BAA Abbreviation of british astronomical association

Baade, (Wilhelm Heinrich) Walter (1893-1960) German-American astronomer who used the large reflecting telescopes at Mount Wilson and Palomar Observatories to make many fundamental discoveries about the Milky Way's stellar populations and the size of the observable Universe. From 1919 to 1931 Baade worked at Germany's Hamburg Observatory, where he made many observations of a wide variety of celestial objects -comets, asteroids, variable stars and star clusters, and galaxies. He discovered the unusual asteroids hidalgo (1920) and icarus (1949). His skills as an observer were recognized by the Rockefeller Foundation, which awarded him a scholarship in 1929 to pursue his work at Mount Wilson Observatory. Two years later he became a staff member there, where he remained until 1959, using the 100-inch (2.5-m) Hooker Telescope and, later, Mount Palomar's 200-inch (5-m) Hale Telescope, to accumulate a large number of high-quality photographic plates of the Milky Way's stars, clusters and nebulae, as well as many galaxies.

In 1934 Baade proposed, with Fritz zwicky, that neutron stars were the end product of supernovae and that many cosmic rays originated from these violent explosions of supermassive stars. With Zwicky and Rudolph minkowski he classified supernovae into two types that differed in their absolute magnitudes and spectral characteristics. With Minkowski, he identified the optical counterparts of the Cygnus A and Cassiopeia A radio sources. Observing extensively during the World War II blackout of Los Angeles, which darkened Mount Wilson's skies, Baade was able to resolve for the first time stars in the core of the andromeda galaxy (M31), as well as in its companions, M32 and NGC 205. In 1944 he announced that the Milky Way consists of younger, metal-rich, highly luminous (usually blue) Population I stars found mostly in our Galaxy's spiral arms, and older, reddish Population II stars of the galactic nucleus and halo. Baade also discovered that there are two populations of cepheid variables. In 1952 and 1953 he used the new Hale Telescope to measure more accurately the brightness of Cepheid and rr lyrae variables. He found no Cepheids in M31, but they should have been visible if the galaxy lay at the distance then currently believed, so he reasoned that M31 had to be at least twice as distant - a discovery that necessitated the doubling of the cosmic distance scale.

Baade's Window Best known of several small regions in the sky where there is a partial clearing of the dust clouds towards the centre of the Milky Way galaxy. Through these 'windows' it is possible visually to observe objects that are within and beyond the central bulge of the Galaxy. Baade's window is about three-quarters of a degree northwest of y Sgr and surrounds the globular cluster NGC 6522. It is 4 east of the galactic centre and so lines-of-sight through the window pass within about 1800 l.y. of the centre. The region is named after Walter baade. Many instruments for the telescopes of Mount Wilson and Palomar Observatories, including diffraction gratings, photometers, guiding electronics, polarizers and spectro-graphs. He was the first (1953) to propose the technique known as adaptive optics.

Babylonian astronomy Astronomy as practised in Mesopotamia by various cultures from early in the 2nd millennium bc to about the 1st century ad. The city of Babylon was the dominant influence in this region for much of this period. It was established by King Hammurabi (1792-1750 bc) on the banks of the Euphrates, about 100 km (60 mi) south of Baghdad. Apart from periods of independence, it was successively controlled by Hittites, Kassites, Assyrians and Persians before its conquest in 311 bc by Alexander the Great. Thanks to the preservation of Babylonian written records on baked clay tablets from around 700 bc onwards, Babylonian mathematics and astronomical observations can be traced further back than those of other peoples. From Babylon came the sexagesimal system of numbers, based on 60 rather than our familiar 10. The subdivisions of the hour into 60 minutes and the minute into 60 seconds, and of the degree into 60 arcminutes and the arcminute into 60 arcseconds, are the legacy of this system.

Like many other ancient peoples, the Babylonians used a mixed luni-solar calendar, with extra months inserted when necessary to correct for the fact that there are between 12 and 13 lunar months in one year. They kept accurate records of celestial events and eventually (c.5th century bc) recognized the metonic cycle (that a times-pan of 235 lunar months is very close to 19 solar years). They set up a predictable calendar based on this relationship. By studying the motions of the planets they were able to identify cycles in their behaviour and could construct predictive almanacs (ephemerides). They also recognized that the annual apparent motion of the Sun appears to be uneven, and they developed a simple model to allow for this when predicting its position.

Although most of the surviving Babylonian records date from after 700 bc, a compilation of celestial omens that supposedly warned of impending disasters, called the Enuma Anu Enlil, contains references to events dating back to the 3rd millennium bc. This text was used to interpret new omens that appeared from time to time and their possible consequences, with a view to advising the king on how to avoid them by performing appropriate religious rituals. Out of the recording and interpretation of celestial omens came 'diaries' of the movements of the Sun, Moon and planets, the discovery of cycles in these movements, and eventually a mathematical model of these movements built around arithmetical sequences.

The precision of the Babylonian observations was complementary to the more speculative nature of Greek astronomy. After Alexander's conquest of Babylon drew their attention to Babylonian achievements, Greek astronomers such as hipparchus became interested in making precise observations and in developing geometrical models that fitted the data.

Bacchus apollo asteroid; number 2063. Radar images show Bacchus to have a smooth, elongated shape. See table at near-earth asteroid

Bacchus Radar images of asteroid (2063) Bacchus have shown it to be about 1.5 km (0.93 mi) long. It also appears to be smooth in texture in comparison with such asteroids as Eros. [S. Ostro (JPL/NASA)]

background noise Intrinsic noise in the detector or the sky background noise. For radio and infrared astronomers the main concern is usually detector noise. For optical astronomers, the night sky itself can be brighter than the astronomical object being observed. In all cases astronomers attempt to measure the background noise so that it can be removed. See also signal-to-noise ratio

backscattering Reflection of light back towards the direction of the light source by an angle greater than 90. Light may be scattered from its direction of travel by fine particles of matter. For particles significantly larger than the wavelength of the incident light, reflection or backscattering of the light occurs. See also forward scattering.

Baikonur Important launch centre situated in a semi-arid desert, north-east of the Aral Sea. Now leased from Kazakstan by Russia, it is the site from which all Soviet/Russian manned spaceflights have been launched. It is also known as Tyuratam.

Bailey, Solon Irving (1854-1931) American astronomer at Harvard College (1887-1931) who pioneered photographic surveys of the southern skies from Peru and South Africa. In 1889 Bailey made the first of several expeditions to South America to find a suitable locale for Harvard's Boyden Station, and eventually chose a site near Arequipa, Peru, at an altitude of 2500 m (8200 ft). From there, Bailey measured the brightness and positions of almost 8000 stars visible only from the southern hemisphere, and studied over 500 variable stars in the globular clusters co Centauri, M3, M5 and M15. In 1908, from Cape Colony (modern South Africa), Bailey completed a photographic survey of the southern regions of the Milky Way.

Bailly Vast lunar enclosure (67S 63W), 298 km (185 mi) in diameter, described as a 'field of ruins'. Though technically the largest of the Moon's walled plains, Bailly is a difficult feature to make out because of its location at the south-west limb and its broken-down walls. Its rims rise to heights of 4250 m (14,000 ft) in some places, but in others are barely discernible. Bailly's floor lacks a central peak, but it does include two sizeable craters, known as Bailly A and B, and many smaller impact sites.

Baily, Francis (1774-1844) English stockbroker and amateur astronomer whose description of the phenomenon visible during annular and total solar eclipses now known as baily's beads, published in 1838, was the first to attract attention. Baily also made the first highly accurate calculation of the Earth's mean density.

Baily's beads Phenomenon usually seen at second contact and third contact of a total solar eclipse, when several dazzling spots of the Sun's photosphere become visible through depressions in the Moon's irregular limb. Baily's beads may also be visible when the Moon at an annular eclipse only just appears too small completely to cover the Sun. Indeed, it was at just such an eclipse, seen from Jedburgh in Scotland on 1836 May 15, that Francis baily first described the 'row of lucid points, like a string of bright beads' around the Moon's limb, from which the phenomenon takes its name.

Baker-Schmidt camera Form of schmidt camera used for photographing meteors; it incorporates design modifications by J.G. Baker. With a typical focal ratio of f/0.67, the Baker-Schmidt camera employs near-spheroidal primary and secondary mirrors, together with a correcting plate, which produce a wide, flat field of view and images that are largely free from chromatic aberration, astigmatism and distortion.

Ball, Robert Stawell (1840-1913) Irish astronomer, the fourth Royal Astronomer for Ireland (1873-92). He discovered several galaxies with the 'Leviathan of Parson-stown', Lord rosse's great 72-inch (1.8-m) reflecting telescope. Ball supported the nebular hypothesis of the Solar System's origin, according to which the Sun and planets condensed from a hot primordial cloud of gas. He believed that meteorites were produced by terrestrial volcanoes and expelled by violent eruptions, falling back to the ground after temporarily orbiting our planet.

balloon astronomy Astronomical research carried out using instruments flown on balloons. In the late 19th century, scientists began using balloons to lift telescopes and other astronomical instruments above the turbulent lower atmosphere. Even after the advent of rocket astronomy and artificial satellites, the convenience and cheapness of balloons ensured their continued use for research and testing new space technology. The development of high-altitude balloons was an important spur for balloon astronomy. Recent applications include study of the cosmic microwave background (CMB); balloons have also been used in planetary exploration.

balloon astronomy The Boomerang balloon is shown here just before launch, with Mount Erebus in the background. At a height of 35 km (22 mi), the balloon flew above 99% of the Earths atmosphere for 10 days, which allowed it to make unprecedented measurements of the cosmic microwave background radiation.

The first astronomical use of balloons was probably in 1874 when Jules janssen sponsored two aeronauts in an unsuccessful attempt to record the solar spectrum with a small hand-held spectroscope. In 1899 July, an automatic photographic spectrograph developed by Aymar de la Baume-Pluvinel (1860-1938) demonstrated that water vapour in the Earth's atmosphere was showing up in the solar spectrum. Victor hess discovered cosmic rays with a balloon-borne electroscope in 1912; further cosmic-ray studies were conducted in the 1930s by twin scientists Jean Piccard (1884-1963) and Auguste Piccard (1884-1962), who ascended in a balloon to above 20,000 m (65,000 ft).

The first use of a balloon-borne astronomical telescope was by Audouin Charles Dollfus (1924- ) on 1954 May 30, during an attempt to detect water vapour in the Martian atmosphere. A few years later, Dollfus and the English astronomer Donald Eustace Blackwell (1921- ) took the first astronomical photographs from the air; their high-resolution images of the Sun's photosphere proved that solar granulation was the product of internal convection. Higher-flying unmanned balloons were pioneered by Martin schwarzschild with Stratoscope I, whose 0.3-m (12-in.) automated Sun-pointing telescope recorded sharper images of the granulation and the photosphere from an altitude of 24,000 m (79,000 ft). Stratoscope II. which carried a 0.86-m (34-in.) telescope, obtained infrared spectra of the Martian atmosphere and red giant stars during its first flights in 1963.

Large, high-altitude, unmanned helium balloons are still used for both astronomical observations and for trials of new technologies and payloads for space missions. Observations at X-ray, gamma-ray and infrared wavelengths continue to be made from balloons. Some of the most significant balloon-borne campaigns of recent years have involved studies of the CMB.

In the Boomerang experiment (1998/1999), a balloon-borne 1.2-m (48-in.) telescope circumnavigated Antarctica, taking measurements at four frequencies to separate faint galactic emissions from the CMB. The resultant map covered approximately 2.5% of the sky with an angular resolution 35 times better than had been achieved by the cosmic background explorer satellite, revealing variations as small as 0.0001 K in the temperature of the CMB. Similar endeavours include MAXIMA (Millimetre Anisotropy Experiment Imaging Array), balloon-borne millimetre-wave telescopes designed to measure fluctuations in the CMB. MAXIMA 1 (1998 August) observed 124 square degrees of sky, while MAXIMA 2 (1999 June) observed roughly twice that area.

The CMB and interstellar dust in our Galaxy were also observed by submillimetre telescopes on the 3-t French PRONAOS (Programme National d'Astronomie Submil-limetrique) stratospheric balloons in 1994, 1996 and 1999. The successor to PRONAOS is the Elisa international project to observe the interstellar medium. 2000 June saw the inaugural balloon flight of Claire, the first gamma-ray telescope to use a separate detector and collector. A follow-up flight with an improved version took place in 2001 June.

Extremely large balloons that can stay aloft for up to 100 days are now being designed. The 30-day maiden flight of NASA's Ultra Long Duration Balloon (ULDB), carrying the Trans-Iron Galactic Element Recorder (TIGER) experiment, began in Antarctica on 2001 December 21. TIGER is designed to measure the abundances of elements from iron to zirconium in galactic cosmic rays. The ULDB is a pressurized balloon made of extremely strong materials, such as polyester, inside which the helium gas is sealed. The balloon maintains a constant volume and so stays at the same altitude. The French pioneered the use of such balloons to explore other worlds. Two helium balloons were delivered by Soviet vega spacecraft into the atmosphere of Venus on 1985 June 11 and 15. Inflated about 54 km (34 mi) above the surface, each lasted for about 46 hours, and travelled more than a third of the way around the planet.

Traditional 'Montgolfier' hot-air or 'zero-pressure' balloons are much lighter and more easily deployed than their helium counterparts, and have potential for flights above other planets. Infrared Montgolfiers could not operate at Mars during the very cold nights, but a novel hot-air venting system could allow repeated, precision soft landings. Although they would be limited to daytime flights - up to 10 hours at lower Martian latitudes, or perhaps months during the long polar summers - such balloons could drag an instrumented 'snake' over the surface or soft-land a large rover, re-ascend, and continue imaging before landing again, possibly with a 'nanorover', at dusk. Other Montgolfier balloon missions that could explore the atmospheres of Venus, Jupiter and Titan are under study.

Balmer lines Series of emission or absorption lines in the hydrogen spectrum resulting from electron transitions down to or up from the second energy level of that atom. The Balmer lines are named with Greek letters: Ha (or Balmer a), which connects levels 2 and 3, falls at 656.3 nm; Hp , which connects levels 2 and 4, falls at 486.1 nm; Hy falls at 434.0 nm; H8 at 410.1 nm, and so on. The series ends at the Balmer limit in the ultraviolet at 364.6 nm.

Bappu, (Manali Kallat) Vainu (1927-82) Indian astronomer and director of the Kodaikanal Observatory who, while at Mount Wilson and Palomar Observatories, discovered with Olin Chaddock Wilson (1909-94) the Wilson-Bappu effect, relating the luminosity of a late-type star to the strength of its calcium K line. He pioneered ultra-low dispersion spectroscopy of stars and galaxies, and made important studies of red stars in the Magellanic Clouds and of planetary ring systems.

bar Non-SI unit by which pressure is measured. It is particularly used in astronomy to describe the pressure in a planetary atmosphere, one bar being equivalent to the average atmospheric pressure on the Earth at sea level. Atmospheric pressure is often quoted in millibars, one bar being equal to 1000 millibars.

barium star Giant gstaror kstarwith an excess of carbon, barium, strontium and other heavy elements (including those from slow neutron capture) in its atmosphere. Barium stars have white dwarf companions. They most likely result from mass transfer that took place when the current barium star was a dwarf and the white dwarf was an evolving, chemically enriched, mass-losing giant. Barium stars are loosely related to CH and dwarf carbon stars.

Barlow lens Concave (negative) lens placed between a telescope objective and eyepiece to increase the magnification, usually by two or three times. The negative lens reduces the angle of convergence of the light cone, effectively making it appear to the eyepiece that the primary has a longer focal length. It was invented in the early 1800s by English physicist Peter Barlow (1776-1872).

Barnard, Edward Emerson (1857-1923) American astronomer noted for his discoveries of comets and Jupiter's satellite amalthea, and for his use of photography to map the Milky Way. While still an amateur astronomer he came to notice for his discovery of eight comets, which allowed him to overcome poverty and gain an education at Vanderbilt University. His observing skills won him a position at the new Lick Observatory in 1887, where he became the first astronomer to discover a comet using photography. At Lick, Barnard used the 36-inch (0.9-m) refractor to prove the transparency of Saturn's C Ring (1889) and to discover Amalthea (1892), the last planetary satellite to be found without the aid of photography. Three years later, he moved to the Yerkes Observatory, where he remained for the rest of his life, making numerous detailed visual observations of the planets and comets with the 40-inch (1-m) refractor. He used Yerkes' 10-inch (250-mm) Bruce Photographic Telescope to take wide-field images of the Milky Way's star clouds, discovering dark nebulae. This work, culminating in his classic A Photographic Atlas of Selected Regions of the Milky Way (1927), continued photographic surveys of the Galaxy that Barnard had started at Lick in the 1890s using the 6-inch (150-mm) Willard wide-field telescope. He also discovered (1916) the star with the largest proper motion, named barnard'sstarin his honour.

Barnard's Loop (Sh2-276) Large, diffuse and very faint arc of nebulosity centred on the Orion ob association, which is the group of hot stars south of Orion's Belt. The nebula is visible only on the eastern and southern sides of the constellation. Barnard's Loop was named after the pioneer astronomical photographer Edward Emerson barnard, who discovered it in 1894. If it were a complete ring, Barnard's Loop would almost completely fill the region between Betelgeuse and Rigel, covering almost 20 of sky. Barnard's Loop appears to be a structure resulting from clearance of dust and gas in the interstellar medium by radiation pressure from the Orion OB association stars.

Barnard's Star Closest star to the Sun after the a Cen-tauri triple system, lying 5.9 l.y. away in the constellation Ophiuchus, with a visual magnitude of only 9.54, well below naked-eye visibility. It is a red dwarf, spectral type M4, with only 0.05% of the Sun's luminosity. Discovered in 1916 by Edward Emerson barnard, it has the largest proper motion of any star, 10".4 per year - so fast that it moves across the sky a distance equivalent to the Moon's apparent diameter in under two centuries. In the 1960s and 1970s the Dutch-American astronomer Peter Van de Kamp (1901-95) reported an apparent wobble in the proper motion of the star, which he attributed to the presence of two orbiting planets similar in size to Jupiter and Saturn. This was not confirmed, and the star's apparent 'wobble' seems to have been due to instrumental error (see also extrasolar planet).

barn-door mount See scotch mount

barred spiral galaxy Spiral galaxy in which the distribution of stars near the nucleus is elongated in the disk plane. Strong bars are reflected in the hubble classification. Weaker bars are common among spirals, with the de Vaucouleurs types indicating that about 60% of spirals have distinct bars. Infrared observations, less sensitive to dust absorption and the confusing influence of young stars, may indicate an even greater fraction of spirals with small or weak bars, among them our Galaxy. Stellar dynamics in a disk can produce a bar because of instabilities, and a bar in turn may dissolve into a ring structure over cosmic time. Indeed, many bars are accompanied by stellar rings. Bars can alter the chemical content of a galaxy's gas, since flow along the bar mixes gas originally located at a wide range of radii, which thus started with different chemical compositions.

barred spiral galaxy NGC 1365 in Fornax has a prominent bar and open arms. The blue areas in the spiral arms are regions of intense star formation, caused by the gravitational influence of the bar.

Barringer Crater See meteor crater

barycentre Centre of mass of two or more celestial bodies. For example, when computing the perturbing effect of the Earth and Moon on other planets it is usual to regard the Earth and the Moon as a single body of their combined mass, located at their barycentre. Similarly when computing the orbits of the five outer planets one would treat the Sun and the four inner planets as a single body of their combined mass located at their barycentre.

barycentric dynamical time (TDB) See dynamical time

baryon Elementary particle that participates in the strong interaction. Two common baryons are protons and neutrons. Other, more massive baryons, such as the lambda and sigma particles, are commonly called hyperons. They have spins of J, and a baryon number of +1, as well as even intrinsic parity.

basalt Volcanic rock (solidified lava) that consists mostly of the minerals pyroxene (Mg,Fe,Ca)2Si2O6 and plagio-clase NaAlSi3O8-CaAl2Si2O8. Basalts are abundant on Earth, the Moon, Venus and Mars, forming vast plains and volcanic constructs. They are probably abundant on Mercury and on Jupiter's satellite Io, where they also form plains. Basalts make up the surfaces of some asteroids, such as vesta. Basalt is formed by solidification of partially melted planet mantle material.

basin Extensive topographically depressed area. In planetary science the term basin is commonly used with the adjective multi-ring. Multi-ring basins are circular depressions of a few tens to about 4000 km (2500 mi) in diameter; they typically have an elevated rim and one or two concentric rings of elevated terrain inside. They are observed on Mercury, Venus and Mars, as well as on large satellites, including the Moon. A multi-ring basin is an impact feature created by a collision with a comet or an asteroid: it is a type of impact crater. The onset diameter at which an impact crater becomes sufficiently large to form a multi-ring basin depends on the character of the target material (for example rock or ice) and on the body's gravitational acceleration. The onset diameter is 40-70 km (20-40 mi) on Venus, 100-130 km (60-80 mi) on Mercury, 130-200 km (80-120 mi) on Mars, and 200-300 km (120-190 mi) on the Moon. The largest such structure known is the 4000-km (2500-m) VALHALLA on Jupiter's satellite CALLISTO. A prominent basin on the Moon's far side, called the SOUTH POLE-AITKEN BASIN, is about 2500 km (1600 mi) in diameter and 12 km (7 mi) deep. Next in size is the HELLAS PLANITIA basin on Mars, which is about 1800 km (1100 mi) in diameter and 5 km (3 mi) deep. Impacts of such size happened in the early history of the Solar System and are not known in later times.

Bayer, Johann (1572-1625) German magistrate of Augsburg and amateur astronomer, who in 1603 published his Uranometria star ATLAS, which identified each constellation's main stars by Greek letters; this convention of so-called Bayer letters is still in use. Uranome-tria, which consisted of 49 constellation charts, with stellar data taken from the work of Tycho BRAHE, is notable as the first star atlas to depict constellations around the south celestial pole. These constellations -Apus, Chamaeleon, Dorado, Grus, Hydrus, Indus, Musca (which Bayer called Apis, the Bee), Pavo, Phoenix, Triangulum Australe, Tucana and Volans - had recently been defined by the Dutch navigator Pieter Dirkszoon Keyser (c.1540-96), based on earlier observations by Amerigo Vespucci and others.

BD Abbreviation of bonner durchmusterung

Beagle 2 UK Mars lander, almost 50% funded by the EUROPEAN SPACE AGENCY (ESA), which will be launched in 2003 June, flying piggyback on ESA's MARS EXPRESS orbiter. The spacecraft will arrive at Mars in 2003 December. The 30-kg Beagle 2 will land at a site in the ISIDIS PLANITIA region at about 13N latitude, which is the best site given the constraints for a safe landing in a smooth area and the scientific objectives of the mission. Beagle will take samples of Martian soil and analyse it for signs of water and bacterial activity, as well as investigating the chemical isotopes present. It will also measure methane in the atmosphere and send back images of the surface. Beagle is being part-sponsored by commercial companies. The project is being managed by the UK's Open University. It will make a bouncy landing encased in inflated balloons, which will be detached as it comes to rest. If Beagle 2 touches down successfully on Mars, it will become only the fourth spacecraft, after the US VIKINGS 1 and 2 in 1976 and MARS PATHFINDER in 1997, to do so.

Beagle 2 This artists impression of the Beagle 2 lander shows it with its solar panels open and its sampling arm deployed. The experiments are intended to discover whether the conditions for life ever existed, or still exist, on Mars. [All Rights Reserved Beagle 2. http://www.beagle2.com]

Becklin-Neugebauer object (BN object) Strong infrared source seen in the sky within the Orion Nebula but actually located behind that nebula inside the KLEIN-MANN-LOW NEBULA. It is thought to be a young B-type star, the optical and ultraviolet radiation of which is absorbed by a dense expanding envelope of dust that surrounds the star and then re-emits the energy as infrared radiation. It was found in 1967 by the American astronomers Eric Becklin and Gerry Neugebauer.

Becrux (Mimosa) The star p Crucis (of which 'Becrux' is a contraction), visual mag. 1.25, distance 353 l.y., spectral type B0.5 III. It is a BETA CEPHEI STAR, a type of eclipsing binary, with a period of 0.2 days; the total range of variation is less than 0.1 mag., which is too slight to be noticeable to the eye.

BeCvar, Antonin (1901-65) Czech astronomer, meteorologist and celestial cartographer who founded and directed (1943-50) the Skalnate Pleso Observatory, which became known for its solar astronomy and photography of meteors using a specially constructed battery of wide-field cameras. Becvar is best known for his celestial ATLASES. Atlas coeli (1950) charts 35,000 objects to the visual magnitude limit of 7 and many star clusters, nebulae and galaxies. This atlas was the first to include the many extraterrestrial radio sources discovered after World War II

Bede, 'The Venerable' (c.673-735) First English astronomer. His arguments for a spherical Earth overturn the myth that all medieval people believed the world to be flat. Much of Bede's astronomy was concerned with updating the calendar and calculating the date of Easter from the Moon's phases, and as such formed part of his wider historical and theological studies. His calendrical formulae came to be used across Europe, and he regularized the use of AD dating (from the birth of Christ).

Beer, Wilhelm (1797-1850) German banker and amateur astronomer who, with Johann Heinrich MADLER, compiled Mappa selenographia (1837), which surpassed all previous maps and catalogues of lunar features. It contained accurate measures for the heights of over 800 lunar mountains/crater rims and the diameters of almost 150 craters. The two German astronomers also produced the first map of Mars to show its albedo features (1830).

Beehive See PRAESEPE

Beijing Astronomical Observatory (BAO) Major research institute of the Chinese Academy of Science, founded in 1958. The observatory has five observing stations of which Xinglong is the main optical/infrared observing site. Situated about 150 km (95 mi) north-east of Beijing at an elevation of 960 m (3150 ft), the observatory hosts China's largest telescope, a 2.16-m (85-in.) reflector. There is also a 1.26-m (50-in.) infrared telescope. Xinglong will be home to the giant LARGE SKY AREA MULTI-OBJECT FIBER SPECTROSCOPIC TELESCOPE (LAMOST). At the BAO's radio astronomy site at Miyun is the Metre-Wave Aperture Synthesis Radio Telescope (MSRT) used for survey astronomy. It consists of 28 dishes 9 m (30 ft) in diameter. see also ANCIENT BEIJING OBSERVATORY

Belinda One of the small inner satellites of URANUS, discovered in 1986 by the VOYAGER 2 imaging team. Belinda is c.68 km (c.42 mi) in size. It takes 0.624 days to circuit the planet at a distance of 75,300 km (46,800 mi) from its centre. It has a near-circular, near-equatorial orbit.

Bellatrix The star y Orionis, visual mag. 1.64, distance 243 l.y., spectral type B2 III. Its name is derived from a Latin term, meaning 'the female warrior', and was first applied by medieval astrologers.

Bell Burnell, (Susan) Jocelyn (1943- ) British astronomer who discovered the first four pulsars. She made the discovery in 1967 with a radio telescope that she built while she was still a graduate student of Antony hewish at Cambridge University, but she did not share the Nobel prize subsequently awarded to Hewish

Benetnasch Alternative name for the star Majoris. See alkaid Ursae

Bennett, Comet (C/1969 Y1) Bright long-period comet discovered by John Caister (Jack) Bennett, South Africa, on 1969 December 28. The comet reached perihelion, 0.54 AU from the Sun, on 1970 March 20, approaching Earth in the following week. Comet Bennett became a prominent object in northern hemisphere skies, reaching peak magnitude +0.5 during April. At this time, Comet Bennett developed a strong dust tail 20 long and a very active, rapidly changing ion tail.

Bepi Colombo European space agency (ESA) mission to be launched in 2009 in conjunction with Japan to explore the planet mercury. After a two-year journey, including a fly-by of Venus, Bepi Colombo should become the second spacecraft to orbit Mercury (after the US messenger) and will deploy a lander and a small sub-satellite to study the magnetosphere. This surface pene-trator will send back data on the structure of the planet. Bepi Colombo will be powered by a solar electric propulsion system and will be protected against temperatures exceeding 673 K.

BeppoSAX (Satellite for X-ray Astronomy) Italian-Dutch gamma-ray and x-ray astronomy satellite. Soon after its launch in 1996 April, as the Satellite for X-ray astronomy, it was renamed in honour of Italian physicist Giuseppe Occhialini, whose nickname was 'Beppo'. It carries four spectroscopes and two wide-field cameras. In 1997 it played a major role in identification of gamma-ray bursts by pinpointing X-ray emissions from these events, so enabling their positions and cosmological distances to be confirmed.

Berkeley Illinois Maryland Association (BIMA) Consortium consisting of the Radio Astronomy Laboratory of the University of California (Berkeley), the Laboratory for Astronomical Imaging at the University of Illinois (Urbana) and the Laboratory for Millimeter-Wave Astronomy at the University of Maryland, which operates the BIMA Millimetre Array at hat creek observatory.

Bessel, Friedrich Wilhelm (1784-1846) German astronomer and mathematician who was the first to measure stellar parallax. In 1804, while employed as a shipping clerk, he calculated the orbit of Halley's Comet, which impressed Wilhelm olbers sufficiently to recommend that he be hired as assistant to Johann schroter. At Schroter's private observatory in Lilienthal, Germany, Bessel observed Saturn and its rings and satellites, and comets. He continued to develop mathematical methods for celestial mechanics; it was while trying to solve the three-body problem of celestial mechanics (1817-24) that he developed the class of mathematical functions that bear his name (see besselian elements). He reduced the positions for 3222 fundamental stars first observed by James bradley at Greenwich Observatory between 1750 and 1762; his 1818 publication of over 63,000 star positions and proper motions, based on his and Bradley's observations, are considered the start of modern astrometry. In 1809 Friedrich Wilhelm III of prussia appointed Bessel director of a new observatory at Konigsberg.

Bessel's most famous achievement was his 1838 measurement of the parallax of the star 61 Cygni. He chose this relatively obscure star because of its high proper motion. Using Konigsberg's Fraunhofer heliometer, Bessel determined that 61 Cygni has a parallax of 0".31 (very close to the modern value of 0".29), implying that the star was 10 l.y. from Earth. The Royal Astronomical Society awarded him its Gold Medal for this achievement, which marked the first step towards accurately measuring stellar distances. Bessel's 1840 paper on perturbations to Uranus' orbit suggested the existence of an eighth planet, six years before the discovery of Neptune by Johann galle. In 1841/1844 he predicted the existence of dark companions of Sirius and procyon after discovering periodic variations in their proper motions - Sirius B was not confirmed until 1862.

Besselian elements These arise in a method devised by Friedrich bessel for calculating the circumstances of an eclipse. For a solar eclipse a reference plane is used that is normal to the line passing through the centres of the Sun and Moon, and which passes through the centre of the Earth. The Earth's limb and the shadow of the Moon are projected on to this plane, and an eclipse will occur if the shadow intersects the limb. The Besselian elements define the location of the centre of the Moon relative to the centre of the Earth and the radii of the umbral and penumbral shadows within this plane, and the direction of projection.

Be star bstar that shows a characteristic B-type spectrum with the addition of hydrogen emission lines. The emissions are commonly doubled, revealing a circumstel-lar disk with one side approaching the observer, the other receding. Be-shell stars have thicker disks and metallic absorptions. Common among class B stars, Be stars are all rapid rotators with equatorial speeds that can exceed 300 km/s (190 mi/s). The origins are contentious, and include rotation, magnetic fields and pulsation.

Beta Centauri See hadar

Beta Cephei star (Beta Canis Majoris star) Short-period pulsating variable star, of spectral type O8-B6, with light and radial-velocity periods of 0d.1 to 0d.6, and amplitudes of mags. 0.01 to 0.3V.

Beta Lyrae (Sheliak) Variable star that shows continuous variations in brightness; it is the prototype for one of the three subtypes of eclipsing binary. The Arabic name, Sheliak, comes from the Arabic for 'harp'. The star lies at a distance of 882 l.y.; its magnitude varies between 3.3 and 4.4. Small telescopes reveal a faint, mag. 7.2 companion. The variability and periodicity of Beta Lyrae were discovered in 1784 by John goodricke. The first complete light-curve, obtained in 1859 by F.W.A. argelander, demonstrated that Beta Lyrae is an eclipsing binary.

All beta lyrae stars show continuous magnitude variation between minima, resulting in a light-curve that has distinct primary and secondary minima. The double-humped light-curve of Beta Lyrae indicates that the stars are deformed into ellipsoids by their mutual gravitational interaction. Because the period is only 12.93854 days, the two stars must be so close to each other it becomes inevitable that significant tidal distortion in their shapes occurs. The light-curve can be explained by the changing total amount of stellar surface area presented to the Earth-based observer as the elongated stars revolve about each other.

Spectroscopic observations in the 1950s established that the primary star is a late-type B giant (B8.5II), but classification of the secondary was confounded by peculiar spectral features, some of which are caused by gas flowing between the stars and around the system as a whole. The secondary star is roughly four to five times more massive than the primary. According to the mass-luminosity relation, however, the more massive a star is, the brighter it is. Astronomers were puzzled because the massive secondary appeared significantly under-luminous, much dimmer than the primary.

Current theories about Beta Lyrae, supported by observations, suggest that the secondary star is enveloped in a thick accretion disk of gas captured from the primary. Material surrounding the secondary so severely subdues its luminosity that the star itself is impossible to observe. The primary star is overflowing its roche lobe, and gas streams across the inner lagrangian point on to the disk at the rate of 10~; solar masses per year. Ultraviolet spectra from the OAO-3 (Copernicus) and Skylab orbiting observatories revealed clouds of gas at the L4 and L5 Lagrangian points. Gas is constantly escaping from the system.

Beta Lyrae star eclipsing binary (subtype EB) that shows a continuous variation in brightness throughout the orbital period, which is generally of one day or more in length. Both primary and secondary minima are always present. Although once thought to be contact binaries (that is, systems that have tidally distorted components with a common atmospheric envelope, as in w ursae majoris stars), the majority of Beta Lyrae stars appear to be semidetached systems in which gas escaping from a bloated primary star is falling on to an accretion disk surrounding the secondary star. Such a configuration is found in beta lyrae, which is the prototype for this type.

Beta Lyrae star The continuous light-curve of Beta Lyrae stars is believed to be caused by an accretion disk of material which surrounds the secondary star and eclipses the primary. This theory also explains why the secondary star appears to be dimmer than the primary, despite being far more massive.

Astronomers now regard Beta Lyrae variables as members of a broad class of double stars known as semidetached binaries, so called because only one of the two stars fills its roche lobe. They are probably binaries near the end of the initial rapid phase of mass transfer. Thus, some Beta Lyrae variables may evolve into Algol-type binaries (see algol star). For example, as the primary star in a Beta Lyrae variable continues to evolve away from the main sequence, it will begin to resemble a red subgiant. As the rate of mass transfer subsides, the accretion disk shrouding the secondary may become transparent and a massive unevolved star will shine forth. The resulting system clearly would resemble algol. Beta Lyrae stars, which account for about 20% of all eclipsing variables, are therefore an important but temporary stage in the early evolution of close binary systems. See also stellar evolution beta particle electron free of the atomic nucleus. It is a common by-product of nuclear reactions and decays.

Beta Pictoris main-sequence A5 star in the constellation Pictor; it has visual magnitude 3.9 and is at a distance of 62 l.y. The star is surrounded by a protoplanetary disk, which is edge-on as seen from Earth. One of the nearest planetary systems, it has been imaged by the Hubble Space Telescope and ground-based telescopes. The disk is about 400 AU in diameter, some four times the diameter of our Solar System. Both dust and hydrogen gas have been detected in the disk. Spectroscopic studies and dynamical modelling indicate that larger solid bodies (planetesimals) are present, and planets may have formed or be in the process of formation.

Beta Pictoris This Very Large Telescope (VLT) falsecolour image of the protoplanetary disk around Beta Pictoris shows that it is not even in composition. The visible kink in the disk is about 20 AU from the star, which corresponds roughly to the distance of Uranus from the Sun.

Beta Regio One of the three main uplands on venus, discovered by the pioneer venus orbiter's radar. Beta Regio's 2000 X 3000 km (1200 X 1900 mi) extent is traversed by the north-south trough of Devana Chasma, generally inferred to be a fault complex comparable with the East African Rift. Superimposed on the rift at the southern end of Beta Regio is Theia Mons, a huge volcano 226 km (140 mi) in diameter and 5 km (3 mi) high. Satellite tracking shows that gravity is high over Beta Regio.

Beta Regio The rift valley Devana Chasma in Beta Regio is as much as 20 km (12 mi) wide and here has cut through an earlier impact crater, which is itself 37 km (23 mi) across. The whole eastern side of the crater has collapsed and fallen into the rift.

Beta Taurids daylight meteor stream active between June 5 and July 18, with peak around the end of June. The stream is associated with Comet 2P/encke and is encountered again in October and November when it produces the night-time taurids. The tunguska airburst of 1908 June 30 may have been produced by a particularly large piece of debris from the Beta Taurid stream.

Betelgeuse The star a Orionis, marking the right shoulder of Orion, distance 427 l.y. It is a red supergiant of spectral type M2 Ib, about 500 times the Sun's diameter and 10,000 times as luminous. Betelgeuse is an irregular variable with a considerable range, varying between about mags. 0.0 and 1.3 over a period of years. At its average value, magnitude 0.5, Betelgeuse is the 10th-brightest star. The origin of the name is often said to be from the Arabic ibt al-jauzd', meaning 'armpit of the central one' but may come from bait al-jauzd', 'house of the twins', a reference to neighbouring Gemini.

Betelgeuse The Hubble Space Telescopes first direct image of a star other than our Sun revealed that Betelgeuse ( Orionis) has a bright spot more than 2000 K hotter than the rest of the stars surface. The exact nature of the spot is not yet understood.

Bethe, Hans Albrecht (1906- ) German-American atomic physicist who discovered the basic nuclear reactions that generate energy inside stars. After receiving an education in theoretical physics and holding teaching positions at the universities in Frankfurt, Stuttgart, Munich and Tubingen, Bethe emigrated to England, then to the USA, where he joined the faculty of Cornell University in 1935. During World War II he made significant contributions to the Los Alamos Science Laboratory's Manhattan Project, which developed the first atomic bomb, but this work convinced him of the importance of nuclear arms control, which he has since strongly advocated.

In the 1930s and 1940s, Bethe developed his models for stellar nuclear processes. He found that for stars of modest mass like the Sun, the most important nuclear reaction converts two protons (atomic hydrogen nuclei) into helium; Bethe also discovered that several protons can also combine with a carbon nucleus inside Sun-like stars to regenerate a carbon nucleus and helium 'ash'. He demonstrated that the powerful gravity of neutron stars, which are much more massive than the Sun, is sufficient to fuse protons and electrons to make neutrons. Bethe also modelled the carbon-nitrogen cycle of reactions, which powers many types of massive stars. In 1947 Bethe explained the Lamb shift observed in the spectrum of the hydrogen atom with a model that laid the foundations of a new branch of physics known as quantum electrodynamics.

Bianca One of the small inner satellites of uranus, discovered in 1986 by the voyager 2 imaging team. Belinda is about 44 km (27 mi) in size. It takes 0.435 days to circuit the planet at a distance of 59,200 km (36,800 mi) from its centre in a near-circular, near-equatorial orbit.

Biela, Comet 3D Comet discovered by Jacques Montaigne at Limoges, France, on 1772 March 8 and again by Jean Louis pons on 1805 November 10. It was next noted by Wilhelm von Biela, an Austrian army officer and amateur astronomer, on 1826 February 27. He realized that it was the same as that which had already been observed in 1772 and 1805. It was subsequently named Biela's Comet and shown to be periodic, with a period of 6.6 years. It was seen again in 1832 and 1845-46.

On 1845 December 19 the comet appeared elongated and by the end of the year had split in two. By 1846 March 3 the two parts were over 240,000 km (150,000 mi) apart. At its next return, in 1852, the separation had increased to 2 million km (1.2 million mi). The comet was never seen again, but in November 1872 and 1885 tremendous showers of andromedid meteors -produced by debris released on the comet's disintegration - occurred on exactly the date when the Earth passed close to the comet's orbit.

Bielids Alternative name for the andromedids

Big Bang theory Theory concerning the explosive creation of the Universe from a single point. The first real strides in the study of cosmology occurred when Edwin hubble and his collaborators noticed that all of the galaxies in the Universe, except for those in our local cluster, were receding from us. This fact was determined by measuring the redshift of the absorption lines in galaxy spectra. It was further noted that the more distant the galaxy, the faster the velocity of recession. If the galaxy velocities were extrapolated back into time, then all galaxies and stars apparently started at some time in the distant past at a single point. This cosmic singularity included not only all of the gas, dust, stars, galaxies and radiation, but all of spacetime as well.

Two types of models came out of these observations and theoretical arguments from general relativity. The steady-state theory pictured the Universe as expanding, with matter created between separating galaxies and clusters at precisely the rate needed to keep the average properties of the Universe the same, that is, homogeneous and isotropic. This idea suggested there was no beginning and will be no end, just constant expansion and filling in. The other major cosmological model, the Big Bang theory, held that the entire Universe emerged from a single point in an explosive event called the Big Bang. general relativity and the idea of a Big Bang are both consistent with the fact that more distant galaxies separate from each other more rapidly, following a mathematical relationship known as the hubble law.

The Big Bang model predicted that the temperature of the very early universe was on the order of 109 K. At the start of the expansion, between T = 0 and T = 10 ~43 s (the planck time), the laws of physics are not well understood; in fact we would need a 'theory of everything' to be able to describe this era. The temperature continued to drop as the universe expanded. Between 10 ~43 and 10 ~35 s the universe had just two fundamental forces - gravity and the gut force. After 10~35 s, the strong force separated from the electroweak force, but the universe was still far too hot for atoms to form. Finally, after about 10~12 s the weak and electromagnetic forces became distinct and the four fundamental forces looked as they do today. At 10 ~6 s the temperature had dropped sufficiently for the nuclei of atoms to form. Significant nucleosynthesis could begin and the cosmic abundance of elements was determined, primarily hydrogen and helium. The next significant event was at roughly T = 300,000 years after the Big Bang. At this time the universe became transparent to photons and the radiation decoupled from matter. As the universe continued to expand, matter evolved separately from the cosmic microwave background (CMB) and atoms formed. Gravity caused small inhomogeneities in the matter distribution to collapse and form stars, galaxies and galaxy clusters.

The Big Bang model made a significant prediction: the Universe should have a temperature, and the temperature could be measured by looking at the CBR that permeates the entire Universe. Calculations within the framework of the Big Bang indicated that as the Universe expanded, the temperature of the CBR should constantly drop and the current temperature should be around 5 K. Two engineers from Bell Labs, Arno penzias and Robert Wilson, were trying to determine why the Holmdel horn antenna, used to communicate with satellites, constantly picked up static at microwave frequencies. They failed to find a terrestrial or electrical explanation for the static and it was finally deduced that the noise was actually the CBR. This discovery, that the Universe has background radiation at a temperature very close to the theoretical predictions of the Big Bang model, was the most convincing piece of evidence that the Universe actually began as a condensed cosmic singularity many billions of years ago. Later observations by balloon-borne telescopes and the NASA satellite COBE precisely mapped this background radiation, even measuring anisotropies and our own motion relative to it.

Along with the successes of the Big Bang theory, there were also some problems. The horizon problem, the flatness problem, the lack of magnetic monopoles and the smoothness of the CBR all presented problems for the Big Bang model. Since the Universe is expanding rapidly in all directions, the two opposite horizons would be out of causal contact, and therefore would not have to show the same level of homogeneity as they do; this is the crux of the horizon problem. The flatness problem suggests that the Universe is so close to critical density that the initial conditions must have been fine tuned to extraordinary precision. The lack of magnetic monopoles present in the Universe was also considered a problem. After completely mapping the CBR, there seemed to be discrepancies between the amount of mass seen in the Universe and the critical density inferred from the smoothness of the CBR. The idea of an inflation epoch in the early universe solved both the horizon and flatness problems. Inflationary theory postulates that in the very early universe, just after T = 10~34 s, the universe expanded rapidly and increased in size by a factor of 1050. This rapid expansion was driven by Higgs field symmetry breaking. Because the universe actually expanded faster than the speed of light, the horizons were actually in causal contact in the early universe, thus disposing of the horizon problem. The flatness and magnetic monopole problems also go away since we see only a small part of the entire Universe. There remained a problem. The density of the Universe derived from the CBR should be equal to the critical density according to the Big Bang theory with inflation. Observations of the luminous matter in the Universe fell short of the critical density by a factor of four or more.

As observations from space platforms improved, scientists concentrated on trying to derive an accurate and consistent value for the hubble constant. Determining this constant required the distances to distant galaxies to be measured independently of the redshift using bright standard candles like supernovae. Results from supernova studies indicated that the Universe was not expanding according to a constant value for the Hubble constant, but is accelerating. Far from invalidating the Big Bang theory, however, it actually explains the aforementioned problems with the Big Bang theory. To explain the acceleration, either the cosmological constant must be non-zero or there exists a different form of unobserved matter that has negative gravity, recently termed quintessence. Either the non-zero cosmological constant or the presence of this exotic quintessence drives the expansion of the Universe.

Betelgeuse The Hubble Space Telescope's first direct image of a star other than our Sun revealed that Betelgeuse (a Orionis) has a bright spot more than 2000 K hotter than the rest of the star's surface. The exact nature of the spot is not yet understood.

Big Bear Solar Observatory (BBSO) Optical solar observatory at Big Bear Lake, California, built by the california institute of technology in 1969. Now managed by the New Jersey Institute of Technology, the observatory monitors the Sun with several optical telescopes, the largest of which has an aperture of 0.65 m (26 in.). Its location in the middle of the lake provides stable daytime observing conditions, eliminating the bad seeing caused by heat haze over land. BBSO participates in the global oscillation network group.

Big Bear Solar Observatory The suite of telescopes in the dome of the Big Bear Solar Observatory observe the Sun in visible, near-infrared and ultraviolet light, thus giving astronomers detailed information about different parts of the solar surface and atmosphere. This information helps them to monitor and understand the way in which features on the Suns surface form and evolve.

Big Dipper Popular (chiefly US) name for the saucepan shape formed by seven stars in Ursa Major, also known as the plough.

BIMA Abbreviation of berkeley illinois maryland association

binary pulsar Pulsar orbiting another star, forming a binary star system. The first binary pulsar to be discovered was PSR 1913+16, in 1974, for which its discoverers, Joseph Taylor and Russell hulse, were awarded the 1993 Nobel Prize for physics. The pulsar, with a period of 0.059 seconds, orbits a neutron star with a period of almost 8 hours. Pulses from the companion neutron star have not been detected, but this might only be the result of an unfavourable viewing angle. The pulses from the pulsar arrive 3 seconds earlier at some times relative to others, showing that the pulsar's orbit is 3 light-seconds across, approximately the diameter of the Sun. Since this is a binary system, the masses of the two neutron stars can be determined, and they are each around 1-3 times the mass of the Sun. Observations have shown that the pulsar's orbit is gradually contracting, due to the emission of energy in the form of gravitational waves, as predicted by Einstein's theory of general relativity, causing the pulsar to reach periastron slightly early. Also, periastron advances 4 per year in longitude due to the gravitational field (thus the pulsar's periastron moves as far in a day as Mercury's moves in a century).

binary star Double star in which the two components are gravitationally bound to each other and orbit their common centre of mass. More than half of stars observed are binary or belong to multiple systems, with three or more components. In a close binary the stars interact directly with each other. Other double stars are optical doubles, which result when two stars appear to be close because they lie almost on the same line of sight as viewed from Earth, but in reality lie at a vast distance from each other. Binary systems are classified in a variety of ways, including the manner in which their binary nature is known.

binary star The two components of a binary system move in elliptical orbits around their common centre of gravity (G), which is not half way between them but nearer to the more massive component. In accordance with Keplers second law of motion, they do not travel at uniform rates. In (2) they are greatly separated and moving slowly. In (6) they are closer and moving more rapidly.

Common proper motion binaries are seen as distinct objects that move across the sky together. No orbital motion is observed because they lie so far apart that their orbital period is very long visual binaries are pairs that are resolved into separate components and for which orbital motion is observed astrometric binaries are systems where only one component is seen. This star is observed to 'wobble' in a periodic manner as it moves across the sky, a movement caused by the gravitational pull of the unseen companion.

Spectrum binaries are systems where a spectrogram of an apparently single star reveals two sets of spectral lines characteristic of two different types of stars.

Spectroscopic binaries are systems where only one star is observed directly, but its spectrum shows one or two sets of spectral lines, which show a doppler shift, indicating orbital motion. These are known as single- and double-lined spectroscopic binaries respectively.

Eclipsing binaries are systems aligned so that one or both members of the system are periodically observed to pass in front or behind the other, causing the total luminosity of the system to fluctuate in a periodic manner.

In general the nature of a binary system can be ascertained by the distance between the pair and thus the orbital period. Widely separated pairs that do not influence each other are known as double stars. Common proper motion pairs and visual binaries have the largest separation and longest orbital periods, which can be up to several million years. Eclipsing and spectroscopic binaries generally have the smallest separation, and their orbital periods can be as short as a few hours. Binaries with non-main-sequence components, such as white dwarfs, neutron stars or black holes, can have orbital periods of as little as a few minutes, although these systems are short-lived, with lifetimes less than about 10 million years.

By convention, systems with orbital periods of less than 30 years are termed close binaries because the short orbital period implies separation of less than about 10 AU (the distance of Saturn from the Sun). Stars that remain within their own roche lobes are termed detached binaries, while semidetached binaries have one star (often a red giant) filling its Roche lobe. In semidetached binaries, material can spill from the lobe-filling star through the first lagrangian point on to its companion, often by transfer via an accretion disk. This mass transfer has considerable effect on the evolution of the two components. If both stars fill their Roche lobes, they are termed contact binaries, and if material has escaped from the lobes to surround both stars, the system is known as a common envelope binary.

The first visual binary star was discovered by Joannes Baptista riccioli in 1650, when he observed mizar and saw that the star was actually double, with a separation of 14" (this was distinct from the known companion alcor). Small numbers of binaries were accidentally found by telescopic observers in the course of the next century, but it was not until 1767 that John michell argued that stars appearing close together really were connected 'under the influence of some general law'. The observational proof of Michell's theory did not appear until 1804. William herschel had originally set out to determine the parallactic shift of the brighter (and, therefore, closer) member of an unequal pair of stars with respect to the fainter component. But for many stars, particularly castor, the motion of the companion with respect to the primary star could be explained only if the two stars were regarded as physically connected, rotating around a common centre of gravity. Later observations showed that the apparent motion of the companion described an arc of an ellipse.

In 1827 the French astronomer Felix Savary (1797-1841) was the first to calculate an orbit that would predict the motion of the companion star with respect to the primary. An important consequence of this was that the masses of the two stars were directly obtainable from elements of the true orbit.

What is actually observed, the apparent orbit, is the projection of the true orbit on to the plane of the sky. The seven elements needed to define the size, shape and orientation of the true orbit are calculated from observations of position angle and separation (see orbital elements). The position angle is the angle that the line between the stars makes with the north point, and the separation is the angular distance between the two stars in arcseconds. By plotting position angle and separation from measurements made at different times the apparent orbit can be drawn. From the elements of the true orbit, the size of the semi-major axis (a) and the period of revolution (P years) will allow the sum of the masses of the two stars to be calculated, provided that the parallax of the binary is known. The relation connecting these quantities is (from Kepler's third law): where M1 and M2 are in terms of the Sun's mass and G is the gravitational constant.

In order to derive individual masses, another relation between M1 and M2 is needed. This can be obtained by plotting the position of each star against the background of fainter, relatively fixed stars in the same field. The two stars will both appear to describe ellipses, which vary only in size depending on the mass of each star.

The determination of masses from spectroscopic binaries is more difficult than for visual binaries because it is not possible to determine the inclination of the apparent orbit unless eclipses occur. The best that can be done is to assume that the inclination is 90 and to calculate from this the minimum values that the masses are likely to take. See also blue straggler; cataclysmic variable; recurrent nova; x-ray binary binding energy Energy that is released when an atomic nucleus is formed; it is also called the mass defect because it is the difference in energy terms between the mass of the nucleus and the sum of the masses of its protons and neutrons. A star's energy is the binding energy that is released as hydrogen is built up to helium, carbon, oxygen, and so on. That process, however, stops at iron, for which the binding energy per nucleon is a maximum; building up heavier nuclei requires more energy than is released by the reaction. The consequent formation of an inert iron core in massive stars leads directly to type II supernova explosions. See also fission; fusion; nucleosynthesis binoculars Two, usually small, telescopes mounted side by side so that both eyes can be used simultaneously. Modern binoculars are usually refracting telescopes with fixed eyepieces and a set of built-in prisms to provide an upright, right-reading view (see porro prism). Binoculars are normally described by two numbers, for example, 7 X 50. The first number is the magnification, and the second number is the aperture in millimetres. Conveniently, the exit pupil in millimetres is calculated by dividing the second number by the first (about 7 mm in this example).

True binocular vision, where both eyes are used to form a stereoscopic view and gauge the distance to an object, is not effective when observing celestial objects. The distance from the objective lenses to the object is far too great compared with the distance between the lenses. However, apparent brightness and resolution are each improved when both eyes are used.

biosphere Shallow region over the surface of a planet that supports life. Earth's biosphere includes the land to about 8 km (5 mi) above mean sea level, the oceans to depths of at least 3.5 km (2 mi) and the atmosphere to a height of about 10 km (6 mi): in total, it is less than 20 km (j2 mi) thick. Recently discovered microbial organisms called extremophiles (see life in the universe) occupy a secondary subsurface biosphere that is believed to extend to 4 km (2.5 mi) beneath the surface of the continental crust and 7 km (4.3 mi) below the seafloor. Specific habitats within a biosphere are known as biomes. Only Earth is known to possess a surface and a subsurface biosphere. Mars, Europa and Titan are targets for future space missions that will search for extraterrestrial subsurface biospheres, and in Titan's case, perhaps a surface biosphere as well.

Biot, Jean-Baptiste (1774-1862) French physicist who co-discovered the Biot-Savart law, which concerns the intensity of a magnetic field produced by a current flowing through a wire. He demonstrated the extraterres-trial origin of meteorites, after investigating the meteorite shower at L'Aigle, France, on 1803 April 26. The following year he accompanied the chemist Joseph-Louis Gay-Lussac on a balloon flight, to an altitude of 4000 m (13,000 ft), in order to collect scientific data about the upper atmosphere.

bipolar flow Non-spherical flow of material from a star. Occasionally stars lose matter at copious rates, for example at the end of their protostellar lives, and during their red giant phases. If these flows are organized into two oppositely directed streams, they are called 'bipolar'. This phenomenon is responsible for the creation of the rapidly moving herbig-haro objects, flying away from the precursors of ttauri stars. Very narrow, highly confined radio-emitting jets characterize this youthful phase; these jets disturb the surrounding dark clouds and sweep up larger volumes of much slower molecular gas, also bipolar in pattern. For older stars the outflows may lead to bipolar planetary nebulae.

Usually the stars that generate bipolar flows are surrounded by extensive, flattened, dusty envelopes - huge toroids - orientated perpendicular to the star's rotation axis. The interplay of stellar rotation and mass outflow is believed to create the bipolar outflows. Binarity of the driving star is implicated in at least some flows, as also may be magnetic fields.

Birr Castle astronomy Astronomy practised at Birr Castle, near Birr, County Offaly (formerly Parsonstown, King's County) in Ireland, with the 'Leviathan of Parsonstown', a 72-inch (1.8-m) telescope completed by William Parsons, Third Earl of rosse, in 1845. Using local workmen whom he trained, Rosse cast mirrors of speculum metal (a highly reflective copper-tin alloy) and constructed first a 36-inch (0.9-m) reflecting telescope. Later came the 72-inch, which remained the largest in the world until 1917, when Mount Wilson Observatory's 100-inch (2.5-m) Hooker Telescope went into service. The Leviathan's 72-inch mirror had a focal length of 54 ft (16.5 m), and the instrument was suspended between two walls 56 ft (17.1 m) high. It could not be effectively driven, and was unsuitable for photography.

Despite these drawbacks, the telescope's great size enabled Rosse to discern the spiral structure of certain nebulae which are now known to be galaxies, most notably the Whirlpool Galaxy (M51), which he sketched. Birr became an astronomical centre, and Rosse himself became a skilful observer, making his results freely available. After his death in 1867, his son Laurence Parsons, later the Fourth Earl, took over the observatory. J.L.E. dreyer was assistant at Birr between 1874 and 1878, during which period he accumulated much of the information that was to be later published in his new general catalogue.

From 1900 activity gradually decreased; the Fourth Earl died in 1908, and the 72-inch was dismantled. In 1916 the last Birr astronomer, Otto Boeddicker (1853-1937), departed, and work ceased. In 1996-98 the 72-inch telescope was restored to working order with a new glass mirror.

BIS Abbreviation of british interplanetary society

Blaauw, Adriaan (1914- ) Dutch astronomer who specialized in the structure of the Milky Way galaxy. His research has covered the processes by which stars form, star clusters and associations, and measuring the cosmo-logical distance scale. Blaauw has done much to advance European astronomy, playing a key role in founding the european southern observatory and establishing the leading journal Astronomy & Astrophysics. He helped to plan the hipparcos astrometric satellite.

black body radiation Radiation emitted by an idealized perfect radiator. It has a continuous spectrum that depends only on the temperature of the source By kirchhoff'slaws the efficiency of the emission by a heated object at a particular wavelength is proportional to the efficiency of its absorption at the same wavelength. Thus an object that absorbs with 100% efficiency over the whole spectrum, known as a black body, will also be the most efficient when it comes to emitting radiation. A good practical approximation to a black body is a small hole in the side of an otherwise closed box.

The emission from a black body, known as black body radiation, is a good fit to the emission from many astronomical objects, including stars and interstellar dust clouds (spectrum lines are mostly minor deviations from the overall emission). Black body radiation follows a bell-shaped distribution given by the planck distribution. The peak of the distribution shifts to shorter wavelengths as the temperature increases (see wien'slaw). This leads to the common experience that at moderate temperatures objects glow a dull red, then change colour successively through bright red, yellow, white to blue-white as the temperature is increased. The total emitted energy increases rapidly with temperature, leading to the ste-fan-boltzmann law.

black drop Optical effect observed during the initial (ingress) and final (egress) stages of a transit of mercury or venus. Once the planet is fully projected on the Sun at ingress, but before its trailing edge breaks apparent contact with the solar limb, the expectation is of instantaneous separation. Instead, the planet seems to lengthen, with a dusky ligament appearing briefly to link it to the Sun's limb. This effect is the black drop, which may be likened to a drop of water before it falls from a tap. It is again seen at egress as the planet's leading edge approaches the opposite limb. The effect may be physiological in origin, but it has also been attributed to atmospheric turbulence and to instrument defects.

black dwarf Ultimate state of a white dwarf star. A white dwarf does not have a means of maintaining its heat, and it therefore cools steadily. Given long enough, it cools to invisibility, and is then called a black dwarf. Such objects are hypothetical: none have been observed, and it is doubtful whether our Galaxy is old enough for any to have cooled sufficiently to enter this state. See also stellar evolution

Black Eye Galaxy (M64, NGC 4826) spiral galaxy in the constellation Coma Berenices (RA 12h 56m.7 dec. + 2141'). It has a prominent lane of dark material close to its nucleus. The galaxy was originally discovered by J.E. bode in 1779. It has apparent dimensions of 9'.2 X 4'.6 and magnitude +8.5. The Black Eye Galaxy's true diameter is 65,000 l.y., and it lies 24 million l.y. away.

black hole Object that is so dense and has a gravitational field so strong that not even light or any other kind of radiation can escape: its escape velocity exceeds the speed of light. Black holes are predicted by Einstein's theory of general relativity, which shows that if a quantity of matter is compressed within a critical radius, no signal can ever escape from it. Thus, although there are many black hole candidates, they cannot be observed directly. Candidates are inferred from the effects they have upon nearby matter. There are three classes of black hole: stellar, primordial (or mini) and supermassive.

black hole Radio jets, spewing from the centre of a galaxy, and lobes of radio emission are the signatures of a supermassive black hole. The giant elliptical galaxy M87 (NGC 4486) houses the radio source Virgo A (false colours indicate the strength of the radio emission).

A stellar black hole is a region of space into which a star (or collection of stars or other bodies) has collapsed. This can happen after a star massive enough to have a remnant core of more than 2.3 solar masses (the Lan-dau-Oppenheimer-Volkov limit for neutron stars) reaches the end of its thermonuclear life. It collapses to a critical size, overcoming both electron and neutron degeneracy pressure, whereupon gravity overwhelms all other forces (see degenerate matter).

Primordial black holes, proposed by Stephen hawking, could have been created at the time of the big bang, when some regions might have got so compressed that they underwent gravitational collapse. With original masses comparable to that of Earth or less, these mini-black holes could be of the order of 1 cm (about half an inch) or smaller. In such small black holes, quantum effects become very important (see quantum theory). It is possible to show that such a black hole is not completely black, but that radiation can 'tunnel out' of the event horizon at a steady rate; such radiation is known as hawking radiation. This then could lead to the evaporation of the hole. primordial black holes could thus be very hot, and from the outside they could look like white holes, the time-reversals of black holes.

It seems that supermassive black holes of the order of 100 million solar masses lie at the centres of active galactic nuclei, extreme examples of which are quasars. It is thought there may also be supermassive black holes at the centres of ordinary galaxies like the Milky Way.

The lifetime of a black hole can be shown to be proportional to the cube of its mass. For black holes of stellar mass, their potential lifetime is of the order of 1067 years. Many primordial black holes will have evaporated away completely by now.

There are various different models of black holes. The most straightforward is that of the Schwarzschild black hole, a non-rotating black hole that has no charge. In nature, however, it is expected that black holes do rotate but have little charge, so the model of a rotating Kerr black hole with no charge is probably the most applicable. A Kerr-Newman black hole is rotating and has a charge. A Reissner-Nordstrom black hole is a non-rotating black hole with a charge.

The radius of a non-rotating Schwarzschild black hole of mass M is given by 2GM/c2, where G is the gravitational constant and c is the speed of light. When a star becomes smaller than this schwarzschild radius, gravity completely dominates all other forces. The Schwarz-schild radius determines the location of the surface of the black hole, called the event horizon. Only the region on and outside the event horizon is relevant to the external observer; events inside the event horizon can never influence the exterior. There is no lower limit to the radius of a black hole. Some of the primordial black holes could be truly microscopic.

When a stellar black hole first forms, its event horizon may have a grotesque shape and be rapidly vibrating. Within a fraction of a second, however, the horizon settles down to a unique smooth shape. A Kerr black hole has an event horizon that is flattened at the poles rather than circular (just as rotation flattens the Earth at its poles). What happens to matter after it crosses the event horizon depends on whether or not the star is rotating. In the case of a collapsing but non-rotating star that is spherically symmetric, the matter is crushed to zero volume and infinite density at the SINGULARITY, which is located at the centre of the hole. Infinitely strong gravitational forces deform and squeeze matter out of existence at the singularity, which is a region where physical theory breaks down. In a rotating Kerr black hole, however, the singularity need not be encountered. Rotating black holes have fascinating implications for hypothetical space travel to other universes.

The density of matter in a star as it crosses the critical event horizon need not necessarily be very high: its density could even be less than that of water. This is because the density of any body is proportional to its mass divided by its radius cubed, and the radius of a black hole is, as we have seen, proportional to its mass. These two facts combined imply that the density at which a black hole is formed is inversely proportional to the square of the mass. Take a supermassive black hole with a mass of from 10,000 to 100 million solar masses - the mass of a black hole that might be found at the centre of certain active galaxies. Such a collapsing mass would reach the black hole stage when its average density was roughly that of water. If the mass of the collapsing sphere were that of an entire galaxy, the average density of matter crossing the event horizon would be less than that of air.

Attempts to discover stellar black holes must rely on the influence of their gravitational fields on nearby matter, and/or their influences on the propagation of radiation in the vicinity of the hole. Black holes within BINARY STAR systems are potentially the easiest to detect because of the influence on their companion. Material is pulled from the companion into the black hole via an ACCRETION DISK. The frictional heating within the disk leads to the emission of X-rays (see X-RAY BINARY). The first candidate where one companion in a binary system is thought to be a black hole is the X-ray source CYGNUS X-1. At the position of this X-ray source lies a SPECTROSCOPIC BINARY star HDE 226868, which has a period of 5.6 days. More recently, all-sky monitors on space-borne X-ray observatories have discovered soft X-ray transients (SXTs); objects that produce rare, dramatic X-ray outbursts (typically separated by decades). Around 75% of SXTs contain black hole candidates.

The existence of supermassive black holes in quasars and as the central sources in active galactic nuclei is generally accepted as the means of explaining the phenomena observed. Many ordinary galaxies like our own show enhanced brightening at their cores, along with anomalously high velocities of objects near the centre, suggesting the existence of a black hole.

There is also the MISSING MASS PROBLEM: the density of the observable matter in our Universe is much less than the theoretically computed value needed to 'close' the Universe (see CLOSED UNIVERSE), and it may be that at least some is in the form of black holes.

Blagg, Mary Adela (1858-1944) English astronomer who catalogued and mapped lunar features. She standardized the nomenclature of the Moon's topographic features (1907-13), collating and correcting thousands of names assigned by previous lunar cartographers. In 1920 the International Astronomical Union (IAU) appointed Blagg to its newly established Lunar Nomenclature Commission; twelve years of further research produced the authoritative Named Lunar Formations (1932, compiled with Karl Muller). With W.H. Wesley, she composed a Map of the Moon (1935), which remained the IAU's official lunar map until the 1960s.

blazar Term compounded from BL LACERTAE OBJECT and QUASAR; it refers to a specific kind of extragalactic object. The blazars are the most active of the galaxies with active nuclei, that is, the galaxies whose central regions are undergoing energetic processes that turn them into SEYFERT GALAXIES, BL Lacertae objects or quasars. The blazars show variable optical brightness, strong and variable optical polarization and strong radio emission. The variations in the optical region may be on timescales as short as days.

Much of the activity in ACTIVE GALACTIC NUCLEI is related to JETS of gas expelled from their central regions with rel-ativistic velocities. The most probable explanation for the exceptional activity in blazars is that with these galaxies we are viewing jets directed straight towards us.

Blazhko effect Periodic change in the light-curves and periods of some RR LYRAE VARIABLE STARS; it was discovered by Sergei Nikolaevich Blazhko (1870-1956). The most likely cause is pulsation in two modes simultaneously, although certain stars appear to be 'oblique rotators', that is, stars in which the magnetic axis does not correspond to the rotational axis.

Blaze Star Popular name for the T Coronae Borealis, the brightest known RECURRENT NOVA. Normally around 11th magnitude, it has flared up to naked-eye brightness on two occasions, once in 1866 when it reached 2nd mag. and again in 1946 when it peaked at 3rd mag. T Coronae Borealis is a spectroscopic binary in which an M3 red giant orbits with a white dwarf every 227.5 days; gas falling from the red giant on to the companion causes the outbursts. A possible additional periodicity of 56.7 days may be caused by a third component.

blink comparator (blink microscope) Instrument that enables two photographs of the same area of sky, taken at different times, to be rapidly alternated to compare them. Any object that has changed position or brightness during the photograph intervals will show up.

The comparator has two optical paths so that the two photographs can be seen together in one viewing eyepiece. By careful adjustment, the separate images are brought into exact coincidence and then alternately illuminated, changing from one to the other about once a second. All features that are identical appear unchanged, but any object that is on only one of the photographs is seen to blink on and off. An object that has changed its position between the times the photographs were taken appears to jump to and fro and an object that has changed in brightness is seen to pulsate.

The eye is very efficient at detecting the few varying objects among what can be tens of thousands of star images. This simple technique makes possible the discovery of stars of large proper motion, minor planets, comets or variable stars, without the need individually to compare every star image on two photographs.

These days blink comparators are particularly used by hunters of novae and asteroids but past examples of their work include: the discovery of PLUTO by Clyde TOMBAUGH; the catalogue of over 100,000 stars brighter than magnitude 14.5 with detectable proper motion, produced by W.J. LUYTEN; and the majority of the nearly 30,000 known variable stars discovered at various observatories around the world.

Blinking Planetary (NGC 6826) planetary nebula located in northern Cygnus (RA 19h 44m.8 dec. + 5031'). The nebula has a compact 25" diameter and overall magnitude + 8.8. The central star is a relatively bright magnitude +10.6, which results in the interesting illusion that if the observer alternates between direct and averted vision, the nebula appears to blink in and out of view.

Blinking Planetary The blink of NGC 6826 is an optical effect rather than anything occurring within the planetary nebula itself.

Bliss, Nathaniel (1700-1764) English astronomer, the fourth astronomer royal (1762-64). Bliss was an able observer who successfully observed the 1761 transit of Venus. Bliss assisted James bradley at Greenwich Observatory, succeeding him as Astronomer Royal, but made little impression on the observatory, dying only two years after his appointment.

BL Lacertae object Category of luminous active galactic nucleus, the defining characteristics of which include very weak or unobservable emission lines and rapid variability. The prototype, BL Lacertae itself, was long listed as a variable star until its extragalactic nature became apparent from observations of the faint surrounding galaxy. When the objects' continuum light is faintest, weak emission lines similar to those of quasars may be detected, and their bright cores often have extensive haloes at radio wavelengths as well. These properties are well explained by a picture in which BL Lacertae objects are quasistellar objects (QSOs) or radio galaxies seen almost along the line of a relativistic jet, so that the radiation seen from the jet is Doppler boosted in frequency and intensity. This geometry also amplifies small changes in the velocity or direction of the jet, accounting for the strong variability of these objects, and fitting with the fact that many of them have small-scale radio jets showing superluminal motion. A few BL Lac-ertae objects have been detected at the highest photon energies, 1 TeV; this implies that their radiation is being beamed into a small angle (as in gamma-ray bursts), otherwise it would be lost to pair production within the source. BL Lacertae objects and the broadly similar optically violently variable (OVV) QSOs are often referred to collectively as blazars.

blooming Natural film that formed on early uncoated lenses. It was noticed the bloom improved the transmission of the lenses and some opticians still use the term to refer to coating. Blooming also refers to the loss of focus of a camera sensor because of excessive brightness and the term may occasionally have this meaning in astronomy.

blue moon Occasional blue colour of the Moon, due to effects in the Earth's atmosphere. It can be caused by dust particles, from volcanoes or forest fires, high in the upper atmosphere, which scatter light, making it appear blue. The expression sometimes refers to the occurrence of a second full moon in a calendar month, something which occurs about seven times every 19 years, and it is used in everyday speech to denote a rare event.

blue straggler In a globular cluster, star that appears to be younger than the others because it still lies on the main sequence after the majority of other cluster members have evolved off. It is observed to be on the main sequence beyond the turnoff point: it is bluer in spectral terms than the stars at the turnoff point. Being younger and more massive than the other stars in the cluster poses a problem as to the blue stragglers' origin. They are thought to be either stars in close binaries that have been rejuvenated by mass transfer from their companions, or stars that were produced by a stellar collision.

blue straggler Surrounded by older yellow stars in the globular cluster 47 Tucanae is a massive young, blue star (arrowed). This blue straggler is thought to be the product of the slow merger of two stars in a double star system.

The Hubble Space Telescope has imaged many blue stragglers situated in the cores of globular clusters. They have also been found in open clusters and in the dwarf galaxy companions to the Milky Way. See also stellar evolution

BN object See becklin-neugebauer object

Bode, Johann Elert (1747-1826) German astronomer, director of Berlin Observatory from 1772. He did much to popularize astronomy, founding the highly regarded Astronomisches Jahrbuch in 1774, which he edited for over half a century. In 1801 he published Uranographia, the most beautiful star atlas ever drawn, with a catalogue of over 17,000 stars and non-stellar objects. Bode's name is most famously associated with the empirical relationship between planetary distances known as bode'slaw, though he did not in fact discover it.

Bode's law (Titius-Bode law) Simple numerical relationship, first noticed by Johann Titius of Wittenberg, but popularized by Johann Elert bode in 1772, which matches the distances of the then known planets from the Sun. The formula is produced by taking the numbers 0, 3, 6, 12, 24, 48, 96 and 192 (all, apart from the first two, being double their predecessor) and then adding four, giving the sequence 4, 7, 10, 16, 28, 52, 100, 196. If the distance of the Earth from the Sun is then taken to be 10, it is found that Mercury falls into place at 3.9, Venus at 7.2, Mars at 15.2, Jupiter at 52.0 and Saturn at 95.4. The discovery of Uranus at 191.8, by Sir William Herschel in 1781, initiated a hunt for the missing planet between Mars and Jupiter at 28. This led to the discovery in 1801 of the first minor planet, Ceres, and subsequently the asteroid belt. Neptune, at 300.7, does not fit the sequence although Pluto, at 394.6, does. The relationship is now widely regarded simply as a mathematical curiosity, rather than indicating anything significant about the physical properties of the Solar System.

Bok, Bartholomeus Jan ('Bart') (1906-83) Dutch-American astronomer best known for his studies of the Milky Way and his discovery of bok globules. At Harvard University (1929-57), Bok worked closely with his wife, Priscilla Fairfield Bok (1896-1975), to map the spiral arms of our Galaxy by 'star-counting' methods. In the course of this work, the two astronomers made highly detailed studies of the Carina region from Harvard's southern station in South Africa. During his tenure at Harvard, Bok was an early advocate of radio astronomy research, and he was instrumental in setting up Mexico's National Observatory at Tonantzintla. At Mount Stromlo (Australia) Observatory (1957-66), where he was director, they carried out similar surveys for the Magellanic Clouds. He was especially interested in the Galaxy's many different kinds of gas and dust clouds, and he was the first to identify the opaque, cool (10 K), tiny (0.6-2 l.y.) condensations of gas and dust now named after him that give rise to low-mass stars. The Boks wrote the classic book The Milky Way, which had run through five editions as of 1981. From 1966 to 1974, he directed the Steward Observatory of the University of Arizona: through his efforts Steward acquired a new 2.3-m (90-in.) reflector at Kitt Peak.

Bok globule See globule

bolide Term often used to describe a major fireball that produces a sonic boom. Such events are frequently associated with the deposit of meteorites.

bolometer Instrument to measure the total radiation received in a telescope from a celestial body by its effect upon the balance of an electrical circuit.

bolometric magnitude Measure of the total radiation of all wavelengths emitted by or received from a star expressed on the stellar magnitude scale.

Bolton, John Gatenby (1922-93) English scientist who spent much of his career in Australia as a pioneer of radio astronomy. Bolton used a radio interferometer to identify the extraterrestrial radio sources Taurus A (the Crab Nebula), Centaurus A (the galaxy NGC 5128) and Virgo A (the galaxy M87). His team was the first to confirm that the centre of the Milky Way galaxy was a strong source of radio emission, which they called Sagittarius A. Bolton founded CalTech's Owens Valley Radio Observatory in the 1950s and later directed Australia's Parkes Radio Observatory.

Boltzmann constant (symbol k) Constant defined as the universal gas constant (R) divided by Avogadro's constant. It is the gas constant per particle and has a value of 1.38066 X 1023 J/K. It is often encountered in equations relating the properties of gases to temperature, for example the gas pressure law, in the form P = nkT, where P is the pressure, n the number of particles per cubic metre, and T the temperature. It also occurs in boltzmann'sequation.

Boltzmann's equation Equation, introduced by the Austrian physicist Ludwig Boltzmann (1844-1906), that gives the relative populations of atoms with electrons in different levels of excitation:

Nb = Sb g-(Bb-Ea)/kT

where Na and Nb are the number densities of atoms in excitation levels a and b, ga and gb are constants for the levels called statistical weights, Ea and Eb are the excitation energies of the levels, k is boltzmann's constant and Tis the temperature. The equation is used in understanding stellar spectra and processes occurring in stellar atmospheres.

Bond, George Phillips (1825-65) American astronomer who in 1859 succeeded his father, William Cranch bond, as director of the harvard college observatory. He made many contributions to the studies of double stars and stellar parallax. With his father, he discovered Saturn's Crepe Ring (the C Ring) and its eighth satellite,

Hyperion, independently of William lassell. Bond argued against the then-popular theory that Saturn's rings were solid, having observed stars through the Crepe Ring. He used the new technique of photography (using 'wet' plates) to study the Moon and the planets and was the first to photograph a star, Vega, in 1850 and the first to image a double star, Mizar, in 1857.

Bond, William Cranch (1789-1859) American astronomer who founded the harvard college observatory and was its director from 1839, being succeeded by his son George Phillips bond. A clock-maker by trade, he designed and built sophisticated chronometers for navigation and astronomy. He transferred his own private observatory to Harvard; Bond procured a 15-inch (380-mm) refracting telescope, then the largest in the world, for the observatory. With this, he and his son discovered Saturn's satellite Hyperion in 1848, and the planet's semitransparent Crepe Ring (the C Ring) in 1850. Bond also made detailed studies of sunspots and the Orion Nebula (M42).

Bondi, Hermann (1919- ) British cosmologist and mathematician, born in Austria, who co-originated the steady-state theory of cosmology. After working for the British Admiralty during World War II, Bondi was appointed professor of applied mathematics at King's College, London in 1954. In 1948, with Fred hoyle and Thomas gold, he constructed a new cosmological theory calling for the 'continuous creation' of matter at the rate of 10-10 nucleons per cubic metre per year. This process, explained Bondi, allowed the Universe to maintain a constant average density of matter that counterbalanced flat space expanding at a constant rate. The discovery in 1964 of the cosmic microwave background cast serious doubt on the validity of the steady-state theory, which has since been abandoned in its original form. The work of Bondi and Hoyle on stellar structure, especially of the formation of heavier elements inside stars, was a valuable by-product of this theory. Bondi later demonstrated that gravitational waves are real, and not just theoretical, consequences of Einstein's general theory of relativity, and his work has defined the physical properties of these phenomena.

Bonner Durchmusterung (BD) Catalogue containing data on 324,000 stars down to magnitude 9.5, published in 1859-62 by Friedrich Argelander and extended southward in its coverage in 1886 by Eduard Schonfeld, adding a further 133,000 stars. The stars are numbered in declination zones from +90 to 23, and are cited in the form 'BD +52 1638'.

Bootes See feature article

Borrelly, Comet 19P Short-period comet discovered on 1904 December 28 by Alphonse Borrelly of Marseilles. The comet is usually quite faint, reaching magnitude +8 at favourable returns. Its orbital period is 6.86 years, with the most recent return to perihelion coming on 2001 September 14. At this return, comet Borrelly was visited by the deep space 1 probe, becoming the second comet (after 1P/halley) to have its nucleus imaged. Results from the spacecraft's September 22 flyby showed the nucleus to be a dark, elongated (6 X 3 km/3.7 X 1.9 mi) body with three emerging gas jets.

boson Elementary particle that obeys Bose-Einstein statistics. Bosons are symmetric particles, which have integral spins (0/1). Common bosons are the helium nucleus, the pi meson and the photon.

Boss, Lewis (1846-1912) American astronomer who in 1895 began an ambitious programme to measure star positions and magnitudes with unprecedented accuracy, culminating in two important star catalogues, the Preliminary General Catalogue of 6,188 Stars (1910) and the General Catalogue of 33,342 Stars (1937), the latter work completed decades after his death by his son Benjamin Boss (1880-1970). Boss observed the stars visible from Earth's northern hemisphere from New York's Dudley observatory, where he became director in 1876, and the southern stars from Argentina.

Bowditch, Nathaniel (1773-1838) Self-educated American scientist, author and translator of Laplace's massive Traite de mecanique celeste (1829-39). Bowditch made many observations of meteors, comets and the Moon, which he wrote up between 1804 and 1820 - some of the first publications based upon original astronomical observations to appear in America.

Bowen, Edward George (1911-91) Welsh scientist who used radar and other equipment salvaged at the end of World War II to help found Australian radio astronomy. He played a key role in the design and construction of the Parkes Radio Observatory's 64-m (210-ft) radio telescope, which was used for several important surveys of radio sources. Bowen pioneered the use of radio telescopes to detect the radio-wavelength echoes produced by meteors.

Bowen, Ira Sprague (1898-1973) American astrophysicist who in 1927 showed that previously unidentified lines in the spectra of nebulae were due not to a new element, 'nebulium', but to so-called forbidden lines of ionized oxygen and nitrogen. As long-time director of the Mount Wilson and Palomar Observatories (1946-64), Bowen oversaw the construction of the 200-inch (5-m) Hale Telescope and the 48-inch (1.2-m) Oschin Schmidt telescope used to make the palomar observatory sky survey.

bow shock Sharp boundary standing in the solar wind flow upstream of a planetary magnetosphere or other obstacle, such as a cometary or planetary ionosphere. The flow of the solar wind is supersonic and indeed faster than the various characteristic speeds associated with the magnetic field and plasmas of which it is constituted. As a consequence, information that the solar wind is approaching a planetary magnetosphere is unable to propagate upstream into the flow, and a curved standing shock wave is formed. At this shock, the solar wind plasma is rapidly decelerated and deflected around the magnetosphere; the upstream flow kinetic energy is converted into plasma heating. Thus a region of hot, slow-flowing, turbulent solar wind plasma and magnetic field appears downstream in the planet's magnetosheath (the region between the bow shock and the magnetopause).

Since the solar wind plasma is collisionless, it was originally assumed that a classical bow shock wave could not form. However, the magnetic field imparts a collective behaviour to the plasma, which thus acts in much the same way as do molecules in a classical gas. The magnetic field, therefore, has a strong influence on the structure of the bow shock. A quasi-perpendicular bow shock, in which the magnetic field points mostly at right angles to the shock surface normal, tends to be a very abrupt boundary. A quasi-parallel bow shock, in which the magnetic field is closely parallel to the shock surface normal, allows any very energetic particles to escape back into the upstream solar wind; it is thus a more diffuse boundary. Bow shocks and shock waves in general are efficient accelerators of particles, and they may be responsible for energization processes around planetary magnetospheres and indeed in many wider astronomical phenomena.

Boyden Observatory Optical observatory dating from 1887, situated 26 km (16 mi) east of Bloemfontein, South Africa. The observatory transferred from Peru to its present site in 1926 and was then equipped with a 1.5-m (60-in.) reflector, which remains the largest telescope on the site. Originally a southern station of harvard college observatory, Boyden has been operated since 1976 by the University of the Free State.

brachinite Subgroup of the achondrite meteorites. Bra-chinites are olivine-rich igneous rocks with approximately chondritic bulk composition. They have oxygen isotopic compositions similar to the howardite-eucrite-diogen-ite association (HEDs). Like the acapulcoite-lodran-ite association, brachinites are considered to be primitive achondrites.

Brackett series Series of infrared emission or absorption lines in the hydrogen spectrum resulting from electron transitions down to or up from the fourth energy level of that atom. The Brackett lines are named with Greek letters: Brackett a, which connects levels 4 and 5, lies at 4.0512 jm; Brackett p, which connects levels 4 and 6, lies at 2.6252 jm, and so on. The series ends at the Brackett limit at 1.4584 jm.

Bradford Robotic Telescope Autonomous 400-mm (18-in.) optical telescope located on the moors of West Yorkshire, England. It was the first instrument in the world to provide remote access via the Internet, principally for education.

Bradley, James (1693-1762) English astronomer and clergyman, Savilian professor of astronomy at Oxford and, later, at Greenwich Observatory. In 1742 he succeeded Edmond halley, becoming the third astronomer royal. While searching for stellar parallax, by making meticulous measurements of the star y Draco-nis, Bradley discovered the aberration of starlight in 1728; this was the first direct observational evidence of the Earth's orbital motion, confirming the Copernican model of the Solar System. His value for this constant was 20".5, the modern value being taken as 20".47. Bradley calculated the time it takes sunlight to reach the Earth as 8 minutes 12 seconds, just 7 seconds shorter than the currently accepted value. From observations covering a complete revolution of the nodes of the Moon's orbit (1727-18), he found a periodic nodding or nutation of the Earth's axis. At Greenwich, Bradley catalogued the positions of thousands of stars - because he took into account the effects of aberration and nutation, his catalogues contained more accurate stellar positions than those of his predecessors. He also made accurate measurements of Jupiter's diameter and carefully observed the eclipses and other phenomena of its satellites.

Brahe, Tycho (1546-1601) Danish astronomer who used pre-telescopic instruments of his own design to obtain planetary and star positions of unprecedented accuracy, later used by his assistant Johannes kepler to derive his three fundamental laws of planetary motions. Tycho was born into a noble family, his father Otto Brahe serving as a privy councillor and governor at Hels-ingborg, Denmark (now in Sweden). He was raised by his uncle, Jorgen Brahe, who left him a substantial inheritance. As a thirteen-year-old boy, Tycho was greatly impressed by astronomers' accurate prediction of the total solar eclipse of 1560 August 21. From 1559 to 1570, he attended the universities at Copenhagen, Leipzig, Wittenberg, Rostock and Basel, studying law and the humanities but maintaining a strong interest in astronomy. In 1566 Tycho duelled with a rival student, losing the tip of his nose, which he covered for the rest of his life with a metal prosthesis. During this period, his teachers helped him make astronomical globes and simple cross-staffs, forerunners of the more sophisticated instruments he would later use, and he read Ptolemy's Almagest, the only astronomy book then available.

Brahe, Tycho The Danish astronomers model of the Solar System, with a static Earth at the centre. He thought that the Moon and Sun orbited the Earth and the other planets moved around the Sun.

In 1563 August, Tycho had observed a conjunction of Jupiter and Saturn, noticing that the Copernican tables were grossly in error (by several days) in predicting this event. He decided to devote the rest of his life to improving planetary and stellar positions, acquiring a large quadrant and building a private observatory at Skane in 1571. The next year, Tycho independently discovered a supernova in Cassiopeia, and his report, De stella nova (1573), made him famous. The Danish king, Frederick II, gave him the Baltic island of Hveen in 1576, where Tycho built two observatories, uraniborg and stjerneborg, carrying out 20 years' worth of very accurate observations with instruments such as the 6i-ft (2-m) mural quadrant. He described these instruments and his work at Hveen in two magnificent books, Astronomiae instauratae mechanica (1598) and Astronomiae instauratae progymnasmata (1602).

Whereas previous astronomers had contented themselves with observing the planets at opposition and quadrature, Tycho obtained positions at many intermediate points in their orbits, accurate to 30" - the best previous planetary positions were accurate only to 15'. He was the first astronomer to take atmospheric refraction into account in correcting observed planetary and stellar positions. Tycho made the first truly scientific studies of comets, observing the position, magnitude, colour and orientation of the tail of the Great Comet of 1577. These observations led him to conclude that the comet's orbit, which he determined must have an elongated shape, lay beyond the Moon - a novel idea at the time. Tycho's careful measurements of the Sun's apparent movement allowed him to determine the length of a year to within 1 second, forcing 10 days to be dropped from the Julian calendar in 1582 after it was shown that the Julian year exceeded the 'true' year by this amount of time. He also compiled a catalogue of 777 stars.

Tycho was not a Copernican, but created a tychon-ian system with the planets revolving around the Sun, and the Sun and Moon revolving around a fixed Earth. This model was widely accepted by many astronomers until the mid-17th century. His observations of the comet of 1577, which demonstrated that it moved among the planets, defeated the Aristotelian notion that the planets were contained within solid crystalline spheres through which a body like the comet could not possibly move. Tycho ended his days as Imperial Mathematician under the Holy Roman Emperor Rudolf II in Prague, where Kepler was one of his assistants. Kepler later reduced much of Tycho's data (which appeared in the Rudolphine Tables of 1627) to discover the three fundamental laws of planetary motion (1609-19).

Brahmagupta (598-c.670) Indian astronomer and author of the Brahmasphutasiddhanta ('The Opening of the Universe'), a scholarly discussion of algebra, geometry and astronomy in verse form. Brahmagupta was an expert on the phases of the Moon, eclipses of the Sun and Moon, and the positions and movements of the planets.

Braille mars-crossing asteroid (number 9969) visited by the deep space 1 space probe in 1999 July. Braille is an irregularly shaped asteroid c.2.2 km (c.1.4 mi) long. Its perihelion distance is 1.32 AU and its orbital period 3.58 years.

Brans-Dicke theory Alternative theory of gravity to Einstein's general theory of relativity. It was proposed by Princeton physicists C.H. Brans and R.H. Dicke in 1961. Based on the ideas of Ernst Mach about reference frames being connected to the distribution of matter in the Universe, Brans and Dicke modified general relativity, introducing a scalar field, to include Mach's proposal. The strength of this field is governed by an arbitrary constant w, but experiments limited the Brans-Dicke parameter w to values closer and closer to one, at which the Brans-Dicke theory became indistinguishable from general relativity.

Brashear, John Alfred (1840-1920) American telescope-maker who founded a renowned optical company in Pittsburgh, Pennsylvania, named after himself, today known as Contraves Corp. Brashear made the 30-inch (0.76-m) Thaw refracting telescope for Pittsburgh's Allegheny Observatory. Besides telescopes, Brashear's firm made spectroscopes, prisms and gratings and other highly specialized astronomical equipment, including George Ellery hale's spectroheliograph.

Braun, Wernher Magnus Maximilian von (1912-77) German-American rocket scientist who played a major role in the US space programme. His early career was spent at the rocket development and test centre at Peen-emunde in north-eastern Germany, which produced a series of rockets culminating in the A-4, renamed the V-2 when it and the team were absorbed into Germany's war effort. At the close of World War II, von Braun and many other German rocket scientists were relocated first to White Sands (New Mexico) Proving Grounds, then to Huntsville, Alabama. At White Sands, the V-2s were put to peaceful use in high-altitude atmospheric and astronomical research. At Huntsville, von Braun led teams that built and launched the Jupiter and Redstone missiles and, on 1958 January 31, the USA's first artificial satellite, Explorer 1. He oversaw the development of the Saturn I, IB and V launch vehicles for the Apollo programme.

breccia Rock comprising angular fragments set in a fine-grained matrix. Breccias form as a result of rocks being crushed by meteoroid impacts, tectonic faulting, violent volcanic eruptions and certain other geological processes. Some meteorites are breccias formed by collisions of their parent bodies. Impact-generated breccias, which may also contain glass, are the most abundant rocks in lunar highlands, and they were predominant among the rock samples returned by the Apollo astronauts.

bremsstrahlung See free-free transition

bright nebula Interstellar gas or dust cloud that may be seen in the visible by its own light, in contrast to dark nebulae, which can only be seen when silhouetted against a bright background. Bright nebulae include all types of emission nebulae plus reflection nebulae.

Bright Star Catalogue (BS) Catalogue of all naked-eye stars, first compiled by Frank schlesinger and published as the Yale Bright Star Catalogue by Yale University in 1930; it is now maintained by Dorrit hoffleit. The current (5th preliminary, 1991) edition, in electronic form, lists 9110 stars brighter than mv = 6.5 and gives their position, parallax, proper motion, magnitude, spectral type and other parameters.

B ring Second major ring of saturn, lying inside the a ring at a distance between 82,000 km (51,000 mi) and 117,600 km (73,100 mi) from the planetary centre.

B ring Voyager 2s images of Saturns rings revealed that they were made up of many thousands of narrow ringlets. The dark markings within the B ring are known as spokes and are thought to be caused by the planets magnetic field.

British Astronomical Association (BAA) Main organization co-ordinating the work of amateur astronomers in the UK, with a membership (2001) of around 2500. It was founded in 1890 as an egalitarian alternative to the Royal Astronomical Society. The BAA has a number of observing sections, each dedicated to a specific area of astronomy. Reports are published in a bimonthly Journal, and the annual Handbook provides ephemerides for astronomical phenomena.

British Interplanetary Society (BIS) World's longest-established organization that is devoted solely to supporting and promoting the exploration of space and astronautics; it was founded in 1933. Though its headquarters are in London, the society has a world-wide membership. Its monthlyjournal, Spaceflight, first appeared in 1956, the year before the launch of the first artificial satellite, and it is regarded as an authoritative source on international space programmes and the commercial exploration of space.

Brocchi's Cluster See coathanger

Brooks, Comet (C/1893 U1) comet discovered in 1893, reaching peak magnitude +7.0. Its complex, rapidly changing ion tail was one of the first to have its activity photographed.

Brorsen, Comet 5D/ Faint short-period comet discovered by Theodor Brorsen, Germany, on 1846 February 26. With an orbital period of 5.5 years, it was seen at five returns. At the last of these, in 1879, it underwent a dramatic fade and almost certainly disintegrated. Comet 5D/Brorsen has not been seen since.

Brorsen-Metcalf, Comet 23P/ Short-period comet discovered by Theodor Brorsen, Germany, on 1847 July 20. The comet reached peak magnitude + 6.5 in mid-August but was only observed over a short arc, leading to uncertainties as to its orbital period. The comet was rediscovered by Joel H. Metcalf on 1919 August 21, reaching magnitude + 4.5 around perihelion on October 17. The comet next returned to perihelion, 0.48 AU from the Sun, on 1989 September 11, reaching magnitude + 5, and proving a readily observable object during this apparition. Comet 23P/Brorsen-Metcalf appears to be subject to considerable non-gravitational force, which modifies the orbital parameters. The comet's current period is 70.54 years.

Brouwer, Dirk (1902-66) Dutch-born expert in celestial mechanics. He spent his career at Yale University, where he directed its observatory for a quarter-century and edited the Astronomical Journal. He was among the first to use computers to perform the complex and laborious calculations demanded by the methods of celestial mechanics that he developed for improving the orbital elements of planets, asteroids and comets. Brouwer extended these methods to calculate the orbits of the first artificial Earth satellites.

Brown, Ernest William (1866-1938) English mathematician, a specialist in celestial mechanics, who moved to America in 1891. Working at Yale University, Brown constructed tables of the Moon's motion accurate to 0".01, based in part on a century and a half of observations made from England's Greenwich Observatory. His advances in lunar theory showed that the Earth's variable rotation rate explained much of the irregularity in the Moon's movements. Brown also made major contributions to our knowledge of the Trojan asteroids.

Brown, Robert Hanbury (1916-2002) English astronomer, born in India, who in the 1950s applied his expertise in radar developed during World War II to the new science of radio astronomy. His major accomplishment was the invention of the intensity interferometer, with which he was able to measure the diameters of stars, something impossible with optical telescopes. Brown played a major role in conducting radio surveys of the sky from Jodrell Bank, and spent much of his later career at the University of Sydney.

brown dwarf Star with mass greater than about 0.01 solar mass but less than 0.08 solar mass; its core temperature does not rise high enough to start thermonuclear reactions. It is luminous, however, because it slowly shrinks in size and radiates away its gravitational energy. As its surface temperature is below the 2500 K lower limit for red dwarfs, it is known as a brown dwarf.

The first unambiguous detection and image of a brown dwarf, Gliese 229B (GL229B), was made in 1995.

GL229B is a small companion to the cool red star Gliese 229, which is located 19 l.y. from Earth in the constellation of Lepus. It is estimated to be 20 to 50 times the mass of Jupiter. Infrared spectroscopic observations, made with the 200-inch (5-m) Hale telescope at Palomar, show that the brown dwarf has an abundance of methane, making it similar in composition to Jupiter and the other gas giant planets in the Solar System. More than 100 brown dwarf candidates have since been discovered in infrared surveys, many of which are warmer than GL229B but cooler than M dwarf stars. A new class of L dwarf (see lstar) has thus been added to the spectral classification system.

brown dwarf The brown dwarf Gliese 229B (next to the cool red star Gliese 229), in images from the Hale Telescope (top) at Palomar and the Hubble Space Telescope (bottom). It is estimated to be 100,000 times dimmer than our Sun.

The Hubble Space Telescope (HST) has carried out a survey of brown dwarfs, showing that there are more lowmass brown dwarfs than high-mass ones. Brown dwarfs are far more abundant than previously thought, but they are probably not abundant enough to contribute significantly to the missing mass. The HST survey also showed that many brown dwarfs are 'free-floating' isolated bodies rather than members of close binary systems, suggesting that they form in a manner similar to stars, not planets. However, some of the more massive extrasolar planets may well be brown dwarfs.

Bruno, Giordano (1548-1600) Italian philosopher and monk. A sharp critic of the cosmology of Aristotle and an early supporter of Copernicus' heliocentric theory, he was burned at the stake in Rome for theological heresy and suspicion of magical practices. Bruno postulated a cosmology in which the universe was infinite, as elucidated in his 1584 work De I'infinito universo e mondi.

BS Abbreviation of bright star catalogue

B star Member of a class of blue-white stars, the spectra of which are characterized by neutral helium and strong hydrogen lines. main-sequence B dwarfs have a great range of stellar properties. Temperatures range from 10,000 K at B9.5 to 31,000 K at B0, zero-age masses from 2.3 to 14 solar masses, zero-age luminosities from 25 to 25,000 solar luminosities, and lifetimes from 800 million to 11 million years. B giants and supergiants have increasingly narrow hydrogen lines, from which the Mor-gan-Keenan class can be determined. Average dwarf rotation speeds maximize at B5 at over 250 km/s (160 mi/s). be stars are fast rotators surrounded by disks that radiate emission lines. Slower rotators can be chemically peculiar stars, appearing as mercury-manganese stars. ap stars extend into the class as Bp.

Because of their youth, B stars are found in the galactic disk and in regions of star formation. Temperatures at B0 and B1 are high enough to produce diffuse nebulae. Light scattered by dust from cooler B stars creates reflection nebulae. Along with ostars, B stars form loose, unbound ob associations. While rare (one for every 1000 or more stars), B stars are thousands of times more common than O stars, and help to outline the spiral arms of galaxies and to give them their bluish colours.

O dwarfs evolve into B supergiants, some of which turn into luminous blue variables, which can have luminosities up to two million times that of the Sun and mass loss rates up to a hundredth of a solar mass per year (Eta Carinae, for example). B dwarfs evolve into giants and may appear at some point as cepheid variables. Class B also contains the Beta Cephei variables, all giants or sub-giants that pulsate with multiple periods of hours and amplitudes of a few hundredths of a magnitude. Bright examples of B stars include Achernar B5 IV, Regulus B7 V, Rigel B8la and Spica BI V.

Bullialdus Magnificent lunar crater (21S 22W), diameter 63 km (39 mi), in the middle of Mare Nubium; it has massive 2500-m-high (8000-ft) terraced walls. A broad, bright ejecta blanket flanks the ramparts of Bullialdus, highlighting a system of gullies flowing down the crater's outer slopes. The crater's squared-off interior walls contain many landslips. The floor features a complex group of central mountains, the highest reaching over 1000 m (3000 ft).

Burbidge, Geoffrey Ronald (1925- ) and Burbidge, (Eleanor) Margaret (1919-) English husband-and-wife astrophysicists noted for their studies of the nucleosynthesis of elements inside stars, investigations into active galactic nuclei (AGNs) and research in cosmology. Working at Cambridge University, the California Institute of Technology, and the University of California at San Diego, Geoffrey Burbidge's research has involved the physics of non-thermal radiation processes, quasars and other AGNs, and alternative cosmological models, including the 'quasi-steady-state theory'. This model, unlike the original steady-state theory, explains the cosmic microwave background. It proposes that matter is created by a series of 'mini Big Bangs' at a rate of 1016 solar masses on timescales of duration c.1/H0 (the Hubble time). Burbidge's cosmology suggests that the isotope 4He is produced, not by the traditional Big Bang, but by stellar hydrogen burning, as are other 'light' isotopes, up to 11B. He has also directed Kitt Peak National Observatory.

Margaret Burbidge has worked at most of the same institutions as her husband, and has served as director of Royal Greenwich Observatory (1972-73). In much of her work she has applied spectroscopy to determine the masses and rotation rates of galaxies. In 1957 she, her husband, William fowler and Fred hoyle published a seminal paper in which they set forth the basic processes that govern stellar nucleosynthesis. Their theory (known from the initials of its proponents as the B2FH theory) explained that younger stars convert their abundant supplies of hydrogen into helium and light, and as they age, the helium is gradually burned into carbon, oxygen and other chemical elements. Heavier nuclei are formed when the carbon and oxygen atoms combine with protons or He nuclei - Mg, Si, S, Ar and Ca can be synthesized this way.

The Burbidges have made extensive investigations of quasars and AGNs, finding that the two often coincide. They concluded that AGNs emit enormous amounts of energy from relatively confined regions, and suggested that black holes are the power source. Margaret Bur-bidge also helped develop the Faint Object Spectrograph for the hubble space telescope, which she has used to study the ultraviolet flux of AGNs.

Burnham, Sherburne Wesley (1838-1921) American observer and cataloguer of double stars who worked at Lick Observatory (1888-92) and Yerkes Observatory (1894-1914), discovering over a thousand new pairs. Burnham was able to use the largest refracting telescopes of the day, including Lick's 36-inch (0.9-m) and Yerkes' 40-inch (1-m). His life's work culminated in the publication of the massive two-volume General Catalogue of Double Stars Within 121 Degrees of the North Pole, which contained data for over 13,000 doubles.

Butterfly Cluster (M6, NGC 6405) Bright open cluster containing 80 stars of magnitude +6 and fainter together with associated nebulosity; it is located a few degrees north of the 'Sting' asterism marking the tail of Scorpius (RA 17h 40m.1 dec. -3213'). With an integrated magnitude +4.2, the Butterfly Cluster is visible to the naked eye under good conditions. It was catalogued by ptolemy in the 2nd century ad as a 'little cloud'. The cluster lies 2000 l.y. away in the direction of the galactic centre. It takes its popular name from the distribution of its stars in two fan-shaped 'wings'. The Butterfly Cluster has an apparent diameter of 33'.

butterfly diagram (Maunder diagram) Plot of the distribution in solar latitude of sunspots over the course of the approximate 11-year solar cycle. At the start of a cycle, sunspots tend to appear at solar latitudes of up to 40 north and south of the equator. Their numbers increase and their latitude distribution moves toward the equator as the cycle progresses. This graphical representation of sunspot latitudes against dates of observation was first plotted by the English astronomer Edward Walter maunder in the early 20th century. The plot for each cycle resembles the wings of a butterfly, which gives the diagram its popular name. The diagram is a graphical representation of sporer'slaw.

butterfly diagram During the solar cycle, the latitude at which sunspots form changes. This is related to changes in the Suns magnetic field during its cycle. Cycles usually overlap, with the first spots of a new cycle forming concurrently with the last spots of the old cycle.

Butterfly Nebula (M76, NGC 650 & 651) See little dumbbell

BY Draconis stars (BY) extrinsic variable star, consisting of an emission-line dwarf star (with dKe-dMe spectra) that shows variations of 0.01-0.5 magnitude in luminosity. The changes arise from axial rotation of stars with markedly non-uniform surface luminosity, caused by large 'starspots' and chromospheric activity. The spots are larger than those on the Sun (covering up to 20% of the stellar surface) and have little or no penumbra. Periods range from a fraction of a day to about 120 days. A number (approximately 20) of these red dwarf stars also exhibit flare activity, in which case they are classed as flare stars.

Byurakan Astrophysical Observatory Optical observatory situated some 40 km (25 mi) north-west of Yerevan, Armenia, at an elevation of more than 1500 m (4900 ft). The observatory was founded in 1946 by the Armenian Academy of Sciences, on the initiative of viktor ambartsumian. Its largest telescopes are a 1-m (40-in.) Schmidt and a 2.6-m (102-in.) reflector, opened in 1960 and 1976 respectively. The Schmidt is notable for the discovery of markarian galaxies.