manned launches of the original Soyuz, 14 of the Soyuz T and, to the end of 2001, 33 of the Soyuz TM. The spacecraft, which weighs approximately 7 tonnes, is launched on a booster of the same name. It carries a maximum of three crew, who are housed in the flight cabin-descent module in the centre of the spacecraft. It has at its rear a service module with an in-orbit manoeuvring engine and two solar panels, and at the front it has an orbital module with a docking system at the forward end. Although earlier Soyuz modules flew independent flights, the TMs remained docked to the MIR space station for months, providing immediate availability for a return to Earth. A Soyuz TM is always docked to the INTERNATIONAL SPACE STATION (ISS) to provide emergency evacuation for the early three-person crews. Later, the ISS may have two Soyuz TMs available at all times for the six crew.

Space Infrared Telescope Facility (SIRTF) Last of the NATIONAL AERONAUTICS AND SPACE ADMINISTRATION (NASA) Great Observatory series spacecraft, after the HUBBLE SPACE TELESCOPE (1990), the COMPTON GAMMA RAY OBSERVATORY (1991) and the CHANDRA X-RAY OBSERVATORY (1999). To be launched in 2002, it is expected to be named after a notable infrared astronomer. SIRTF is much smaller than originally planned and will be placed into an Earth-trailing solar orbit, providing excellent sky visibility and a benign thermal environment. It will continue the work conducted by the INFRARED ASTRONOMICAL SATELLITE, COSMIC BACKGROUND EXPLORER and the INFRARED SPACE OBSERVATORY. Its liquid-helium-cooled telescope is expected to operate for about three years before the cryogenic coolant is depleted.

Spacelab Manned laboratory developed by the EUROPEAN SPACE AGENCY to fly scientists and experiments into space in the cargo bay of the SPACE SHUTTLE. There were two basic elements: a pressurized laboratory, where scientists worked in a 'shirt-sleeve' environment; and one or more external pallets, which carried instruments that were directly exposed to space. Various combinations of these elements were possible. Several astronomy missions were flown during the 22 Spacelab flights between 1983 and 1998.

space probe Space vehicle launched to investigate the Moon, the planets and their satellites, comets and asteroids, or to study interplanetary space.

Space Shuttle Probably the most famous space vehicle today. The USA's Space Shuttle had flown over 100 missions by the end of 2001, 20 years since its maiden flight. The Shuttle has made space travel almost look routine, albeit not quite as regular an occurrence as was hoped when the programme began life in 1972 with President Richard Nixon's signature. At the time 50 missions a year were envisaged, rather than the seven to eight actually flown.

Space Shuttle Columbia was first launched on 1981 April 12 and, after a major refit, was brought back into service in 2002 March for the third Hubble servicing mission.

The Space Shuttle comprises three main elements. The orbiter is equipped with three main engines called the SSMEs, which are fuelled by liquid oxygen and liquid hydrogen fed from a large brown external tank attached to the belly of the orbiter. The external tank (ET) is the only part of the Shuttle that is expendable, being abandoned to re-enter the Earth's atmosphere when the orbiter has reached initial orbit. Attached to either side of the ET are two solid rocket boosters (SRBs), which supplement the SSMEs during the first 2 minutes of flight and which are recovered from the sea after each flight. Most parts of the SRBs are used again on future Shuttle flights. The Shuttle's payloads are carried primarily in the payload bay, which is 18.3 m (60 ft) long and 4.6 m (15 ft) wide. Additional cargo and experiments, as well as equipment and consumables for the crew, are located in the mid-deck, which is under the flight deck. The mid-deck also acts as the wardroom, kitchen and temporary gym and is equipped with a toilet. The maximum payload capability of 24,947 kg has not yet been carried on any Shuttle mission. The payload capability depends on the orbital inclination taken by a mission. Each degree higher than 28 takes 226 kg off the payload weight. Missions from the Kennedy Space Center can fly up the eastern seaboard of the USA into a 57 orbit - on one reconnaissance satellite deployment mission, this was extended to 62.

Six Shuttle orbiters have been built, starting with the Enterprise in 1977. This was not a spaceworthy craft and was used for atmospheric glide tests before the space missions could be attempted. The first orbiter to make an orbital spaceflight was Columbia in 1981 and the second, Challenger, in 1983. This was followed by Discovery in 1984 and Atlantis in 1985. After Challenger was lost in 1986, a replacement orbiter, Endeavour, was built , which flew for the first time in 1992. The orbiter is 37.24 m (122.2 ft) long with a wingspan of 23.79 m (78.06 ft) and is 17.27 m (56.67 ft) high from the undercarriage to the top of the vertical stabilizer. The Space Shuttle on the launch pad and fully loaded for lift-off weighs 2,041,186 kg, of which the orbiter accounts for about 113,399 kg. The SRBs are 45.46 m (149.15 ft) and 3.70 m (12.14 ft) in diameter. The ET is 47 m (154 ft) long and 8.38 m (27.5 ft) wide. The total length of the whole stack from the tip of the ET to the tail of the SRBs is 56.14 m (184.2 ft).

Most flights of the Space Shuttle carry the Remote Manipulator System (RMS), which is a sophisticated robot arm controlled from the flight deck by specialist RMS operator astronauts, who use a hand controller, a computer, TV cameras on the RMS and the view out of the rear window of the flight deck. The RMS is used to deploy and retrieve payloads and as a mobile carrier for space-walking astronauts who stand on a foot restraint at the end of the arm. The 15.24-m (50-ft) long RMS is fitted with a 'shoulder', 'elbow', 'wrist' and 'hand', to enable it to be moved into all kinds of positions. The RMS has proved invaluable to the Shuttle programme and its technology is being utilized for the INTERNATIONAL SPACE STATION.

At the end of a mission, after the retrofire, the Shuttle begins its transition from a spaceship to a glider as it starts re-entry at about 121,920 m (400,000 ft), approximately 8000 km (5000 mi) from the landing site at a speed of about Mach 25. There are six types of thermal material to protect parts of the orbiter that experience varied heat levels, the wing leading edges and underside bearing the brunt of the re-entry heating (1530 K), while the upper part of the fuselage is less exposed to heat. The approach to the landing site, usually at the Kennedy Space Center, is made at an angle seven times steeper and a speed 20 times faster than an airliner, and at touchdown the speed is over 320 km/h (200 mph).

space station Large orbiting structure with substantial living and working accommodation which is designed to be permanently or intermittently manned.

Space stations have many potential roles. These include experimental and observational work in the pure sciences (astronomy, physics, life sciences); environmental monitoring; research and development work in applied sciences; industrial activity utilizing the space environment (virtual zero gravity, ultra-high vacuum) for materials processing; providing a servicing base for satellites and free-flying structures; and providing a base for further constructional work in space. There are also various potential military applications, such as surveillance, servicing of satellites and anti-missile activity. Space stations may eventually provide a base for the launching and return of interplanetary exploration missions, manned or unmanned.

The first space station to be placed in orbit was the Soviet SALYUT 1 in 1971. The lone US competition came in 1973 with the launch of SKYLAB. A major advance came with the introduction of the second generation of Soviet space stations and the assembly of separate units in orbit. The first demonstration of this technique was the linking of the COSMOS 1267 module to Salyut 6 in 1981. However, the real breakthrough came in 1986 February, when the Soviet Union launched the core module of its MIR ('Peace') station. Mir was equipped with four radial docking ports in addition to the usual access at the front and rear. Five additional modules and a US-made docking unit were attached to the station in 1987-96.

Even before Mir re-entered the atmosphere in 2001 March, construction of an INTERNATIONAL SPACE STATION had begun. Under the leadership of the USA, 16 nations came together to contribute to the largest space structure ever placed in orbit. Assembly is scheduled to be completed by 2006.

Space Telescope Science Institute (STScl) Astronomical research centre responsible for operating the HUBBLE SPACE TELESCOPE. Located at the Homewood campus of the Johns Hopkins University in Baltimore, Maryland, the STScI was founded in 1980 and is operated by AURA under contract to NASA. Besides its service role to the astronomical community, it is also concerned with the further development of the HST, and the pursuit of new space- and ground-based initiatives.

spacetime Four-dimensional CONTINUUM made up of three spatial dimensions and one time dimension. The laws of physics proposed by Isaac Newton in the late 17th century were based on the supposition that three-dimensional space was fundamentally different from time. In fact, time played a role of only increasing at a constant rate everywhere in the Universe. In SPECIAL RELATIVITY, time lost its special role and became another dimension, not quite equivalent to the other three, but could not be assumed to tick off at the same rate for every observer. Thus, relativity transformed our picture of the Universe from three spatial dimensions to a four-dimensional spacetime continuum. GENERAL RELATIVITY attributes gravity to the warping or bending of spacetime due to the presence of mass and energy.

space velocity True speed and direction of a star through space, relative to the Sun. From observations of a star's position it is possible to measure its PROPER MOTION - its movement across our line of sight - and from that to calculate its TANGENTIAL VELOCITY. Observations of the Doppler shift in its spectrum reveal how fast it is travelling away from us, in other words its RADIAL VELOCITY. Because the two lie at right angles to one another, simple trigonometry can be used to calculate their resultant, which gives a measure of the star's actual speed and direction through space.

Spacewatch ASTEROID and COMET search programme based at the University of Arizona, USA, employing telescopes at Kitt Peak. Spacewatch, founded by Tom Gehrels (1925- ), was the first NEAR-EARTH OBJECT project to make use of charge-coupled devices. Many comets discovered in this project have been given the name Spacewatch.

space weather Responses of Earth's local space environment to variations on the Sun, such as CORONAL MASS EJECTIONS, FLARES, solar particle events and associated interplanetary shocks. Monitoring and predicting these variations is becoming more widespread because many modern technological systems are increasingly susceptible to these effects. For example, these severe solar disturbances propagate out to the Earth and drive GEOMAGNETIC STORMS in the MAGNETOSPHERE. These in turn cause dramatic increases in the particle populations in the VAN ALLEN BELTS and in the electrical currents flowing in the magnetosphere and IONOSPHERE. Navigation, communication and weather satellites orbiting the Earth may be damaged or destroyed if they pass through these enhanced radiation environments. Hazardous radiation levels may also be experienced by astronauts, and in extreme cases by passengers in aircraft flying high over the poles of the Earth.

Communication between spacecraft and the ground may be affected by disturbances in the ionosphere. The ionospheric drag on low-Earth-orbit satellites may increase, thus reducing the satellites' operational lifetime. Ionospheric currents generate magnetic fields on the ground, which may disrupt the operations of large power-distribution grids. For example, such magnetic fields were the cause of a 9-hour blackout affecting 9 million people in Quebec, Canada, in 1989 March. These fields can also introduce significant errors in geomagnetic surveys used in the commercial exploration of natural resources, such as drilling for oil.

Space weather effects are particularly prevalent during the maximum of the SOLAR CYCLE, although they may occur at any time. Various agencies around the world are engaged in routine monitoring of the Sun, the solar wind and the terrestrial magnetosphere and attempt to predict which events will have major impact on the Earth environment. Such predictions allow appropriate protective measures, such as powering down susceptible systems, to be implemented before damage occurs.

spallation NUCLEAR REACTION wherein a nucleus is struck by a particle whose energy is greater than 50 MeV. Usually this is a COSMIC RAY impact with the nucleus of an atom in the interstellar medium. It leads to fragmentation of the nucleus and the production of light elements (see ASTROCHEMISTRY and NUCLEOSYNTHESIS).

Spallation is also the ejection of debris from the back of a surface, or around the point of impact, for an object hit at high speed by a macroscopic particle, such as micro-meteoroid impacts on spacecraft.

Special Astrophysical Observatory (SAO) Research institution of the Russian Academy of Sciences, established in 1966, and the only Russian centre for ground-based astronomy. The observatory is located near the town of Zelenchukskaya in the North Caucasus. Its two facilities are the BTA (Bolshoi Teleskop Azimutal'ny, or Large Altazimuth Telescope), completed in 1976, and -20 km (12 mi) to the south-east - the RATAN-600 (for 600-m Radio Telescope Antenna), opened in 1977. The BTA is at an elevation of 2100 m (6890 ft) and, with its 6.05-m (238-in.) mirror, was for 15 years the largest optical telescope in the world. The RATAN-600 is similarly generously proportioned, with a 0.6-km (0.4-mi) diameter multi-element ring antenna that can be pointed by tilting the individual panels of the ring. The SAO carries out research over the full range of modern astrophysics, and has branches in St Petersburg and Moscow.

special relativity Theory of mechanics proposed by Albert EINSTEIN in 1905 that correctly describes the motions of objects moving near the speed of light. Einstein proposed a version of mechanics based on the LORENTZ TRANSFORMATIONS. Einstein's theory returns results identical to Newton's laws for small velocities relative to the speed of light, but predicts substantial differences for objects moving close to the speed of light. A primary postulate for special relativity is that we live in a four-dimensional SPACETIME continuum, and that time is relative to the reference frame of the observer. A second postulate is that observers always measure the speed of light at the same value: lightspeed is an invariant quantity in relativity.

Lorentz transformations predict that time runs slower in the reference frame of the object as it moves relative to a stationary observer: time slows down with increasing velocity. These propositions also show that in the reference frame of a stationary observer, the object in motion is physically smaller in the direction of motion, being smaller when closer to the speed of light.

Finally, special relativity predicts that mass increases with velocity. These important considerations led to the famous equation E = mc2, spelling out the equivalence of mass and energy. The mass (m) multiplied by the speed of light (c) squared yields a tremendous amount of energy (E), since the speed of light is huge (roughly 300,000 km/s or 186,000 mi/s). This discovery allowed astronomers to explain energy generation in the Sun as caused by NUCLEAR REACTIONS converting hydrogen to helium. Every aspect of special relativity has been confirmed to a high degree of accuracy numerous times.

speckle interferometry Technique for improving the resolution of astronomical images by removing the blurring caused by turbulence in the Earth's atmosphere. When the light from a star passes through the atmosphere, it is refracted in random directions over periods of tens of milliseconds by constantly moving pockets of air of varying density and temperature. This refraction produces multiple images of the star that appear to dance around (twinkle) many times a second and be blurred. The effect is known as SEEING.

The light refracted from the individual atmospheric cells reaches different parts of the telescope's optics at different times and from different directions, resulting in either constructive INTERFERENCE (peaks of separate waves coincide) or destructive interference (peaks and troughs coincide). This produces a series of bright and dark regions, giving a highly contrasted mottled or grainy appearance. The bright regions are known as speckles, several hundred of which may be seen, and the speckle pattern represents the random distribution of atmospheric irregularities. Because the atmosphere is turbulent, this pattern changes quite rapidly with time so that images of stars taken with exposures of about a second or more become smeared into what is known as the 'seeing disk'. For short time-exposures, however, the atmospheric motion is frozen and the full speckle pattern is revealed.

Speckle interferometry involves the use of a detector such as a CCD to take a sequence of short-exposure (10-15 milliseconds) 'snapshots' of an object, freezing its apparent motion. Each snapshot produces an instantaneously distorted image and the sequence can then be combined by computer image-processing, electronically removing the atmospheric effects, to produce a distortion-free image.

The technique is not limited to stellar observations. In recent years, speckle interferometry has been successfully applied to various solar phenomena and has also been used to map surface features on Saturn's moon Titan.

spectral classification System of classifying the spectra of stars (see SPECTRUM). In the mid-19th century, when astronomers began to observe the brighter stars with SPECTROSCOPES, they discovered a rich variety of spectra that required some system of classification. Of several original schemes, that by Father Angelo SECCHI was the most widely adopted. His system contained five types that were based on both the stars' colours and the details within their spectra: Type I contained blue-white stars with strong hydrogen ABSORPTION LINES (for example, Sirius and Vega); Type II referred to yellow and orange stars with numerous metallic spectrum lines (the Sun, Capella, Arcturus); Type III contained orange-red stars with metallic lines and bands, now known to be caused by titanium oxide (TiO), that shaded to the blue (Betelgeuse, Antares); Type IV included deep-red stars that had dark carbon bands shaded to the red; Type V was reserved for stars with bright EMISSION LINES.

As spectroscopes improved and photography became available, better spectroscopic detail required a more comprehensive description. A system devised between 1890 and 1901 by Harvard's E.C. PICKERING and his assistants (Williamina FLEMING, Antonia MAURY and Annie J. CANNON) originally used letters from A to Q based primarily on the strengths of the hydrogen lines such that A-D belonged to Secchi's Type I, E-L to Type II, M to Type III, carbon-line N to Type IV, and O-Q to Type V.

With improved observations, several letters were dropped as unnecessary, while the work of Maury and Cannon showed better continuity of all the lines if B were to come before A and O before B. The final result was the classic spectral sequence OBAFGKM. Cannon then added for each class simple numerical subtypes from 0 to 9, such that B0 to B9 is followed by A0 to A9 and so on (the modern system beginning at O3). Cannon's classification of about 225,000 stars was published between 1918 and 1924 in the henry draper catalogue. (A later extension increased the count to 359,082.)

Based in part on the presence of emission lines, M was originally subdivided into Ma to Md and O from Oa to Oe, but these were eventually made numeric as well ('e' and 'f' are now descriptive terms for emission lines). R STARS, which are the warmer versions of class N CARBON STARS, were added in 1908. SSTARS, which are intermediate carbon stars with bands of zirconium oxide, were added in 1924.

The system was quickly seen to correlate with colour that progressed from blue to red, and thus with temperature, class O to M ranging from 50,000 to 2000 K. The development of physical theory demonstrated that the spectral sequence O to M represents an ionization and excitation sequence, not one of chemical composition. R, N and S, however, are different, containing various enhancements of carbon. Though some carbon stars are DWARF STARS (with their own classification schemes), all the R, N and S stars are GIANT STARS. Temperatures in the classes R and N (now combined into class C) characterize the track on the Hertzsprung-Russell diagram of ordinary stars evolving from mid-G to late M, whereas class S temperatures track only class M.

Improved infrared technology in the last decade of the 20th century began to turn up stars that were not classifiable on the original scheme. In 1999 class L (see L STARS) was added to account for deep-red (really infrared) stars in which the strong TiO bands of class M ( see MSTARS) were replaced by powerful absorptions of hydrides and the alkali metals, the temperatures falling from 2000 to 1500 K. TSTARS, at the end, near 1000 K, are defined by methane bands. Class L is a mixture of RED DWARFS and BROWN DWARFS, while T is reserved for low-mass brown dwarfs.

Various comments are commonly added to the basic spectral types to describe details. Dwarfs and giants were originally prefixed by 'd' and 'g' (the Sun, for example, is a dG2 star, Arcturus is gK1). A separate classification by Maury used a, b and c to describe line widths. Sharp-lined stars, class c, were later seen to be SUPERGIANTS. The descriptive is still in use: Lc stars, for example, are irregular supergiant variables. Also, 'e', 'm', 'p', 'wk', respectively, stand for 'emission', 'metallic', 'peculiar' and 'weak-line' (Sirius is an A1m star).

The HARVARD SYSTEM was not adequate to deal with the differences between dwarfs, giants and supergiants, that is, stars of different luminosities at a given temperature. In 1943 William MORGAN, Philip Childs Keenan (1908-2000) and Edith Marie Kellman (1911- ) redefined the spectral types and introduced a two-dimensional classification system. The MORGAN-KEENAN CLASSIFICATION (MK or MKK) scheme retained the decimally subdivided OBAFGKM temperature sequence. To each of the classes is added a Roman numeral that describes the luminosity of the star, I to V standing for supergiant to dwarf, the Sun (a G2 dwarf) is thus classified as G2 V. Keenan later added Arabic '0' to define super-supergiants (hypergiants). The MK system gives precise details of which ratios of spectrum line strengths are to be used to determine the spectral types, and it provides photographic examples of bright stars that act as classification standards. Summaries of both Harvard and MK classes are given in the accompanying tables.

For accurate work, finer subdivisions are necessary, such as Iab (intermediate between Ia and Ib) and IIa or lib (respectively on the bright or faint side of class II). The decimal divisions can also be more finely divided, B0.5 falling between B0 and B1. Intermediate luminosity classes are expressed by hyphenated Roman numerals, 09 IV-V falling between subgiant and giant.

The MK system is applicable only to stars of normal (that is, solar) chemical composition. Roman numeral VI for metal-poor SUBDWARFS is therefore inconsistent. WHITE DWARFS were originally classified on a pseudo-Harvard scheme, with D preceding the type: DA white dwarfs have strong hydrogen lines; DB and DO strong helium lines; and DC only a continuum. These types bear more relation to odd chemical compositions than they do to temperature. DA now refers to white dwarfs with hydrogen envelopes, DB or non-DA to the others (those with helium-rich atmospheres). A more complex scheme is now available. For stars that contain spectral abnormalities, an MK type may be assigned together with an indication of the strength of the peculiarity; for example K0III-CN3 shows that the star has anomalously strong bands of CN, and K2II-Ba5 indicates that the K2 giant is an extreme BARIUM STAR. Carbon content is also added to the spectral class, for example C2,4, where the latter number indicates band strength.

To be properly classified on the MK system, the spectrum of a star must be photographed in the blue-violet with a SPECTROGRAPH of prescribed type and with an appropriate classification dispersion. Higher dispersions can reveal lines that are not seen under low-dispersion conditions and can lead to mis-classification. It is common, however, to calibrate a different system of absorption lines in a different part of the spectrum (red, ultraviolet) against the MK standards. Such calibration is also necessary for classifications achieved with modern CCD (charge-coupled device) detectors, whose data are rendered graphically rather than photographically. A variety of quantitative and computer-aided schemes are also available. For consistency, however, all need to be calibrated against the original MK standards.

spectral type Series of divisions into which stars are classified according to the appearance of their SPECTRUM. A star's spectral type is ultimately based on temperature, luminosity and to some degree chemical composition. See SPECTRAL CLASSIFICATION

spectrogram Recording of a SPECTRUM. Spectrograms were originally recorded on photographic plates. With the electronic revolution in astronomy, in particular with the advent of charge-coupled devices (CCDs), spectrograms are now recorded digitally for graphical and computer-aided display. Photographic spectrograms are difficult to calibrate in terms of light intensity, while the electronic (digital) versions easily lend themselves to analysis.

spectrograph Device that takes and records SPECTROGRAMS. Spectrographs are the permanent-recording versions of the original spectroscopes through which astronomers made direct visual examinations of spectra. The first spectrographs used prisms as dispersing elements and recorded the spectra on photographic plates.

Modern spectrographs are large and complex pieces of equipment. They are often positioned at a stationary focus of a telescope, such as the COUDE or NASMYTH FOCUS. Light from a star or other celestial object enters a narrow slit and passes through a collimator, producing a parallel beam. This is directed on to a DIFFRACTION GRATING, a piece of reflective material ruled with thousands of parallel lines, to disperse the light. The resultant spectrum is then imaged on to a detector such as a CCD. The use of a narrow entrance slit means that only an image of this, rather than the star image, which will be distorted by atmospheric effects, is recorded. This prevents adjacent wavelengths being smeared out and keeps the spectrum sharp.

Spectrographs contain different types of diffraction gratings to produce either high-, medium- or low-resolution spectra, depending on the required application. An Echelle spectrograph uses an ECHELLE GRATING, which produces spectra with a high degree of resolution over a narrow band of wavelengths. The grooves of an echelle grating are stepped and their spacing relatively wide, producing a number of high-resolution overlapping spectra, which are then separated by a second, lower-dispersion grating. An astronomer might use a variety of different spectrographs to study an object. For example, if he or she were attempting to identify the optical counterpart of a known X-ray source, a low-resolution spectrograph might be used to obtain rapidly the spectra of all the possible candidates. From this survey, the X-ray object could be identified by its spectral properties, at which point an intermediate-resolution instrument would be used to obtain more detailed information, isolating features of interest. If even greater detail were required, a high-resolution instrument would be brought into use. Modern, large telescopes are equipped with a variety of such instruments for this very purpose.

An examination of some of the spectroscopic instruments available for astronomers to use on the twin Keck telescopes reveals the diversity of today's equipment. ESI, an Echellette Spectrograph and Imager, is used for obtaining high-resolution spectra of very faint galaxies and quasars from the blue to the infrared in a single exposure. It can also be configured to record lower-resolution spectra of several objects simultaneously. HIRES, as its name suggests, is a high-resolution instrument that operates in the 0.3-1.1 um range and can measure the precise intensity of thousands of separate wavelengths. LRIS is a low-resolution imaging spectrograph that can take spectra of up to 30 objects simultaneously, making it suitable for studying stellar populations of distant galaxies as well as galactic clusters and quasars. LWS is a long-wavelength spectrograph, designed for the study of planetary nebulae, protostellar objects and galactic cores. Finally, NIRSPEC, the near-infrared spectrometer, is an echelle spectrograph that operates in the 1-5 u m range and is designed for the study of very high redshift radio galaxies and the motions of stars near the galactic centre. It can also be used to study active galactic nuclei, interstellar chemistry and stellar physics. See also SPECTROSCOPY; SPECTROMETER

spectroheliograph Instrument used for imaging the Sun in light of a particular wavelength (monochromatical-ly). In principle, the spectroheliograph operates like a SPECTROHELIOSCOPE with a photographic plate or digital detector placed very closely behind the exit slit. An image of the Sun is projected from the telescope on to the narrow entrance slit, which is used to select the part of the solar disk or CHROMOSPHERE to be observed. A DIFFRACTION GRATING disperses the light into a SPECTRUM, and a second slit is then used to select the exact wavelength of light required, usually corresponding to one of the Sun's main elements, such as hydrogen (which produces the spectral line Ha) or calcium (K). A whole image of the Sun, known as a spectroheliogram and showing the distribution of that element, can be obtained by scanning the seen entire solar disk with the entrance slit. Increasingly fine detail can be resolved as the slit widths are reduced, but this lessens the final image brightness so that the speed of the scan has to be reduced proportionately.

The spectroheliograph was invented in the 1890s independently by George hale and Henri deslandres. As with the spectrohelioscope, the apparatus is usually so bulky that it is mounted with its entrance slit at the focus of a fixed telescope served by a heliostat or coelostat and second mirror.

spectrohelioscope High-dispersion spectroscope that provides a visual image of the Sun in monochromatic light. The spectrohelioscope is the visual equivalent of the spectroheliograph, in that an image of the solar disk is projected from a telescope into a narrow entrance slit. A diffraction grating disperses the light into a spectrum, and the exact wavelength to be observed is set using a second slit. The observer can thus select the element within the Sun and the depth in the solar atmosphere that they wish to view. By rapidly scanning the solar disk and its prominences with a repetition rate exceeding ten per second, by persistence of vision the observer sees a stationary monochromatic image. Scanning is commonly achieved by vibrating the two slits synchronously at high frequency, but some instruments have fixed slits that are scanned optically.

This apparatus is usually so bulky that it is mounted with its entrance slit at the focus of a fixed telescope served by a heliostat or coelostat and second mirror. The focal ratio of the spectrohelioscope must be the same as the fixed telescope so that, although the optics are invariably folded to put the entrance and exit slits adjacent to each other, the complete system is longer than twice the focal length of the fixed telescope.

The wide range of possible settings and its suitability for the measurement of radial velocity of violent solar disturbances make the spectrohelioscope a more versatile monochromatic instrument than optical filters. The detail seen increases as the slit width is reduced, but this necessitates a high scanning rate to ensure adequate image brightness. Wider slit widths show limb prominences only but disk detail is not visible.

spectrometer Instrument used for observing and measuring features of a spectrum, such as the spectral lines, by direct observation. The spectrometer incorporates a spectroscope, to split light from a celestial object into its component wavelengths, producing a spectrum, together with a device such as a fabry-perot interferometer, to measure accurately the positions and intensities of the emission and absorption lines within the spectrum. See also spectroscopy

spectrophotometer Instrument used for measuring the relative intensities of absorption and emission lines in different parts of a spectrum. The line profile of the spectrum that the spectrophotometer produces is a graph of how intensity varies with wavelength. This measure of the amount of radiation of a particular wavelength that has been absorbed or emitted by a celestial object is an indication of the relative concentration of chemical elements present within the object. The spectrophotometer can be used for the study of spectra at ultraviolet and infrared as well as visible wavelengths.

spectroscope Instrument that splits electromagnetic radiation into its constituent wavelengths, producing a characteristic spectrum. The spectrum can be observed visually or recorded in some form, in which case the instrument is known as a spectrograph. Since all astronomical data is now recorded for future analysis, the term spectrograph is almost universally used to describe this sort of instrument.

spectroscopic binary binary star that is too close for its components to be resolved optically; its orbital motion is deduced from periodic shifts in its spectral lines, indicating variable radial velocity. A spectroscopic binary is likely to be a close binary with a short orbital period and relatively high radial velocity. Binaries cannot be detected spectroscopically if their orbit is perpendicular to the Earth.

spectroscopic binary Redshift is used to ascertain the relative motions of stars in close binary pairs. When a star is moving away, its spectrum is shifted to the red, whereas the spectrum of a star moving towards us will be blueshifted. When both stars are moving at right angles to our line of sight their spectra are not displaced.

If the components have similar brightness, both sets of spectral lines are observed, and the system is called a double-lined spectroscopic binary. A composite spectrum binary has a spectrum that consists of two sets of lines from stars of dissimilar spectral type. Systems with only one set of spectral lines observable are known as single-lined spectroscopic binaries.

The radial velocity of the star (or stars) is determined by measuring the doppler shift of the spectral lines. In double-lined spectroscopic binaries, the relative velocities (v) of the two components can be determined and hence the relative masses (M). The individual masses can be determined if the inclination of the orbit is known, for example when the system is also an eclipsing binary.

spectroscopic parallax Method of obtaining the distances of stars from their spectral types (see spectral classification) and apparent magnitudes. Analysis of the spectrum reveals the full spectral class in the morgan-keenan classification system. These classes have all been calibrated in terms of absolute visual magnitude through the measurement of the distances of stars by trigonometric parallax and by main-sequence cluster fitting. The spectral class of any star thus gives its absolute visual magnitude. Comparison with the apparent visual magnitude gives the distance modulus and therefore, through the magnitude equation, the distance itself.

The method is dependent on an assessment of the degree of interstellar extinction to the star. This can be found from the degree of reddening through comparison of the actual colour with that inferred from the spectral class. The chief unknown is the ratio of total to selective extinction (that is, the relation between reddening and total absorption), which can vary from place to place depending on the nature of the absorbing dust grains.

spectroscopy Practice of obtaining, and the study of, the spectrum of an astronomical object. Astronomical spectroscopy is the key technique by which the physical properties of astronomical bodies are revealed. Spectra are obtained with a spectrograph, and recordings are made of the distribution of light relative to wavelength for the object concerned.

Current instruments allow the precise measurement of the following: the flux distribution (energy per unit area per unit time per unit wavelength) of the continuum; the fluxes, wavelengths and shapes of emission lines; and the wavelengths, strengths (the amounts of energy extracted from the continuum) and the detailed profiles (exact distribution of radiation with wavelength) of absorption lines.

Reduction of spectrographic data to actual radiative fluxes requires calibration of the sensitivity of the detector, and also requires calibration with standard astronomical sources to correct for wavelength-dependent absorption by the Earth's atmosphere (if the telescope is ground based) and the telescope/spectrograph optics. The determination of precise wavelengths for radial velocities requires observation of wavelength standards. In older spectrographs, these were iron arcs set within the spectro-graph. The iron spectrum was displayed photographically on either side of the spectrum of the source. In modern spectrographs, the standards are gas emission tubes whose spectra are digitally compared with those of the source.

The data then allow accurate physical analysis of astronomical sources. For example, the energy distribution of the continuum reveals the nature of the source, whether thermal (produced by a hot gas) or non-thermal (perhaps synchrotron emission from a supernova remnant). Emission line fluxes and wavelengths of nebulae, galaxies, quasars and a variety of other sources allow the determination of temperatures, densities, velocities (both radial velocities and those related to internal motions) and chemical compositions. Absorption line strengths and profiles allow the deduction of temperatures, densities, surface gravities, magnetic fields, rotation speeds and chemical compositions of stars.

These principles extend to all wavelength domains. Gamma-ray, X-ray and ultraviolet spectra are observed by space-borne telescopes and spectrographs. Infrared as well as visual spectra are observed with both ground-based and orbiting spectrographs. Radio telescopes are also fitted with spectrographs that use electronic techniques for the observation of emission, absorption and continuous spectra from a great variety of radio sources, from molecular clouds to galaxies.

spectrum Distribution of electromagnetic radiation with wavelength; a 'map' of brightness plotted against wavelength (or frequency). A continuous spectrum is an unbroken distribution over a broad range of wavelengths. At visible wavelengths, for example, white light may be split into a continuous band of colours ranging from red to violet. An emission spectrum comprises light emitted at particular wavelengths only (emission lines), and an absorption spectrum consists of a series of dark absorption lines superimposed on a continuous spectrum.

spectrum All elements absorb or emit radiation at particular wavelengths. This partial spectrum shows a few of the absorption lines in the the atmosphere of a star.

The spectrum of a star normally consists of a continuous spectrum together with dark absorption lines, though in some cases emission lines are also present. Emission line spectra are typical of luminous nebulae. Astronomical spectra extend to all wavelengths, from the gamma-ray domain through X-ray, ultraviolet, optical (visual) and infrared, to radio. High-energy gamma-ray and X-ray spectra are usually plotted against energy (kiloelectron volts, keV, or megaelectron volts, MeV) rather than wavelength. Wavelength in nanometres (1 nm = 10~9 m) or angstroms (1 A = 10~8 m) is used for ultraviolet and optical, whereas infrared observations use micrometres (1 um=10~6 m). Radio spectra, on the other hand, use frequency units (cycles per second, or hertz, kilohertz, megahertz or gigahertz, expressed as Hz, kHz, MHz, GHz respectively).

The term 'spectrum' is also applied to a distribution of particle energies: that is, an energy spectrum is a plot of the numbers of particles with particular energies against the range of possible energies.

spectrum variable Star showing low amplitude light variations, but having a spectrum with absorption lines that vary in strength by larger amounts, sometimes in a clearly periodic manner. The class of spectrum variables includes peculiar stars of spectral type A (Ap) and early F (Fp). The brightest and best studied example is the Ap star alpha2 canum venaticorum. In this star, variations in spectrum, brightness and magnetic field are all cyclical, with a period of 5.46939 days, equivalent to its rotation period. Models derived from observations suggest that spectrum variable stars are objects in which quantities of particular metallic elements are concentrated in patches in the stellar atmosphere; absorption lines due to specific elements are most prominent when these patches are presented in line of sight by the star's rotation. Some spectral type B stars also show such variations.

spectrum All elements absorb or emit radiation at particular wavelengths. This partial spectrum shows a few of the absorption lines in the the atmosphere of a star.

speculum metal See reflecting telescope

spherical aberration Imperfections in an image produced by lenses or mirrors having spherical surfaces. When light parallel to the optical axis is focused by a spherical lens or mirror, the light close to the axis will be focused at a different point from light farther from the axis, thus producing an imperfect image. Spherical aberration is the main aberration affecting simple optical systems; it can be reduced by the use of aspheric optics such as paraboloids.

spherical astronomy Largely obsolete term describing the mathematical calculations necessary to determine the positions and distances of celestial objects on the celestial sphere. The sky can be thought of as the inside of a sphere surrounding the Earth; to locate positions on the sphere requires the precise measurement of the angular displacement between the object in question and a set of reference points. Distances to nearby objects may be determined using trigonometric parallax. See also annual parallax; astrometry; celestial coordinates; parallax

spheroid Shape formed by rotating an ellipse about one of its axes. If the major axis is chosen, the spheroid is prolate; if the minor is chosen, it is oblate. A self-gravitating, rotating planetary body is distorted by centrifugal force into an oblate spheroid, with equatorial radius greater than polar. The shape, called a Maclaurin spheroid, depends on the body's density and rotation rate.

Spica The star a Virginis, visual mag. 0.98 (but slightly variable), distance 262 l.y., spectral type B1 V. It is a spec-troscopic binary with a period of 4 days that owes its light variation of just under 0.1 mag. not to eclipses but to tidal distortion in the shape of the orbiting components. The name is Latin for 'ear of grain'.

spicules Narrow, predominantly vertical, short-lived jets of solar material, extending from the chromosphere into the CORONA. They have a spiky appearance. Spicules are seen in spectrograms or SPECTROHELIOGRAMS, especially those taken in the red wing of the HYDROGEN-ALPHA LINE. Spicules change rapidly, having a lifetime of five to fifteen minutes and velocities of about 25 km/s (16 mi/s). Typically, spicules are about one kilometre thick, and thousands of kilometres long. They are not distributed uniformly on the Sun, being concentrated along the cell boundaries of the SUPERGRANULATION pattern.

spider Thin, usually wire or metal support holding the SECONDARY mirror in the tube of a REFLECTING TELESCOPE. The support may commonly have three or four 'legs', DIFFRACTION effects from which give rise to the cross patterns often seen in astronomical images.

spin casting Technique for manufacturing large parabolic telescope mirrors by spinning molten glass in a rotating furnace and allowing centrifugal forces to produce the required curvature in the surface. Astronomical mirrors are traditionally cast flat, and large quantities of glass are then ground out and discarded to create the correct shape, in a time-consuming and costly process. Spin casting involves heating blocks of borosilicate glass in a honeycomb mirror mould to around 1453 K. As the furnace rotates, the surface of the molten glass assumes a paraboloidal shape. It is then allowed to cool and solidify whilst still spinning.

spiral galaxy GALAXY that has a thin disk of stars, gas and dust, in which a more or less continuous spiral pattern appears. Most spiral galaxies also contain a central spherical bulge of old stars. The spiral pattern may be maintained as a DENSITY WAVE moving through the disk, or by differential rotation shearing star-forming regions into locally tilted segments. Thus spirals range from grand design patterns, with between two and four arms traceable through complete turns around the galaxy, to flocculent galaxies, in which only small, T spiral galaxy Spiral discontinuous pieces of the spiral pattern exist. Most galaxies vary in shape and spiral galaxies have at least a weak bar, an elongation of structure, but all have a the nuclear bulge, in the plane of the disk. Spirals are nucleus containing older stars classified into various subtypes in the HUBBLE surrounded by a disk where CLASSIFICATION. Most maintain active star formation Spitzer, Lyman Jr (1914-97) American astrophysicist who studied stellar dynamics and interstellar matter, and originated the concept of a telescope in Earth orbit. After helping to develop sonar during World War II, he succeeded Henry Norris RUSSELL as head of the astronomy department and observatory director at Princeton University, where he spent the rest of his working life.

spiral galaxy Spiral galaxies vary in shape and structure, but all have a nucleus containing older stars surrounded by a disk where star formation is occurring in spiral arms.

From the late 1930s, Spitzer investigated interstellar matter (ISM), establishing it as a major research field. Observing that elliptical galaxies contained old stars and lacked ISM, whereas spirals contained younger stars and were rich in ISM, he concluded that stars formed from condensing clouds of interstellar gas and dust. He studied the effect of heating and cooling and of interstellar magnetic fields on the ISM.

This interest in magnetic fields led Spitzer to another line of enquiry, the possibility of power generation by nuclear fusion, and the founding, with Hannes ALFVEN and others, of the study of plasma physics. The plasma of which stars are made is held together by the star's gravitation, but to contain plasma in terrestrial laboratories would require powerful magnetic fields. A figure-of-eight chamber constructed for this purpose at Princeton's Plasma Physics Laboratory was the forerunner of today's experimental fusion reactors.

In 1990 Spitzer witnessed the launch of the Hubble Space Telescope. He had proposed an orbiting telescope as early as 1946, since when he had fostered the idea with continual political encouragement and technical advice. He also directed the development of the COPERNICUS orbiting observatory.

spokes Features discovered in Saturn's B RING by VOYAGER. They take the form of radial shadows that are visible projected on the rings when viewed at low angles. They are thought to be the shadows of particles held above the plane of the rings by electrostatic forces. Since the discovery of the spokes, a reanalysis of drawings has shown that shadings have been recorded for over three centuries when conditions have been favourable; these are thought, in part, to represent observations of the spokes.

sporadic meteor Completely random METEOR that does not belong to any recognized METEOR SHOWER, being produced instead by the atmospheric entry of debris from the general dust-cloud filling the inner Solar System. Most sporadics originate from long-defunct meteor streams, which have been dispersed by gravitational perturbations and solar radiation effects. Sporadic activity varies over the course of the year, being highest in the autumn months. Rates also show a diurnal variation, and are highest just before dawn. Depending on the time of night and time of year, between three and 12 sporadic meteors may be seen per hour.

Sporer, Gustav Friedrich Wilhelm (1822-95) German solar astronomer who co-discovered the Sun's differential rotation and first noticed the pattern of sunspot drift called SPORER'SLAW. He joined Potsdam Observatory in 1874, working with Johann ENCKE on the motions and distribution of sunspots. Sporer discovered, independently of Richard CARRINGTON, the Sun's differential rotation: spots near the Sun's equator rotate faster than spots nearer its poles. By carefully plotting sunspot positions throughout the approximately 11-year solar cycle, Sporer discovered the patterns of sunspot migration collectively known as Sporer's law. His study (1887) of historical records revealed a scarcity of sunspots between 1645 and 1715, a period now known as the Maunder minimum after E. Walter MAUNDER, who publicized it.

Sporer's Law Appearance of sunspots at lower latitudes over the course of the SOLAR CYCLE. It is named after Gustav SPORER, who first studied it in detail. It is also depicted graphically in the BUTTERFLY DIAGRAM. A new cycle starts with sunspots at high latitudes of around 40 north and south of the equator. As the cycle progresses, the spots appear at decreasing latitudes until some occur within 5 north or south of the equator at solar minimum. Before minimum has been reached, high-latitude spots of the new cycle can appear, causing an overlap of the cycles.

spring equinox Alternative name for vernal equinox

spring tides High-level high tides that occur when the Sun and Moon lie in the same or opposite directions (that is at new moon or full moon).

s process Method of creating heavy stable nuclei within the interiors of stars by successive capture of neutrons. In the s process, neutrons are added slowly to nuclei, which have time to decay before the next neutron is added. It is the process that occurs in the interiors of S-type red giants. See also nucleosynthesis; pprocess; rprocess

Sputnik (Fellow Traveller) Name given to the first ten satellites launched by the Soviet Union. Sputnik 1, launched on 1957 October 4 was the first artificial satellite. It weighed 83.6 kg and orbited the Earth in 96 minutes. Sputnik 2 (launched 1957 November 3) carried the dog Laika, the first living creature in space. Sputnik 3 (1958 May 15) was a highly successful scientific satellite. Sputniks 4, 5 and 6 tested the vostok re-entry capsules, with Sputnik 5 successfully returning the dogs Belka and Strelka from space. Sputniks 7 and 8 were failed Venus missions. Sputniks 9 and 10 were also Vostok test flights with dogs on board.

Square Kilometre Array (SKA) International project to design and ultimately build a centimetre-wavelength radio telescope with an effective collecting area of 1 sq km (0.4 sq mi). It will synthesize an aperture with a diameter of approximately 1000 km (625 mi). Such a telescope would probe the gaseous content of the very early Universe, complementing planned facilities in other wavebands such as the next-generation space telescope. A consortium representing the United States, Europe, Australia, Canada and Asia is planning to select a site having minimal interference from radio communications by 2005.

Square of Pegasus A large asterism, over 15 across, formed by the stars a, p and y Pegasi, and a Andromedae; this latter star was in former times regarded as common to Andromeda and Pegasus, when it had the alternative designation 8 Pegasi. The Square is high in the sky on northern autumn evenings. Despite its considerable size, there are remarkably few naked-eye stars within it.

SS Cygni star See ugeminorum star

SS433 Very unusual binary star system that lies at the centre of the supernova remnant W50 in aquila. SS433 was discovered in 1976 by the Ariel 5 X-ray satellite. It has strong emission lines from hydrogen and helium, and it was classified as object 433 in a catalogue of emission-line stars published (1977) by Bruce Stephenson and Nicholas Sanduleak (1933-90) of Case Western Reserve University. The emission lines have broad and narrow components; the broad lines vary slightly with a period of 13.1 days, while the narrow lines vary significantly in wavelength over a period of 164 days.

SS433 The unusual binary pair SS433 is thought to consist of a neutron star and a red giant. Material from the red giant is being accreted by the neutron star and some of it is being expelled in twin jets. The jets are observed to precess in a 164- day period probably because of gravitational interactions between the two stars.

It is generally accepted that SS433 is a binary system composed of a hot, massive star (of 10 to 20 solar masses) and a neutron star (about 1.5 to 3 solar masses), which orbit each other every 13.1 days. The neutron star is probably the stellar remnant from the supernova that produced W50. A stellar wind from the hot star produces the 'stationary' emission features. Matter is transferred from the massive star to the neutron star via an accretion disk. Not all the material reaches the surface of the neutron star, however, and some is ejected at high speed, probably by radiation pressure, via two finely collimat ed jets. The jets sweep around the sky every 164 days, producing the peculiar drifting spectral features that appear to make SS433 unique in astronomy.

S star Member of a class of giant stars with the same temperature range as mstars, but with absorption bands of zirconium oxide (ZrO) rather than titanium oxide (TiO). The overabundance of zirconium, as well as carbon and many heavy elements, is a result of convective mixing that brings by-products of nuclear reactions (principally those of hydrogen fusion, helium fusion and slow neutron capture) up to the stellar surfaces.

S stars are the evolutionary intermediaries between M giants and carbon stars. As M giants within a particular (but uncertain) mass range dredge carbon from below, and in the process become carbon stars, they pass through the S star state. While the atmospheres of M giants have more oxygen than carbon and carbon stars more carbon than oxygen, S stars have approximately equal amounts of the two. The carbon readily combines with the oxygen. What little free oxygen is left combines more readily with the enhanced zirconium (a slow neutron-capture product) to make zirconium oxide rather than titanium oxide. M stars with weak ZrO bands are assigned the intermediate spectral type MS, while SC stars are intermediate between classic S stars and carbon stars.

S stars often show hydrogen emission lines in their spectra and commonly pulsate as mira stars, the brightest being chi cygni, which can peak at third magnitude.

Stadius Lunar ghost crater (11N 14W), 70 km (44 mi) in diameter. It originally appeared as a complex crater. After its formation, however, lunar basalts flooded the region to a depth greater than the height of Stadius' rim. Only a few of the highest rim peaks are visible above the lava. Numerous secondary craters from the ejecta of copernicus cover the region.

standard epoch Particular date and time chosen as a reference point against which to measure astronomical data in order to remove the effects of precession, proper motion and gravitational perturbation. The standard epoch currently in use is called 2000.0 and all celestial coordinates published in star catalogues are quoted as being correct on 2000 January 1. It is customary to change the standard epoch every 50 years.

standard model (1) Mathematical description of the solar interior, based on a spherical Sun; it specifies the current variation with radius of density, temperature, luminosity and pressure. The Sun is presumed, for this purpose, to have condensed from a cloud of primordial gas composed primarily of hydrogen and helium. Theoretical models use appropriate equations describing nuclear energy generation by hydrogen burning (see PROTON-PROTON REACTION) in the central core of the Sun, hydrostatic equilibrium that balances the outward force of gas pressure and the inward force of gravity (that is, the Sun is in a steady state and perfect gas laws can be used), energy transport by radiative diffusion and convection, and an opacity determined from atomic physical calculations. Computers are used to integrate these models over the 4.6-billion year lifetime of the Sun. A standard model is arrived at that best describes the Sun's observed luminosity, size and mass. Theoretical fluxes of solar neutrinos are also calculated from the model. The standard model is consistent with helioseismology measurements of the Sun's internal temperatures.

standard model (2) Reigning model of elementary particle physics; it successfully combines the weak and strong forces into a consistent quantized theory. The standard model utilizes quarks to build protons, neutrons and other subatomic particles.

standard time Legal time kept in any community. This is likely to be based on its local time zone, but it may differ slightly for reasons of convenience. Many countries also observe daylight saving time, advancing their clocks by one hour during the summer months. The standard time in the UK is universal time (UT), popularly known as greenwich mean time (GMT). In summer, the standard time is UT +1h (British Summer Time).

star Gaseous body that emits radiation generated within itself by nuclear fusion. Mainly composed of hydrogen and helium, a star is a fine balancing act between the force produced within itself and the gravitational force. Many stars belong to binary or multiple systems.

A star begins its life when the temperature and pressure within a collapsing cloud of interstellar material cause nuclear reactions to start. Below about 0.08 solar mass, conditions do not reach the point where nuclear fusion can occur (see brown dwarf). Above about 150 solar masses, a star becomes unstable. The mass of a star has a great influence on its evolution (see stellar mass). A star's evolution is dictated throughout its life by the balancing act between gravitation and the energy produced by the nuclear reactions within the star. As a star evolves, its chemical composition changes. Stars belonging to close binary systems often have their evolution altered by interactions with their companion.

The composition of stars is initially mainly hydrogen and helium, with a proportion of heavier elements, depending on the material from which it is forming. Stars produce energy within their core by nuclear fusion (see stellar interiors). The energy released has to force its way out through opaque gas, and at every point within the star the gravitational pull of the parts within exactly balances the outward thrust and the star is in hydrostatic equilibrium (see stellar atmosphere).

The internal structure of a star cannot be observed directly. The equations of stellar structure, on which computer models of the conditions within stars depend, are based on known laws of physics. Physical data such as mass, luminosity, radius, chemical composition and effective temperature can be measured for some stars, and the observed stellar conditions are satisfactorily predicted by the equations of stellar structure.

Stars can be broadly classified by their mass and stage of evolution and also by their effective temperature. A star's spectral type is an indication of its temperature, but it also gives information about its chemical composition. Stars are also classified by the percentage of heavy elements in their composition (see populations, stellar). The percentage of heavy elements affects a star's evolution.

Many stars are observed to fluctuate in brightness (see variable stars). These fluctuations can be due to a star belonging to an eclipsing binary system or a close binary system, or they may be a result of instabilities caused by its evolutionary phase (see ttauri stars, mira stars).

starburst galaxy galaxy that is undergoing a strong, temporary increase in its rate of star formation. Such activity may span the whole galaxy, but is often confined to the nucleus. Starbursts may appear as ultraviolet-bright galaxies or may be identified by strong optical emission lines or excess radio emission, but the largest number to date have been found from far-infrared emission (as by the Infrared Astronomical Satellite). This is because many starbursts are quite dusty, and when the dust absorbs the ultraviolet and visible light from the massive stars, it radiates in the far-infrared (typically at 0.03-0.1 mm). Starbursts can be triggered by tidal interactions and mergers between galaxies, and the energy output of the massive stars and supernovae can be strong enough to drive a wind sweeping gas completely out of the galaxy (and terminating the starburst). Starburst galaxies are useful guides for the conditions expected in genuinely young galaxies in the early Universe, when star formation was more intense than is usually found in present-day galaxies.

star clouds Parts of the MILKY WAY, especially in Sagittarius and its surrounding constellations, where the light from millions of faint stars combines to give the appearance of irregular faintly glowing clouds. The star distribution is actually fairly uniform and the irregularities result from the stars' obscuration in some regions by DARK NEBULAE such as the COALSACK and the PIPE NEBULA.


star diagonal Flat mirror or prism enabling an object to be viewed at right angles to the direction in which a telescope is pointing. The image is reversed left-to-right, but is the correct way up. Star diagonals are popular with users of REFRACTING TELESCOPES or SCHMIDT-CASSEGRAIN TELESCOPES, in which viewing of objects at high altitudes might otherwise prove uncomfortable.

Stardust NASA Discovery programme spacecraft, launched in 1999 to rendezvous with the comet Wild 2 in 2004 January and to return to Earth in 2006 January with samples of dust from the comet's coma. Stardust, which will also return with samples of INTERPLANETARY DUST, is equipped with two collectors, on either side of a mast, consisting of ultra-low-density silica aerogel material. One hundred particles of interplanetary dust between 0.1 and 1 um and 1000 particles of comet dust larger than 15 um will be returned to Earth. The collector will be deposited into a capsule, which will enter the Earth's atmosphere as Stardust makes a return to its home planet, to be recovered in Utah. The samples will be analysed at the Planetary Material Curatorial Facility at the NASA Johnson Space Center in Houston, Texas.

Star of Bethlehem Celestial portent of the birth of Jesus Christ, mentioned in the biblical book of Matthew. The precise nature of the Star of Bethlehem has been of great interest to Christians and astronomers alike. Many books and articles have been written on the subject, some authors regarding it as a mythical or miraculous event and others supposing that it was a real astronomical phenomenon that ordinary mortals could have seen.

The first problem encountered when trying to relate the star to a real astronomical event is that the precise year of the nativity is uncertain, though it is believed to have been between 7 BC and 1 BC. A census of allegiance to Augustus Caesar, which may have been the census mentioned in the Bible, took place in 3 BC. Numerous astronomical events such as planetary conjunctions and conjunctions of planets with stars occurred during those years; several comets and a nova were also recorded. Although there is no general agreement on the precise events that guided the Magi on their path to Bethlehem, one theory is that successive conjunctions of Jupiter and Venus in 3 BC August and 2 BC June were the 'stars' that led them westwards. Around 2 BC December 25, the planet Jupiter seemed to stand still among the stars because of its change from (apparent) retrograde to prograde motion. Seen from Jerusalem, it would have appeared to be above Bethlehem.

starburst galaxy The bright pink areas in this irregular galaxy are areas of starbirth and the blue areas are newly emerged young stars. Many starburst galaxies are triggered by gravitational interactions between galaxies.

star streaming Grouping of the motions of individual stars, relative to their neighbours, around two opposite preferred directions in space. The effect was discovered by Jacobus KAPTEYN,who made statistical studies showing that, in general, the peculiar motions of stars (that is relative to the average motion of neighbouring stars) appear to be in one of two directions in space, rather than moving in random directions. The phenomenon is caused by the rotation of the Galaxy and many later studies of distances and spatial arrangements of the stars in the Milky Way arose from this discovery.

star trailing Extension of the images of stars into arcs caused by the rotation of the Earth, and hence the movement of stars across the sky, on a photograph taken with a stationary camera or telescope. Images of stars near either celestial pole form tight arcs, while at the equator star trails are straight lines. Lengthening the exposure time produces longer trails. Trailing is avoided by driving the camera to track the stars (see TELESCOPE DRIVE).

star trailing Because of the Earth's rotation, the light from stars will form trails on unguided astronomical images. This image is of star trails around the southern celestial pole.

stationary point Point at which the motion of a SUPERIOR PLANET, as observed from Earth, appears to change direction relative to the background stars due to the Earth catching it up in its orbit and overtaking it. See also RETROGRADE MOTION

steady-state theory Theory, originated by Hermann BONDI, Thomas GOLD, Fred HOYLE, Jayant NARLIKAR and others, designed to satisfy the PERFECT COSMOLOGICAL PRINCIPLE, which states that the local properties of the Universe - when averaged over some suitable distance -are the same from whatever point in space and time they are determined. This principle is consistent with the model derived by Willem DE SITTER from Einstein's field equations. The steady-state universe is expanding, but a constant density of matter is maintained by invoking CONTINUOUS CREATION of matter at all places and all times by a so-called C-field. It avoids the problem, associated with the BIG BANG THEORY, of an initial SINGULARITY - a fixed starting point and, apparently, some instrument of creation.

For nearly two decades, steady state and Big Bang were rival theories. There were two main reasons for the demise of the steady-state theory: the discovery of the COSMIC MICROWAVE BACKGROUND in 1965, and the realization that galaxies have evolved over time, neither of which it could readily explain. Its lasting importance was the stimulus it provided to astrophysics and cosmology, leading, for example, to the first detailed work on NUCLEOSYNTHESIS. Ironically, the steady-state universe strongly resembles some versions of INFLATION, the model proposed to account for the initial rapid expansion that followed the Big Bang.

Stebbins, Joel (1878-1966) American astronomer who pioneered photoelectric photometry. Stebbins spent most of his career (1918-48) at the University of Wisconsin's Washburn Observatory. In 1910 he attached a primitive photocell to a telescope and discovered the secondary minimum in the light curve of the eclipsing binary star Algol. Stebbins devised methods for reducing the huge amounts of data generated by his photoelectric instruments, which he used to study 'regular' stars, various types of variable stars, the solar corona, globular clusters and galaxies. With Albert Edward Whitford (1905-?1983) he developed increasingly sensitive photoelectric cells that eventually supplanted photography as a means of measuring star magnitudes. The superiority of the photoelectric method (photographic emulsions are usually overly sensitive to either blue or red light) allowed Stebbins and his colleagues to quantify the 'reddening' effects of interstellar matter and to adjust stellar magnitudes accordingly.

Stefan-Boltzmann constant (symbol) Constant relating the energy emitted by a BLACK BODY to its temperature (see STEFAN-BOLTZMANN LAW). It has the value 5.6697 X 10~8 W mT2 K~4.

Stefan-Boltzmann law Law giving the total radiant energy (flux) emitted by a BLACK BODY (see also RADIATION). It was formulated by the Austrian physicist Joseph Stefan (1853-93) and by Ludwig BOLTZMANN.

stellar association See ASSOCIATION, STELLAR

stellar atmosphere Atmosphere of a STAR; it consists of the layers from which radiation is directly observed. Stellar atmospheres comprise plasmas composed of many types of particles, including atoms, ions, free electrons, molecules and sometimes dust grains. The inner boundary of a stellar atmosphere is with the stellar interior, about which no direct information is available, while the outer boundary is taken to be where it merges with the interstellar medium.

Although the atmosphere of a star can extend a long way out into space, the majority of radiation emitted from it comes from a relatively thin layer called the PHOTOSPHERE. Generally, a stellar photosphere is about one thousandth of the star's radius, although hotter stars have relatively thicker photospheres. The photosphere is the innermost part of a stellar atmosphere.

The outer layer of a stellar atmosphere is the STELLAR WIND, a region where the outflow velocities are comparable to or larger than the local speed of sound. In a few stars there is a further, more remote layer: some very young stars may still be enveloped by the material from which they formed.

The structure of the atmosphere above the photosphere differs markedly between cool and hot stars. Cool stars, defined as stars with an EFFECTIVE TEMPERATURE of less than about 7500 K, have been observed to have a CHROMOSPHERE and CORONA similar to that of the Sun. The atmospheres of cool stars are very complex and are characterized by increasing temperature with altitude in their outer layers. The mechanism causing this temperature increase is not fully understood, but heat conduction becomes an important process in regions of low density such as in stellar coronae. Energy may also be transported by MAGNETOHYDRODYNAMIC or acoustic waves, which are responsible for heating the corona non-radiatively. These waves are generated by strong, geometrically complex magnetic fields, which in turn are created by a dynamo effect in the convection zone beneath the photosphere. Coronal heating is believed to be the source of the X-rays detected from these stars.

The high coronal temperature in cool MAIN-SEQUENCE stars results in a large gas pressure, which induces a weak pressure-driven stellar wind. Fast, but very tenuous, this wind is so optically thin throughout that it cannot be detected spectroscopically; it is often not included in the atmosphere of such stars. In evolved cool stars, however, a much denser wind is observed. These stars have low surface gravity so the mass-loss rate of a coronal wind is larger. In addition, other mechanisms, such as RADIATION PRESSURE on dust grains formed in the outer layers or acceleration in pulsating layers, become efficient drivers of the stellar wind.

Strong, fast winds typify the atmospheres of early-type main-sequence stars (O and early B-type) and O, B and A-type SUPERGIANTS. These fast, supersonic winds are driven by radiation pressure generated by absorption of photons. The X-ray emission detected in O and early B-type stars is attributed to shock-heated material in the wind and not to the corona. The winds of red supergiants are slow.

stellar diameters Stars range in size from having diameters of a few tens of kilometres (NEUTRON STARS) to several hundreds of millions of kilometres (SUPERGIANTS). Generally the sizes of stars are expressed in terms of the solar radius (Ro).

The diameter or radius of a star, along with its mass, LUMINOSITY and EFFECTIVE TEMPERATURE, helps to determine its position on the HERTZSPRUNG-RUSSELL DIAGRAM. When a star's radius is very large or very small, its effect on the luminosity can dominate over the effect due to temperature. A star's diameter can be measured directly if its angular diameter and distance from Earth can be measured. Measuring the angular diameters of stars directly, however, is difficult because of the large distances involved. The diameters of some stars can been measured when they are occulted by the Moon.

Optical INTERFEROMETRY can also be used, and as the resolution of interferometers improves, the diameters of smaller stars can be measured. Another technique used is SPECKLE INTERFEROMETRY. The diameters of some stars can be obtained indirectly, for example if they are members of eclipsing binary systems. If the effective temperature (Teff) can be measured from its spectrum, and a value of its luminosity (L) determined from its apparent brightness and distance, then a value of the star's radius (R) can be inferred from L = 4TrR2Some stellar diameters have been obtained when a star is involved in microlensing events.

stellar evolution Development of a star over its lifetime. The theory of stellar evolution revolves around mathematical models of stellar interiors and current laws of physics. It gives a fairly detailed account of the evolution of any particular star, and it can be checked by observation of stars at each of the predicted phases. The evolutionary stages of stars are represented on the hertzsprung-russell diagram (HR diagram). Timescales for evolution lie between hundreds of thousands, and thousands of millions of years; hence the evolution of a particular star cannot be directly observed, although very occasionally changes in a star over the course of a human lifetime can be observed as it passes through a rapid phase, as with fg sagittae. The evolution of a star in a close binary system can be affected when mass transfer occurs from its companion, because changing the star's mass alters conditions in its interior.

Stars form from a dense interstellar cloud known as a giant molecular cloud. A large cloud is able to collapse under its own gravity and will subsequently fragment into several hundred smaller clouds, each of which contracts further and heats up to become a protostar. Protostars are very red and can be observed at infrared wavelengths; they are found in regions of our Galaxy where there is an abundance of gas and dust. The progress of a pre-main-sequence star on the HR diagram is represented by the hayashi and henyey tracks.

A protostar will continue to collapse under gravity until its centre is hot enough (about 15 million degrees K) for nuclear fusion reactions to commence. As it collapses, its outer layers fall in to take up a normal star-like form and it becomes bluer in colour. ttauri stars are low-mass stars in this final phase.

Once nuclear fusion has commenced in the core of a star, the star adopts a stable structure, with its gravity being balanced by the heat from its centre (the photons created in the nuclear fusion reactions diffuse outwards, exerting radiation pressure). The controlled nuclear fusion reaction is the fusion of hydrogen atom nuclei (protons) to form helium nuclei. This period is known as the main-sequence phase of a star's life; it is the longest phase in the life of a star, and consequently at any particular moment most stars are in the main-sequence period of their existence. The Sun has a main-sequence lifetime of some ten thousand million years, of which only a half has expired. Less massive stars are redder and fainter than the Sun and stay on the main sequence longer; more massive stars are bluer and brighter and have a shorter lifetime. As a star ages, the composition of its core changes from mainly hydrogen to mainly helium, causing it to become very slightly brighter and bluer; on the main sequence its position moves slightly upwards and to the right of its zero-age position (see zero-age main sequence).

The internal structure of a main-sequence star depends on its mass. Massive stars have a convective core and a radiative mantle (see convection and radiative transfer). Low-mass stars, including the Sun, have a radiative core and a convective mantle. The stirring that occurs in the core of a massive star causes it to have a uniform composition within that core, which affects the details of the next phase of evolution.

Eventually, the hydrogen supply in the core of the star runs out. With the central energy source removed, the core will collapse under gravity, and heat itself up further, until hydrogen fusion is able to take place in a spherical shell surrounding the core (hydrogen shell burning). As this change occurs, the outer layers of the star expand considerably, and the star becomes a red giant, or in the case of the most massive stars, a red supergiant. The speed of evolution to the red giant phase depends on the mass. A low-mass star changes gradually to become a red giant as the hydrogen exhaustion spreads outward from its centre. A high-mass star evolves quickly to become a red giant because its whole convective core runs out of hydrogen at the same time. Giant stars are sufficiently cool to have a spectral type of K or M.

While it is a red giant, the star's central temperature will reach 100 million K, and the fusion of helium to carbon will commence in the core. In the case of low-mass stars, the onset of helium fusion is sudden (the helium flash), and the star reduces its radius to become a bluer but fainter horizontal branch star as observed in globular clusters; it may subsequently return to being a red giant. In high-mass stars, the onset of helium fusion occurs more gradually, and the star remains a red giant.

The evolution of a star beyond the helium fusion phase is more difficult to calculate with certainty. When the helium fuel in the core has been converted into carbon, the core of the star will again collapse and heat up. In the case of a low-mass star, the central temperature will not rise high enough to initiate carbon fusion, and the outer layers of the star will contract, then gradually cool down, so that the star becomes a white dwarf. It is likely that before a white dwarf cools appreciably, it throws off its outer layers in the form of a planetary nebula. White dwarf stars are plentiful, but they are difficult to observe because of their faintness. It is believed that white dwarfs cool forever, becoming black dwarfs - mere cinders of stars.

In the case of a high-mass star, the contraction of the carbon core will lead to further episodes of nuclear fusion, during which the star will remain very bright. Many of these stars fluctuate in brightness as mira stars. At this stage the internal chemistry can change, or gas of a different chemical mix deep within can rise to the surface by convection. These stars may then appear as carbon stars (C stars). Most of these objects are extremely red and thus rather faint.

Once a star reaches the point where no more nuclear fusion can occur, it will collapse to become a white dwarf, unless the core exceeds the chandrasekhar limit of 1.4 solar masses, in which case further contraction occurs, producing a neutron star. When the core is composed of elements close to iron in the periodic table, no further fusion is possible. At this point, it is most likely that the core will collapse explosively, and the star will throw off its outer layers in a supernova explosion, becoming for a few weeks bright enough to outshine the galaxy in which it is situated. The dead core of the massive star will remain as a neutron star or, if the core is massive enough, it will collapse further to become a stellar black hole.

stellar interferometer optical interferometer used to measure the angular diameter of stars or the angular separation of the components of double stars. An interferometer operates by splitting and re-combining a beam of light using mirrors in order to examine the interference fringes produced by the re-combined beam. Because a star image is a disk, not a point source, it is possible to adjust the separation of the two mirrors, d, until the fringes disappear, bright fringes from one side of the stellar disk coinciding with dark fringes from the other. At this point, d = 1.22 X/0, where X is the wavelength of light and 0 the angle subtended by the stellar disk.

stellar interior Internal structure of a star. Knowledge of stellar interiors is gained by observing external properties and formulating a stellar model using known laws of physics to explain the properties observed. The equations that are used to construct models of stellar interiors are known as the equations of stellar structure.

The determination of the density, temperature, composition and luminosity at any point within a star requires the solutions to the conservation laws of energy, mass and momentum at that point. Different conditions exist in stars of different mass and at different stages of evolution, the main differences being the form of energy transport and the composition.

stellar wind The Bubble Nebula is caused by the strong stellar wind from the star at the bottom of the image. When young stars undergo a phase called the T-Tauri phase their extremely strong stellar winds blow away much of the nebula in which they were formed.

stellar wind The Bubble Nebula is caused by the strong stellar wind from the star at the bottom of the image. When young stars undergo a phase called the TTauri phase their extremely strong stellar winds blow away much of the nebula in which they were formed.

CONVECTION and RADIATIVE TRANSFER are the principal means of energy transport in stellar interiors. For MAIN-SEQUENCE stars below about 1.5 solar masses, the temperature gradient within the core is too low for convection to occur. Energy transfer within these stars is primarily by radiative transfer, with convection occurring in shallow regions near the surface known as convective shells. More massive main-sequence stars have convective cores.

Most stars at a stable point in their evolution are assumed to be in hydrostatic equilibrium, whereby the forces at any point within a star balance and the internal pressures counteract the gravitational pull. For main-sequence stars the internal pressure is a combination of gas and radiation pressure. For evolved stars like WHITE DWARFS and NEUTRON STARS the internal pressure is produced by the DEGENERATE MATTER in their cores.

stellar mass The mass of a STAR is a very important factor in its evolution and determines its basic structure and lifetime. Stellar masses are generally expressed in terms of the Sun's mass (Mo).

Stellar masses range from 0.08 solar mass to about 150 solar masses. Below 0.08 solar mass, temperatures at the core do not become sufficiently high for nuclear FUSION to begin. Stellar-like objects with mass less than 0.08 solar mass are called BROWN DWARFS.

Upper theoretical limitations suggest that stars with masses above about 150 solar masses are unstable because of the effect of RADIATION PRESSURE. The most massive MAIN-SEQUENCE stars observed are within the R136 cluster in the 30 Doradus Nebula of the LARGE MAGELLANIC CLOUD. These stars are around 155 solar masses; they show signs of high mass loss similar to that observed in WOLF-RAYET STARS. Stellar masses can be determined directly from BINARY SYSTEMS. They can also be determined indirectly, using stellar models, if a star's LUMINOSITY, EFFECTIVE TEMPERATURE and distance are known.

According to the VOGT-RUSSELL THEOREM, a star's properties and evolution are determined by its initial mass and chemical composition. Since the composition of stars varies relatively little, it is the initial mass that dictates basic structure and evolution. Higher-mass stars have shorter lifespans because the higher temperatures at their cores enable them to burn their fuel quicker and thus evolve faster. See also INITIAL MASS FUNCTION; MASS-LUMINOSITY RELATION; STELLAR EVOLUTION

stellar nomenclature System for assigning designations to stars. The brightest stars have proper names originally given to them by Greek, Roman or Arab astronomers, for example Sirius, Capella and Aldebaran. In addition to any proper name, stars down to around 5th magnitude within each constellation are identified by a letter or number followed by the name of the constellation in the genitive (possessive) form (see constellations table, pages 94-95). The brightest stars are indicated by Greek letters, as in a (Alpha) Lyrae or e (Epsilon) Eridani. These letters are known as Bayer letters because they were assigned by Johann BAYER in his star atlas Uranometria of 1603, usually in order of brightness. Stars that lack Bayer letters are prefixed by a number, as in 61 Cygni. These are known as Flamsteed numbers because they were assigned to stars charted in John FLAMSTEED's Historia coelestis britannica, in order of right ascension (RA). Capital and lower-case roman letters are also used, particularly for stars in the southern hemisphere. VARIABLE STARS have their own system of nomenclature.

Fainter stars (and deep-sky objects) are identified according to their listing in any of a number of other catalogues, by the catalogue's abbreviation followed by its designation in the catalogue. The designation scheme can be a single sequence of numbers (often in order of RA), as in HD 74156 or NGC 6543, or the numbering can be based on zones, as in BD +31 643. Modern designations tend to use truncated coordinates: for example, PSR 0531 +21 is the designation of a pulsar having its 1950.0 coordinates RA 05h 31m, dec. +21; while in PSR J0534 + 2200, the 'J' indicates truncated 2000.0 coordinates. A list of the various designation letters is in a Dictionary of Nomenclature of Celestial Objects maintained under the auspices of the International Astronomical Union (IAU).

There are commercial organizations from which star names can be bought. Although this is not illegal, and individuals might like to name a star after a loved one, such star names have absolutely no status and are not recognized by the IAU.

stellar wind Stream of charged particles, mostly PROTONS and ELECTRONS, that is continually emitted from the surface of a STAR, including the Sun (see SOLAR WIND).

Young stars evolving towards the MAIN SEQUENCE have powerful stellar winds, sometimes up to a thousand times stronger than the SOLAR WIND. These winds crash into the surrounding gas clouds and ionize them, producing expanding shock waves in the interstellar medium. While on the main sequence, high-mass stars lose a significant fraction of their mass to the interstellar medium. A star of around 120 solar masses at birth could lose as much as 50 solar masses of material during its main-sequence lifetime. A star of around 60 solar masses at birth could lose around 12 solar masses. Old stars evolving off the main sequence to become RED GIANTS also have strong stellar winds.

Stephano One of the several small outer satellites of URANUS; it was discovered in 1999 by J.J. Kavelaars and others. Stephano is about 20 km (12 mi) in size. It takes 674 days to circuit the planet, at an average distance of 7.98 million km (4.96 million mi). It has a RETROGRADE orbit (inclination near 144) with a moderate eccentricity (0.228). See also CALIBAN

Stephan's Quintet (NGC 7317, NGC 7318 A and B, NGC 7319, NGC 7320) Compact group of galaxies in the constellation Pegasus (RA 22h 36m.0 dec. +3358'); the group was discovered in 1877 by the French astronomer Edouard Stephan (1837-1923). All the constituent galaxies are about 13th magnitude and appear within an area of sky only 3'.5 across. Four of the galaxies lie at a distance of 270 million l.y., and two - NGC 7318 A and B - are interacting with each other. The fifth galaxy, NGC 7320, is very much closer; it is not a true member of the group but simply a relatively nearby galaxy in the line of sight.

Stephans Quintet The gravitational interactions between the member galaxies of Stephans Quintet have spawned massive star formation. The stars in the bright regions are between 2 million and 1 billion years old.

step method See argelander step method; pogson step method stereo comparator Type of comparator that enables two photographs of the same area of sky, taken at different times, to be viewed simultaneously to reveal objects that have changed position or brightness. The instrument has two optical paths so that both photographs can be viewed together with binocular vision. Discordant images then appear to stand out from the plane of the picture.

Steward Observatory Major US astronomical institution operated by University of Arizona, Tucson. It incorporates the University's Department of Astronomy. During the 1980s, the observatory pioneered novel techniques for manufacturing large telescope mirrors, and its Mirror Laboratory has now produced optics for some of the world's biggest telescopes. Today, the instruments operated with the participation of the Steward observatory include the 6.5-m (256-in.) telescope of the mmt observatory, the two 6.5-m magellan telescopes and the 10-m (33-ft) heinrich hertz telescope. The twin 8.4-m (331-in.) instruments that comprise the large binocular telescope are due to become operational in 2004. The Steward Observatory also runs the 2.3-m (90-in.) Bok Telescope at kitt peak national observatory.

Stingray Nebula (Hen-1357) Youngest-known planetary nebula, with estimated age 200 years. Located in the southern constellation of Ara (RA 17h 16m.4 dec. 5929'), the Stingray is 18,000 l.y. away and has an apparent diameter of less than one arcsecond. Its actual diameter is about 130 times that of the Solar System.

Stjerneborg Second observatory constructed by Tycho brahe in about1584; the name means 'Castle of the Stars'. Its observing chambers were mainly below ground level to provide stability and were covered by rotating roofs equipped with openable shutters. See also uraniborg

Stockholm Observatory Observatory founded by the Royal Swedish Academy of Sciences in Stockholm in 1748. In 1931 it moved to Saltsjobaden, 20 km (12 mi) south-east of Stockholm. New instruments were added, including a 1.0-m (39-in.) reflector. In 2001, with its scientists using overseas facilities such as the european southern observatory and the nordic optical telescope, the observatory moved again to become part of the Stockholm Centre for Physics, Astronomy and Biotechnology near the city centre. The telescopes remain at Salt-sjobaden, and are used for public outreach.

Stofler Lunar crater (41S 6E), 145km(90 mi) in diameter. It is an ancient crater, as can be seen by its walls, which are deeply incised from later impacts, and its rim, which has been rounded by impact erosion. It has central peaks. Stofler sits astride several even older crater units. Under high Sun illumination, bright rays from tycho are visible on its floor.

stony-iron meteorite meteorite with approximately equal proportions of silicate minerals and iron-nickel metal. Stony-irons are subdivided into two big groups, mesosiderites and pallasites, which have very different origins and histories.

stony meteorites meteorites that are made from the same elements as terrestrial rocks, dominantly silicon, oxygen, iron, magnesium, calcium and aluminium.

Like terrestrial rocks, stony meteorites are assemblages of minerals, such as pyroxene, olivine and plagioclase, but unlike terrestrial rocks they also contain metal and sulphides. Stony meteorites are subdivided into two big groups, chondrites and achondrites. The former have not melted since aggregation of their original components, and thus they are unfractionated with respect to the Sun. The latter formed from melts on their various parent bodies.

Strasbourg Astronomical Observatory A laboratory of the Universite Louis Pasteur at Strasbourg, France. Besides teaching and research activities, it hosts the Strasbourg Astronomical Data Centre (centre de donnees astronomiques de strasbourg).

stratosphere Layer in the Earth's atmosphere extending above the tropopause (the upper limit of the troposphere) as far as the mesosphere. There is an atmospheric temperature minimum at the tropopause, above which the temperature at first remains steady with increasing height. Above 20 km (12 mi), it begins to increase, reaching a a maximum of about 273 K at an altitude of 50 km (31 mi), which marks the stratopause, the top of the stratosphere. The heating is primarily through the absorption of solar ultraviolet radiation by ozone. The ozone layer (sometimes called the ozonosphere) lies at an altitude of between about 15 and 50 km (9 and 31 mi) and is essentially identical with the stratosphere. The highest concentration of ozone occurs at a height of about 20-25 km (12-16 mi).

Stratospheric Observatory for Infrared Astronomy (SOFIA) Boeing 747 aircraft modified to accommodate a 2.5-m (100-in.) reflecting telescope - the largest airborne telescope in the world - for infrared astronomy. The aircraft is being modified by a US-German team and will fly in 2002 from its base at NASA's Ames Research Center in California.

strewnfield Area over which material from a specific impact event is scattered. It is most often applied to tektites and to meteorite shower falls. Strewnfields are usually elliptical in shape and indicate the direction from which the meteorite came. Following atmospheric disruption, larger meteorites are slowed down by frictional heating less than smaller ones, and so are carried further distances; smaller meteorites are deposited at the incoming direction of the strewnfield, larger ones at the far end.

Stromgren, Bengt George Daniel (1908-87) Danish astrophysicist, born in Sweden, who explained the nature of ionized hydrogen (HII) gas clouds that surround hot stars. He succeeded his father, Svante Elis Stromgren (1870-1947), as director of Copenhagen Observatory (1940), and later worked at Yerkes and McDonald Observatories and Princeton University's Institute for Advanced Study. Stromgren became particularly interested in the astrophysics of the Orion Nebula and Milky Way's other HII regions. He developed theoretical models of what were later called STROMGREN SPHERES, which he used to estimate the size and density of a hydrogen cloud by measuring the luminosity of its exciting star. These models allowed Stromgren and others, especially William W. MORGAN, to map the spiral structure of the Milky Way.

Stromgren sphere Volume of ionized gas around a hot star (see also HII REGION). The region will only be spherical when the interstellar gas in which it forms is of uniform density. A Stromgren sphere has a definite size that is reached when the number of ultraviolet photons emitted by the star exactly balances the number of ionizations occurring in the gas. The radii of Stromgren spheres were first calculated by Bengt STROMGREN; they range from 1 to 100 l.y. for O5 stars (within high and low density HI REGIONS respectively), and from 0.1 to 15 l.y. for B0 stars.

Struve family German-Russian family that produced astronomers of note across four generations. As a youth, (Friedrich Georg) Wilhelm von STRUVE (1793-1864) left Germany for Latvia, later moving to Russia to establish PULKOVO OBSERVATORY. One of his sons was Otto (Wilhelm) Struve (1819-1905), known to historians as Otto I Struve. Like his father, whom he succeeded as director at Pulkovo in 1862, Otto studied double stars, discovering around 500 of them. He determined an accurate value for the rate of precession, taking into account the motion of the Sun with respect to nearby stars, which he measured. (Karl) Hermann Struve (1854-1920), who became director of Berlin Observatory (1904), and (Gustav Wilhelm) Ludwig Struve (1858-1920), director of Kharkhov Observatory from 1894, were sons of Otto, while Georg Struve (1886-1933) was a son of Hermann; they all studied the Solar System. Otto Ludwig STRUVE (1897-1963), a son of Ludwig, emigrated to the USA in 1921 and became a naturalized American. He is known to historians as Otto II Struve.

Struve, (Friedrich Georg) Wilhelm von (1793-1864) German astronomer, the first major observer and cataloguer of double stars, and one of the first to measure a stellar parallax. He founded the most distinguished of astronomical dynasties - the STRUVE FAMILY. In 1808 he left his homeland to escape conscription into the occupying Napoleonic army, settling in Dorpat (modern Tartu, in Estonia), becoming director of Dorpat Observatory in 1817. From 1835 he helped to establish PULKOVO OBSERVATORY, which he directed until his retirement in 1862.

In 1824 Struve commenced an extensive survey of double and multiple stars with the 9i-inch (240-mm) f/18 Dorpat Refractor, made by Joseph FRAUNHOFER and equipped with a clock drive and a filar micrometer - the largest and best telescope of its day. His measurements of position angles and separations, comparable with modern ones in their precision, were published in a catalogue (1827) and two later supplements, which between them listed 3112 doubles, of which 2343 were discovered by Struve himself. The designations from these catalogues, prefixed by 2 (the Greek capital S) are still in use. The collected results were published in Stellarum duplicium et multiplicium Mensurae Micrometricae per magnum Fraun-hoferi tubum in Specula Dorpatensi (1837).

Between 1835 and 1838 Struve made careful measurements of Vega, seeking to detect a parallax. He completed his analysis shortly before Friedrich Wilhelm BESSEL had done the same for the star 61 Cygni, but Bessel was the first to publish his result. Struve obtained a parallax for Vega of 0".262, equivalent to a distance of 12.5 l.y., half the present-day value. His other achievements included the finding, in 1846, that starlight is absorbed in the galactic plane, which he correctly attributed to the presence of interstellar matter.

Struve, Otto Ludwig (1897-1963) Russian-American astrophysicist, great-grandson of Wilhelm Struve, who applied spectroscopy to the study of binary and variable stars, stellar rotation and interstellar matter. Born in Russia, his work at Kharkov Observatory, where his father, Ludwig Struve, was director, was halted by the turmoil following World War I, and in 1921 he moved to Yerkes Observatory, becoming a naturalized American in 1927. He was director of the observatory and professor of astrophysics at the University of Chicago (1932-47), and later became the first director of McDonald Observatory (1939-50) and of the National Radio Astronomy Observatory (1960-62). Struve's most important contribution (1929) was to show, with Boris Petrovich Gerasimovich (1889-1937), that interstellar matter pervades the whole Galaxy, and is not localized as had been thought. In 1937 he detected interstellar hydrogen.

STS Abbreviation of Space Transportation System - the SPACE SHUTTLE


Subaru Telescope Major optical/infrared telescope with an aperture of 8.2 m (323 in.) operated by the NATIONAL ASTRONOMICAL OBSERVATORY OF JAPAN at MAUNA KEA OBSERVATORY, Hawaii. The telescope's name is Japanese for 'Pleiades'. Construction began in 1991, and the telescope became operational in 2000 with a sea-level base facility in nearby Hilo. The Subaru is notable for its ACTIVE OPTICS, sophisticated drive systems and cylindrical dome, which helps to suppress local atmospheric turbulence. It also has four foci and an auto-exchanger system for instruments at its prime focus, giving it a reputation as the Rolls-Royce of 8-10-metre telescopes.

subdwarf DWARF STAR of class F, G or K with low metal abundance; it appears below the normal MAIN SEQUENCE in the HERTZSPRUNG-RUSSELL DIAGRAM or colour-magnitude diagram. The term is misleading because subdwarfs are not below the standard main sequence but to the left of it. The low metal content yields spectra that make subdwarfs look too early for their temperatures; it decreases the atmospheric opacities to make them seem too hot and blue for their masses. Subdwarfs tend towards high velocities and are considered to be Population II halo stars.

subdwarf (sd) Any member of a group of stars that are less luminous by one to two magnitudes than MAIN-SEQUENCE stars of the same SPECTRAL TYPE; subdwarfs are of luminosity class VI. Mainly of spectral type F, G and K, subdwarfs are generally old halo Population II stars with low metal content. They lie below the main sequence on the HERTZSPRUNG-RUSSELL DIAGRAM. See also POPULATIONS, STELLAR

subgiant Star that has the same SPECTRAL TYPE as GIANT STARS, mainly G and K, but lower luminosity; subgiants are of luminosity class IV. They are stars evolving off the MAIN SEQUENCE in the process of becoming giant stars.

subluminous star Star that is less luminous than a main-sequence star of the same temperature. Subluminous stars are usually old, evolved, metal-poor, Population II stars. Examples are white dwarfs, subdwarfs and high-velocity stars.

Submillimeter Array (SMA) Joint project by the smithsonian astrophysical observatory and the Academia Sinica Institute of Astronomy and Astrophysics (Taiwan) to build an array of eight 6-m (20-ft) submillimetre telescopes at mauna kea observatory, Hawaii. It became operational in 2002.

Submillimeter Telescope Observatory (SMTO) See heinrich hertz telescope

submillimetre-wave astronomy Study of extraterrestrial objects that emit in the region of the electromagnetic spectrum between 350 um and 1 mm. It was originally regarded as part of the radio region or the infrared region - part of the radio region because it was the shortest non-optical wavelength that could be observed from the ground. Alternatively, it can be regarded as very far-infrared because the instruments used to detect submillimetre radiation are very similar to those used for infrared astronomy. The instruments must be cooled with liquid helium, and the telescopes must be at high, very dry sites because atmospheric water vapour easily absorbs submillimetre radiation. Submillimetre-wave astronomy focuses on molecular clouds, star-forming regions, cold dust and molecules. Molecules and cold dust are seen in planetary nebulae and galaxies, and many other types of astronomical object. A far-infrared and submillimetre satellite (the herschel space observatory) will be launched in 2007.

There are several submillimetre observatories, but the james clerk maxwell telescope (JCMT) on Hawaii has made a significant impact in the area of submillimetre astronomy, particularly with the array instrument SCUBA. High-redshift galaxies in the hubble deep field have been found to emit strongly in the submillimetre. Cold dust disks around young and main-sequence stars (such as Vega and e Eridani) have been resolved by JCMT. Spectral emission lines from higher energy levels of molecules can be detected in the submillimetre (and compared with those in the millimetre wave region), so that the warmer and denser regions of star-forming clouds can be observed, probing for the initial stages of the collapse, both for high-mass and low-mass stars. The submillimetre can be used to map cold dust and molecules in the direction of the centre of our Galaxy.

subsolar point Point on the surface of a body in the Solar System, particularly the Earth, at which the Sun would be at the zenith at any given moment. Similar points are also defined for other pairs of bodies.

substellar object Body with mass above that of about ten Jupiter masses but below that of a brown dwarf. The more massive a substellar object, the more closely its characteristics resemble those of a brown dwarf. See also extrasolar planets

Suisei Second of two Japanese space probes sent to Comet halley. Suisei was launched in 1985 August and flew past the comet at a distance of 151,000 km (94,000 mi) on 1986 March 8. It was used to investigate the growth and decay of the comet's hydrogen corona and the interaction of the solar wind with the cometary ionosphere.

summer solstice Moment when the Sun reaches its greatest declination and highest altitude in the sky. In the northern hemisphere this occurs around June 21 when the Sun's declination is 23.5N, marking the northern limit of its annual path along the ecliptic. At this point it is overhead at the tropic of cancer and the hours of daylight are at a maximum, the day of the solstice also marking the longest day of the year. In the southern hemisphere the summer solstice occurs around December 22, when the Sun is overhead at the tropic of capricorn. See also winter solstice

Summer Triangle Popular name for the large and prominent triangle formed by the first-magnitude stars altair (in Aquila), deneb (in Cygnus) and vega (in Lyra). It is overhead on summer nights in northern temperate latitudes, but remains visible well into the northern autumn.

Sun main-sequence star of spectral type G2; it is the central body of the solar system around which all the planets, asteroids, comets and meteoroids revolve in their orbits. The Sun's light and heat are essential for life on Earth.

Sun The Sun is a typical main-sequence dwarf star. Although it is at a stable part of its life cycle, it is not unchanging and undergoes a roughly 11-year cycle, during which its magnetic field winds up and then declines. Sunspots are the most obvious result of these changes.

The Sun's energy source is the nuclear fusion of hydrogen into helium, taking place in the core, which occupies about 25% of the solar radius. solar neutrinos produced by these nuclear reactions are detected at Earth. The structure and dynamics of the solar interior are studied by helioseismology. A radiative zone surrounds the core, and a convective zone occupies the outer 28.7% of the solar radius. The visible disk of the Sun is called the photosphere. active regions may manifest as pores, sunspots and faculae in the photosphere. These regions are associated with strong MAGNETIC FIELDS from 2000 to 4000 Gauss. The white-light photosphere shows fine-scale mottling known as GRANULATION, and a larger-scale SUPER GRANULATION; both are caused by convection. DIFFERENTIAL ROTATION is seen in the photosphere, which has an effective temperature of 5780 K.

The inner solar atmosphere consists of the thin CHROMOSPHERE, lying just above the photosphere. SPECTROHE-LIOGRAMS or SPECTROGRAMS of the chromosphere reveal structures known as PROMINENCES, SPICULES, FIBRILS, PLAGES and FLOCCULI. The Sun's outer atmosphere forms the million-degree CORONA. Observed at X-ray wavelengths, the corona shows features such as CORONAL HOLES and coronal loops.

All forms of activity vary in a roughly 11-year SOLAR CYCLE, including CORONAL MASS EJECTIONS, FLARES and the occurrence of active regions and their associated sunspots. The total flux of solar radiation incident on the Earth (the SOLAR CONSTANT) also varies in step with this cycle; longer-term changes in the solar constant may underpin episodes of climatic change, such as the MAUNDER MINIMUM. Solar activity may vary on millennial timescales. Intense X-ray radiation from flares affects Earth's IONOSPHERE, whilst energetic particles released from them may be a hazard to astronauts and artificial satellites. Coronal mass ejections influence SPACE WEATHER and can lead to magnetic storms, accompanied by intensified displays of the AURORA.

sundial Simple instrument of great antiquity for determining the time of day from a shadow cast by the sun. It usually consists of a flat plate graduated in hours and minutes, and a style or gnomon which casts the shadow. Many variations of this simple pattern exist. Without correction, the dial shows the apparent solar time. However, an accurate sundial may be constructed for a particular location. A simple date-dependent correction for the non-circularity of the Earth's orbit can be applied to obtain the time correct to about a minute.

sundog Popular name for a PARHELION sungrazer COMET that at perihelion passes very close to the Sun. See also KREUTZ SUNGRAZER

sunspot Dark, temporary concentration of strong magnetic fields detected in the white light of the Sun's PHOTOSPHERE. Sunspots are the most visible manifestations of ACTIVE REGIONS, which also include features such as FACULAE and PLAGES. The central magnetic field of a sunspot is vertical, and typically has a strength of 2000 to 4000 Gauss. Most sunspots have a central dark umbra and a lighter, grey surrounding region, called the penumbra, although either feature can exist without the other. A sunspot is cooler than the surrounding material and therefore appears darker. The effective temperature of the sunspot umbra is about 4000 K, compared with 5780 K in the neighbouring photosphere. The penumbra consists of linear bright and dark elements known as filaments, which extend radially from the umbra if the spot is more or less circular; the penumbra in complex spot groups may be more irregular.

sunspot The magnetic field around the region of a sunspot prevents the normal flow of plasma. The spot is cooler than the surrounding regions and so appears dark.

Sunspots vary in size from PORES about 1000 km (600 mi) in diameter to about 1 million km (600,000 mi). The largest spots become visible to the protected naked eye (under no circumstances should an observer look directly at the Sun without using an approved, safe filter). The duration of sunspots varies from a few hours to a few weeks, or months for the very biggest. The number and location of sunspots vary over the SOLAR CYCLE. The number of sunspots is described by the RELATIVE SUNSPOT NUMBER, and their locations change over the cycle in accordance with SPORER'SLAW, shown graphically as the BUTTERFLY DIAGRAM. Heliographic latitudes and longitudes are measured relative to a standard reference frame defined by the CARRINGTON ROTATIONS. Sunspots usually occur in pairs or groups of opposite magnetic polarity that move in unison across the face of the Sun as it rotates. See also EVERSHED EFFECT; WILSON EFFECT supercluster Extensive grouping of galaxies which may include multiple clusters as well as surrounding smaller groups and extensive regions of enhanced galaxy density in the forms of sheets or filaments. We are in the Local Supercluster, centered on the VIRGO CLUSTER about 60 million l.y. away, and occupying a flattened volume roughly perpendicular to the plane of the Milky Way. As is typical, the Local Supercluster is still growing because its dynamical influence has time to be felt over larger and larger regions, so that eventually the LOCAL GROUP of galaxies is expected to fall into the supercluster centre. Other well-known superclusters include the Shapley Supercluster, which is related to the GREAT ATTRACTOR; the Perseus-Pisces Supercluster, which includes seven rich clusters plus outlying filaments, together spanning more than 40 in our sky and about 1 billion l.y. in space; and the Coma/Abell 1367 Supercluster, which is part of the so-called Great Wall of galaxies seen almost edge-on about 300 million l.y. away.

The mass of a supercluster can be estimated from non-Hubble distance estimates of outlying objects on its near and far borders, which may reveal the characteristic pattern of infall under the supercluster's influence. At their outskirts, superclusters blend into the overall LARGE-SCALE STRUCTURE of clumps, walls and filaments of galaxies which spans the observable Universe, on scales of up to 1 billion l.y. Their existence is important for cosmology, since enough time must have elapsed for them to have collected such a massive concentration of material, and the mass distribution in the early Universe must have been uneven enough to start the process of accumulation.

supergiant Largest and most luminous star known. Supergiants occur with SPECTRAL TYPES from O to M. Red (M type) supergiants have the largest radius, of the order of 1000 times that of the Sun. Betelgeuse is a type M supergiant; Rigel is a type B. As there is a an upper limit to the absolute bolometric magnitude of red supergiants, they can be used as distance indicators. Supergiants lie above the main sequence and giant region on the hertzsprung-russell diagram. Instabilities created by radiation pressure mean that supergiants are often variable. See also cepheid variables; eta carinae; s doradus star supergranulation Large convective cells seen in the solar photosphere. They have dimensions of about 30,000 km (19,000 mi), lifetimes of about 20 hours, and internal plasma motion velocities of about 1 km/s (0.6 mi/s). Unlike the granulation, the supergranulation is not visible in the white light of the photosphere and is instead detected by the doppler effect. The dominant flow in the cells is horizontal and outwards from the centre, but there is a weak upward flow at the cell centre and a downward flow at the cell boundaries. The cells are outlined by the magnetic network in the chromosphere, the boundaries of which contain concentrations of magnetic fields and spicules.

superior conjunction Point in the orbit of one of the inferior planets at which it lies on the far side of the Sun, as seen from Earth, and the three bodies are in alignment. conjunction occurs when two bodies have the same celestial longitude as viewed from the Earth. For an inferior planet, this can also occur when it is on the same side of the Sun as the Earth. At this point it is said to be at inferior conjunction. See diagram at conjunction

superior planet Term that is used to describe any planet with an orbit that lies beyond that of the Earth; the superior planets are Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

superluminal expansion Apparent faster-than-light effect. Special relativity asserts that massive particles can never reach or exceed the speed of light. Yet, radio lobes seen in distant radio galaxies and quasars have been measured to be separating from the central source at velocities exceeding the speed of light. This faster-than-light or 'superluminal' expansion is merely an effect of the finite speed of light that occurs in things that are moving close to the speed of light and very close to the line of sight. Time dilation effects coupled with the time for light to travel between the reference object (the quasar or galaxy core) and the emitting object (the knot in the jet that is moving close to the speed of light) can cause the distant observer to overestimate the speed by a large fraction.

supermassive black holes black holes that are thought to reside in the nuclei of galaxies and quasars and to contain 106 to 109 times the mass of the Sun. These black holes are a million to a billion times more massive than black holes that result from stellar evolution. It is not clear how they form in galactic centres, but the evidence for their existence is very strong. The direct evidence for the existence of supermassive black holes comes from observations of high-velocity gas clouds orbiting the centres of radio galaxies. The gravitational potential necessary to bind these clouds in orbit requires 106 to 108 solar masses contained within a very small region of space. The mass density required assures that the supermassive object would immediately collapse to a black hole if it were not one already. Indirect evidence for supermassive black holes is also found in observations of quasars, which suggest that a large amount of energy is generated within a volume the size of our Solar System. The only energy generation mechanism efficient enough to accomplish this is the conversion of gravitational energy into light by a supermassive black hole.

supermassive star Very rare star with mass of the order of 100 solar masses and above. It is not known if there is an upper limit for stellar masses, but some estimates suggest an upper limit of 440 solar masses. The most massive main-sequence stars observed are within the R136 cluster in the 30 Doradus Nebula of the large magellanic cloud. They are around 155 solar masses. See also eta carinae; s doradus star

supernova Stellar explosion that involves the disruption of virtually an entire star. Supernovae are classified by their spectra into two broad types, Type I and Type II, according to the presence or absence of hydrogen in their spectra. Type I supernovae have no hydrogen in their spectra, whereas Type II do. It is now known that the cause of supernovae is not determined by the presence or lack of hydrogen in the spectra, thus further categorization has been made.

supernova Elements heavier than iron cannot be formed in normal stellar fusion, and supernovae are thought to be the mechanism that produces them. Lighter elements are also created during supernovae this image shows the presence of silicon in the supernova remnant Cassiopeia A.

There are two basic models for the cause of super-novae. The 'core collapse supernovae' are massive stars that have exhausted the nuclear fuel in their cores. However, because the mass of the core reaches beyond the chandrasekhar limit further core collapse occurs until neutron degeneracy pressure sets in, and the outer atmosphere is thrown off as a result of shock waves. Other supernovae are thought to occur in close binary systems in which a white dwarf is sent over the Chandrasekhar limit by mass transfer from its companion.

Type Ia supernovae appear in all types of galaxies but are less frequent in the spiral arms of spiral galaxies. They have elements such as magnesium, silicon, sulphur and calcium in their spectra near maximum light, and iron later on. The light-curve of a Type Ia supernova shows an initial rise over about two weeks, then a more gradual decay over timescales of months. A Type Ia supernova is thought to be the explosion, as a result of mass transfer, of an old, low-mass, long-lived star in a binary system. As Type Ia supernovae are so bright, they have been used to estimate distances to faraway galaxies.

supernova The light-curve of a supernova shows a sudden brightening as the outer atmosphere is thrown off after the stellar core has collapsed. This peak is followed by a steady decline for about two months, after which this fading slows further.

Type II supernovae do not appear in elliptical galaxies, but instead occur mostly in the spiral arms of spiral galaxies or sometimes in irregular galaxies. They show ordinary stellar abundances in their spectra. The light-curve of a Type II supernova rises to a peak in a week or so, remains constant for about a month, then drops suddenly over a few weeks, returning to obscurity over a timescale of months. A Type II supernova is believed to be the result of an explosion in the core of a red giant star with a massive extended envelope.

Types Ib and Ic supernovae appear to explode only in the spiral arms of spiral galaxies. Both show evidence of oxygen, magnesium and calcium in their spectra after peak brightness. In addition, Type Ib supernovae show evidence of helium at times near maximum light. The light-curves of both Ib and Ic are similar to Type Ia but dimmer at maximum light. They are often strong radio sources, whereas no radio emissions have yet been detected from Type Ia. Types Ib and Ic supernovae are thought to be produced by explosions of the cores of massive stars that have been stripped of their hydrogen, and, in the case of Type Ic, of their helium as well.

Supernovae produce neutron stars, and many have been observed at the centre of the remains of the disintegrated stellar envelope. The nebula created by the supernova is termed a supernova remnant. Supernova remnants with pulsars at their centres are termed plerions.

Supernovae are relatively rare, discovered at the rate of about one every century in an average galaxy. They are discovered relatively infrequently in edge-on spiral galaxies because their light is dimmed by dust. Only five have been discovered in the Milky Way in the last millennium, with supernova sn 1987a occurring in the close companion galaxy, the large magellanic cloud, in 1987.

Recent technological advances, especially the ccd, becoming available to amateur astronomers have meant a dramatic increase in the number of supernovae discoveries in other galaxies in recent years. Computer-controlled telescopes equipped with CCDs scan a large number of remote galaxies each clear night, and comparisons made with previous images reveal any supernovae explosions.

The supernova of 1054 was identified by Edwin hubble as the progenitor of the Crab Nebula. Like the super-novae of 1006 and 1181, it was recorded by oriental astronomers as they scanned the sky for celestial portents. Chinese, Korean, Japanese, Arabic and European astronomers contributed to records of these supernovae: the 1054 supernova was probably depicted in Native American art.

The supernova of 1572 was carefully observed by Tycho brahe. He recorded data on its unchanging position and its stellar magnitude as it faded day by day. Brahe demonstrated that the supernova, which was circumpolar from Denmark, had no parallax as the Earth rotated. This placed the star well beyond the Moon. Its lack of motion over the 18 months that it was visible meant that it was beyond Saturn, the most distant planet then known. This discovery placed the supernovae among the 'fixed stars' and proved that they were subject to the same laws of change as terrestrial phenomena.

The supernova of 1604 is known as Kepler's star, although Johannes Kepler was not the first to see it. Both Kepler's and Tycho's supernovae inspired a wealth of comment by many writers of the 16th and 17th centuries, including Richard Corbet, Henry More, John Donne, Edmund Spenser and John Dryden.

There is evidence that a supernova exploded in Cassiopeia in around 1680. An expanding gaseous remnant of an exploded star is observed as a powerful emitter of radio radiation; it is known as cassiopeia a. No optical outburst was recorded, but the star may have ejected a great deal of its outer layers before exploding, or been relativity small and compact.

supernova remnant (SNR) What is left after a supernova explosion has occurred. For some supernovae, there are two components to the supernova remnant - the central neutron star, pulsar or black hole and the expanding cloud of gas. Only the last of these, however, is included in the usual meaning of the term SNR.

In appearance SNRs may resemble some hii regions, but they often have more of a filamentary, shell-like morphology than the latter. The radio emissions from the two types of object allow them to be differentiated unequivocally, since recombination lines appear in the radio spectra of HII regions but not in those from SNRs. Furthermore, synchrotron radiation produces much of the emission from SNRs and this is strongly polarized, whereas the emission from HII regions is unpolarized.

Given that they are created by an explosion, it is clear that SNRs must be expanding. The initial expansion velocities can be 10,000 to 20,000 km/s (6000-12,000 mi/s), though this reduces to a few hundred km/s with time. Nonetheless after a century or two, the nebula's physical size is measured in light-years. The crab nebula for example is about 13 l.y. across, the Cygnus Loop about 100 l.y. and the local bubble, which may have resulted from a much older supernova explosion, is some 300 l.y. in diameter. The kinetic temperature of the material initially exceeds 106 K, but this is reduced as energy is radiated away. The particle densities range from 106 m-3 to 1012 m-3, with 109 m-3 being fairly typical for the visible portions of an SNR. The masses of the nebulae range from a fraction to a few times the solar mass.

The development of an SNR occurs in several stages. At first the hot gases exploding outwards at around 10,000 km/s (6000 mi/s) have densities so much higher than that of the surrounding interstellar medium that the expansion is essentially into a vacuum. After about a century, when the SNR is around 1 l.y. or so in size, the interstellar material swept up by the expanding remnant reaches densities at which it starts to impede the expansion. This creates a turbulent shock region, with synchrotron emission coming from the relativistic electrons. The expansion velocity will then slowly decrease until at about 100 km/s (60 mi/s) optical line emission from the heavier elements becomes significant. The last identifiable stage may be as a hot low-density cavity blown into the interstellar medium. Finally, after some few hundred thousand years, the remnant will merge with the interstellar medium.

Most SNRs have the appearance of a thin spherical shell, with much fine structure. Type Ia supernovae, such as that seen by Tycho brahe in 1572, lead to shells that are brighter towards their outer edges at radio and X-ray wavelengths, with only Balmer emission lines detected in the visible. Type II supernovae, such as the one that produced Cassiopeia A, result in shells containing numerous rapidly moving knots of material that are very bright at both radio and X-ray wavelengths, with thermal emission dominating the X-rays. A few SNRs, however, like the Crab Nebula, are filled-in to their centres. These are given the name plerions, and their difference probably arises from the presence of a central pulsar that can continue to supply high-energy electrons to the centre of the nebula. An SNR should be detectable for a period of a few hundred thousand years after the explosion. Since the estimated rate of supernovae in our galaxy is around ten per century, there should be several thousand SNRs in our Galaxy. Only about a hundred are actually known, however, and most of these are detected by their radio emissions. The disparity is probably a result of SNRs' low surface brightnesses and the obscuration of the more distant examples.

The radio emission from SNRs is mostly produced by synchrotron radiation. Initially, the electrons producing this radiation result from the supernova explosion. Later, except in plerions, the electrons probably gain their energies whilst passing through the shock fronts where the expanding SNR meets the interstellar medium. A few SNRs are observable outside the radio and X-ray regions. In particular, the Crab Nebula emits detectable radiation from the infrared to the gamma-ray region. Most of this again appears to be due to synchrotron radiation, as shown by the high degree of optical polarization of the nebula. Some of the visible light, however, results from recombination after the gas has been ionized by the ultraviolet synchrotron emission. The X-ray emission from SNRs results from synchrotron radiation and from electron-ion collisions in the hot gas.

SNRs affect their locality in several ways. Since Type II supernovae originate from short-lived massive stars, they will often occur whilst the star is still inside a dense gas cloud. The pressure of the expanding SNR may then initiate further star formation. The energy of the SNR adds to the energy and turbulence of the interstellar medium. It also enriches it in elements heavier than helium, which were synthesized in the supernova and its preceding star. Finally, as particles are accelerated during interactions between the turbulent high-velocity gases and the magnetic fields of an SNR, cosmic rays may be produced.

Supernova SN 1987A Bright supernova that flared up in the Large Magellanic Cloud in 1987 February, reaching naked-eye visibility. It was the first supernova to do so since Kepler's star of 1604. Supernova SN 1987 A was a Type II supernova, and a flood of neutrinos was detected from its eruption, suggesting that a neutron star was formed. No subsequent evidence of a neutron star has been found, but it may be obscured in some way.

The precursor star (Sanduleak 69202) of the supernova explosion was a compact blue supergiant, rather than the red giant or supergiant that current theories predict should give rise to a Type II supernova. It has been suggested that the star may have had a binary companion that was consumed as the progenitor became a red giant, increasing both its size and temperature. The set of three nested rings that have subsequently been observed may be related to the fact that the system was a binary system, but this remains unclear.

superstring theory Theory of elementary particles that asserts that the most fundamental forms of matter and energy are not particles but minute vibrating strings. The shape, topology and frequency of vibrations of these strings determine the physical properties of the particles. The scale of these strings is very small, of the order of 1033 cm. This in itself is a problem since they are the smallest possible quanta and thus can never be observed. However, if one assumes these strings do exist, and they do make up elementary particles and fields, then one can construct a model that could possibly describe everything in the universe - a theory of everything (TOE).

supersymmetry Principle that attempts to explain the different strengths of the fundamental forces in the standard model. This model accurately describes the three fundamental forces and provides a grand unification. If the interaction strengths of the three GUT forces (see grand unified theory) are extrapolated to high energies, they all come together around 1016 GeV. The interaction strength of each force needs to be specified independently in the standard model; this problem is called the hierarchy problem. Supersymmetry attempts to explain the hierarchy problem in terms of a symmetric principle that assigns a symmetric partner to every known elementary particle and also relates fermions to bosons.

surface gravity Acceleration, g, due to the gravitational force experienced by a freely falling object close to the surface of a massive body.

survey, astronomical Systematic large-scale observation made with one or a small number of dedicated instruments to gather reference data for the whole sky or a significant fraction of it. Examples are the surveys are aimed at systematic observations of the recessional velocities of very large numbers of galaxies.

Surveyor Series of seven soft-landing lunar spacecraft launched by NASA between 1966 May and 1968 January in preparation for the Apollo landings. Five of them were successful in sending back high-resolution images and data on the nature of the lunar surface. Each Surveyor had a steerable television camera with filters. Other payloads included a combined surface-sampler/trench-digger and a simple soil composition analyser. Among the wealth of data returned were panoramic and close-up images, data on the bearing strength, optical and thermal properties of the surface, the content of magnetic material and its major-element chemistry.

Su Song (or Su Sung) (1020-1101) Chinese astronomer, noted for his clock-driven astronomical instruments. In 1090 Su Song finished his two-storey clock tower at Kaifeng. Inside was a clock-driven celestial globe, while moving figures indicating the time were visible from the outside. Surmounting the tower was an armillary sphere fitted with a sighting tube. Globe and sphere automatically followed the rotation of the skies, and were the first astronomical instruments to do so, anticipating Western automatic clock-driven instruments by six centuries.

SU Ursae Majoris star (UGSU) Subtype of cataclysmic variable known generically as a dwarf nova (commonly called u geminorum star). Their behaviour resembles that of the classical U Geminorum/SS Cygni subtype (UGSS) in that there are short and long maxima. In the SU Ursae Majoris stars, however, the latter are known as supermaxima, because they are half to one magnitude brighter than normal outbursts, and remain at maximum for 10 to 20 days (as against 4 days for a normal outburst). For almost all maxima, stars of this type rise to near maximum brightness in 24 hours or less. Some have a short pause at an intermediate magnitude on the rise for an hour or two. They also have periodic oscillations, termed superhumps, superimposed on the light-curve with amplitudes of 0.2 to 0.5 mag. Their period is about 3% longer than the orbital period.

Swan Nebula See omega nebula

Swift-Tuttle, Comet The comet's most recent return was in 1992 and was not particularly spectacular. It will be much closer to Earth at its next approach in 2126.

Swasey, Ambrose See warner & swasey co.

Swedish ESO-Submillimetre Telescope (SEST) Radio telescope of 15-m (49-ft) aperture operating in the frequency range 70-365 GHz. Built in 1987, it is located at la silla observatory and operated in a partnership between the european southern observatory (ESO) and onsala space observatory, which is responsible for receivers and computer software. Hardware and mechanical components are maintained by ESO.

Swedish Extremely Large Telescope (SELT) Proposal for a 50-m (164-ft) optical telescope made by astronomers at lund observatory and elsewhere during the late 1990s. The telescope would require multi-conjugate adaptive optics to achieve its potential resolution, and would use the segmented-mirror technology pioneered at the w.m. keck observatory. An alternative is to spin-cast the mirror as a monolithic structure at the observing site. By 2002 it seems likely that the SELT proposal would be subsumed into the overwhelmingly large telescope project.

Swift, Lewis (1820-1913) American astronomer who, between 1855 and 1901, discovered more than 1200 star clusters, nebulae and galaxies, ranking him third behind only the English astronomers William herschel and John herschel. Swift also found 13 comets, including 109P/swift-tuttle, which produces the annual per-seid meteor shower. He served as director of the H.H. Warner Observatory in Rochester, New York and the T.S. Lowe Observatory on Echo Mountain, near Pasadena, California.

Swift-Tuttle, Comet 109P/ Short-period comet discovered by Lewis Swift on 1862 July 16, and independently three days later by Horace Tuttle (1837-1923). The comet became a naked-eye object, at mag. +2.0 in early September, when the tail reached a length of 25-30. The last observations in the discovery apparition were made at the end of October. Uncertainties over the orbital period remained until the next return - ten years later than some predictions - in 1992.

Comet 109P/Swift-Tuttle returned to perihelion on 1992 December 12, reaching mag. + 5.0. As at the previous return, the comet's nucleus showed considerable jet activity, a source of non-gravitational force, which partly explains the earlier difficulty in deriving an orbit. The period is now taken to be close to 135 years, and the comet is, as originally proposed by Brian Marsden (1937- ) of the Harvard Smithsonian Center for Astrophysics, identical with Comet Kegler of 1737. Its next return in 2126 will see 109P/Swift-Tuttle make a close approach to Earth. The comet is the parent of the perseid meteor shower, which showed enhanced activity around the time of its 1992 perihelion.

Sword Handle See double cluster

SX Phoenicis Pulsating variable star, 6.5 west of a Phoenicis; it exhibits periods of 79 and 62 minutes, with a beat period of 278 minutes. The visual range is from mag. 7.1 to 7.5. Its distance is about 140 l.y. Its behaviour closely resembles that of a delta scuti star, and it has been taken as the prototype (designated SXPHE) for subdwarf stars that belong to globular clusters and the older regions of the Galaxy and have multiple periods.

Sycorax One of the several small outer satellites of uranus; it was discovered in 1997 by Philip Nicholson (1951- ) and others. Sycorax is about 120 km (75 mi) in size. It takes 1289 days to circuit the planet, at an average distance of 12.18 million km (7.57 million mi). It has a retrograde orbit (inclination near 159) with a substantial eccentricity (0.523). See also caliban

symbiotic star (variable) binary star that exhibits the spectral characteristics of two grossly different temperature regimes. Typically there are absorption bands such as those found in giants and supergiants of late spectral classes around 3000 K, together with emission from a hot star of about 20,000 K. In addition, emission from hot, surrounding nebulosity is often detectable. Symbiotic stars exhibit a wide range of characteristics. They may all be classed as cataclysmic variables, in which gas from the cool star is accreted by the smaller companion, giving rise to outbursts. z andromedae stars resemble dwarf novae, but with a giant (rather than main-sequence) secondary. In certain systems, such as RR Telescopii, there is a high rate of mass transfer, producing systems with extremely extended outbursts. Such systems were formerly known as 'very slow novae' but are now more commonly called 'symbiotic novae'. Some systems - most notably R Aquarii - appear to consist of a pulsating mira star transferring mass to an accretion disk and thence to a condensed object that is ejecting material in a pair of luminous jets. This is very similar to the configuration found in the strange object SS 433, where the condensed component appears to be a neutron star, with an accretion disk and exceptionally strong polar jets.

symbiotic star This peculiar structure is thought to be the result of at least three successive nova explosions on a red giant/white dwarf pair. The hourglass structure in the centre results from the most recent explosion, while the insect-like structure is the remnant of the previous episode.

synchronous orbit Orbit in which a satellite's period of revolution is the same as the planet's period of axial rotation. For Earth, the radius of the orbit is 42,164 km (26,200 mi). The satellite appears to hover over one point on the equator, neither rising nor setting. The main significance for natural satellites is that the direction of tidal evolution is inwards or outwards according to whether the satellite is below or above the synchronous orbit. Many artificial satellites of the Earth have been placed in such orbits, but the only natural example among the planets is Pluto's satellite, charon. It is an ultimate state resulting from tidal evolution; it requires that the secondary is a substantial fraction of the mass of the primary, in order that it can absorb the spin angular momentum of the primary. Eventually the Moon will reach this state. The Earth's rotation is slowing as the Moon recedes from the Earth (see also secular acceleration), and this will continue until the day has lengthened to be the same as the month (but this will take about 10 billion years).

synchronous rotation Coincidence of the rotational period of a celestial body with its orbital period. This has the effect of causing a planetary moon, for example, always to present the same face towards the planet it orbits, as is the case with the Earth and the Moon. The phenomenon, also known as captured rotation, is caused by tidal friction. See also tide

synchrotron radiation electromagnetic radiation emitted by charged particles (usually electrons) moving in magnetic fields at large fractions of the speed of light. The charged particles follow helical paths along magnetic lines of force and emit radiation because of the accelerations to which they are subjected. The process is closely related to free-free radiation. The wavelength of the emitted radiation decreases as the energy of the electrons increases, and is strongly polarized. Synchrotron radiation produces much of the emission from the crab nebula, pulsars and radio galaxies.

Strictly, a term that means 'pertaining to two successive conjunctions of celestial bodies'; it is more commonly used to describe a single cycle of any phase of a celestial body viewed from a given point. For example, a synodic month is the period between successive new or full moons.

synodic month (lunar month) Period between successive new or full moons. This is the same duration as one lunation and is equivalent to 29.53059 days of mean solar time. See also anomalistc month; draconic month; month; sidereal month; tropical month

synodic period Interval between successive oppositions, or conjunctions, of a planetary body. It is a measure of that body's orbital period as observed from the Earth, as opposed to its sidereal period, which provides a true measure.

Syrtis Major Planitia Most conspicuous feature on mars; it is centred near 9.5N 290.5W. It is dark, wedge-shaped and an easy telescopic object. Formerly known as the Syrtis Major, it was first drawn by Christiaan huygens as long ago as 1659. It is now known to be a plateau rather than a vegetation-filled depression, as was once thought.

Systems I and II Grouping by rotation period of the clouds of jupiter, which do not rotate uniformly. The groups are: System I, 9h 50m 30s, found in equatorial regions; and System II, 9h 55m 41s, found above latitude 10N or S. A third, System III, defined by radio emissions, has a rotation period of 9h 55m 29s.37.

syzygy Approximate alignment of the Sun, Earth and Moon, or the Sun, Earth and another planet. Syzygy thus occurs when the Moon is either new or full or, for a planet, at conjunction and opposition. The term is also used to describe the alignment of any three celestial bodies.