HA Abbreviation of hour angle habitable zone See ecosphere

Hadar (Agena) The star p Centauri, at visual mag. 0.61 the 11th-brightest star, although it is actually a pulsating variable of p Cephei type with a range of a few hundredths of a magnitude and a period of 3.8 hours. It lies 525 l.y. away and its spectral type is B1 III. The name comes from an Arabic expression referring to one member of a pair of stars, although the origin of the designation is unknown. Its alternative title, Agena, is also of obscure origin, although it may come from the Latin word genu, meaning 'knee', since it marks one of the knees of the centaur, Centaurus.

Hadley cell One of the primary circulation cells in the Earth's atmosphere, in which air rises over the tropics and descends at the subtropical high-pressure regions located approximately at latitudes 30N and 30S. At the surface, the Hadley cells are represented by the trade winds on either side of the equator. The circulation was originally proposed by George Hadley (1685-1768) in 1735, as a single cell in each hemisphere, with warm air ascending at the equator and cold air descending at the poles. For dynamical reasons such a circulation cannot exist at tropospheric levels on Earth, although a similar circulation does occur in the stratosphere in winter. It has been proposed that the atmospheric circulation on Venus may be of this form.

hadron Elementary particle that interacts through the strong force. All hadrons have nucleon numbers of 1, 0 or 1, and can be divided into the subclasses baryons, antibaryons and mesons. Examples of hadrons are protons, anti-protons and pions.

Haedi Nickname given to the stars and ir\ Aurigae, near Capella. The name is Latin for 'kids' - in mythology, they were the offspring of Capella, the She-Goat. The goat and kids are visualized as being carried on the arm of Auriga, the charioteer.

halation Spreading of a point image on a photograph caused by light being reflected, scattered and diffused within the emulsion during exposure. The image of a star can spread into a small disk, and sometimes a halation ring is produced around it.

Hale, George Ellery (1868-1938) American pioneer of solar astrophysics who discovered the magnetic fields of sunspots and facilitated the construction of very large telescopes: the 40-inch (1-m) refractor at Yerkes Observatory, the 60-inch (1.5-m) and 100-inch (2.5-m) reflectors at Mount Wilson Observatory, and the 200-inch (5-m) reflector on Mount Palomar, later named the hale telescope in his honour.

After graduating from the Massachusetts Institute of Technology in 1890, where he constructed the first spec-troheliograph (1890-96), Hale carried out solar research from his private Kenwood Observatory, equipped with a fine 12-inch (300-mm) refractor. He brought these and other instruments to the University of Chicago, where a new, major observatory was being built - Hale had convinced Chicago streetcar magnate Charles Tyson Yerkes (1837-1905) to fund the world's largest refracting telescope and the observatory to house it. The 40-inch refractor was first displayed at the 1893 Columbian Exposition, then permanently installed at yerkes observatory when that facility opened four years later.

In 1905 Hale determined that sunspots are cooler than the surrounding chromosphere, and by observing the zee-man effect, a splitting of the Sun's spectral lines, he discovered (1908) that sunspots are associated with strong magnetic fields. He went on to show (1919) that solar magnetic fields reverse their polarity twice every 22-23 years. As early as 1904 Hale had persuaded the Carnegie Institution to build an observatory devoted to solar research atop Mount Wilson, near Pasadena, California. The snow telescope was the first major instrument installed at Mount Wilson Observatory, which Hale directed from 1904 to 1923. Mount Wilson became the world's leading observatory after it acquired, through Hale's fundraising efforts, the 60-inch reflector in 1908 and, in 1919, the 100-inch hooker telescope.

Hale-Bopp, Comet (C/1995 01) Long-period comet, the brightest of the 1990s, discovered independently by American amateur astronomers Alan Hale (1958- ) and Thomas Bopp (1949- ) on 1995 July 22-23. As with Comet kohoutek in 1973, the comet was remarkably bright (mag. +10) at discovery, given its distance at that time of 7 AU from the Sun. Comet Hale-Bopp, however, lived up to the optimistic forecasts for its performance around perihelion, reached on 1997 April 1 at a distance of 0.91 AU from the Sun. The comet brightened steadily during 1996 and, following conjunction with the Sun, re-emerged into the morning sky in February 1997 as an already-prominent naked-eye object with a distinctive, fanned dust tail and a fainter ion tail. From mid-March, Comet Hale-Bopp became a magnificent object in early evening skies, peaking in brightness at mag. 0.5 around perihelion, at which time the dust and ion tails reached lengths of 20 and 25 respectively. Close examination of the coma revealed concentric 'shells' of material being ejected from the nucleus, indicating a rotation period of 11.5 hours. The nucleus is large, with an estimated diameter of 40 km (25 mi). Comet Hale-Bopp showed a strong antitail around 1998 January 5. In early 2001, the comet had faded to mag. + 14.5, but it continued to show strong dust emission activity at a distance of 13 AU (midway between the orbits of Saturn and Uranus) from the Sun. The comet has a very highly inclined (89.4) orbit, with a period of about 2400 years.

Hale Telescope palomar observatory's famous 200-inch (5-m) reflector, for more than a quarter of a century the world's largest optical telescope. It is named after George Ellery hale, whose organizational genius lay behind the project's commencement in the 1920s. Although he witnessed the successful casting of the mirror blank in 1934 December (at the second attempt), Hale died a decade before the telescope was finally declared finished in 1949 November. The telescope has had a distinguished career in all areas of astronomy and continues to provide front-line service, despite increasing light pollution from nearby San Diego.

half-life (symbol ti) Time taken during an exponential decay process, such as radioactivity, for half the available reactions to have occurred. Half-lives can range from tiny fractions of a second to billions of years. For example, the oxygen-15 isotope that is involved in the carbon-nitrogen-oxygen cycle has a half-life of 124 seconds. The term can be used for related processes, such as a neutron splitting into a proton and electron (half-life of 12 minutes). The average, or mean, lifetime of a particle is 1.44 times its half-life.

Hall, Asaph (1829-1907) American astronomer who, in 1877 August, discovered Phobos and Deimos, the two tiny satellites of Mars. He used the United States Naval Observatory's 26-inch (0.66-m) refracting telescope, at the time the largest in the world, to detect the satellites visually. He had previously used the same instrument to discover a white spot in Saturn's atmosphere, allowing him accurately to determine that planet's rotational period. Hall also found that the orbit of Saturn's satellite Hyperion was precessing by about 20 every year.

Halley, Comet 1P/ Brightest and probably most famous short-period comet; it is the only one to have been seen regularly with the naked eye. Comet 1P/Halley has been observed at each return since 240 bc, and it was first reliably noted in Chinese annals from the winter of 1059-1058 bc. The comet has a highly elliptical orbit, with a period of about 76 years. Its most recent perihelion, 0.587 AU from the Sun, between the orbits of Mercury and Venus, was on 1986 February 9. Aphelion carries 1P/Halley out beyond the orbit of Neptune, to a distance of 35.295 AU from the Sun. The orbit is retrograde, with an inclination of 162.2.

Halley, Comet 1P/ The elongated elliptical orbit of Halleys Comet takes it from aphelion beyond the orbit of Neptune to perihelion between the orbits of Venus and Mercury. The comets orbit is retrograde, and 1P/Halley was last at perihelion on 1986 February 9.

The comet is named after the second British Astronomer Royal, Edmond Halley, who was first to calculate its orbit, making the connection between a comet he had seen in 1682 and previous recorded apparitions in 1531 and 1607. Halley successfully predicted its next return, in 1759.

Historically, Halley's Comet has made a great impression on observers at a number of past returns. Its appearance in 1066, when it passed 0.1 AU from Earth and reached mag. 4, shortly preceded the Norman Conquest of England, and the comet is depicted in the Bayeux Tapestry with the Saxon courtiers looking on in horror as King Harold totters on his throne. In 1301 the comet was seen by the Florentine painter Giotto di Bondone, who used it as a model for the Star of Bethlehem in his 1304 fresco Adoration of the Magi in the Arena Chapel in Padua.

The comet's apparitions vary considerably in terms of how favourably it may be viewed. In ad 837, 1P/Halley passed 0.034 AU (510,000 km/320,000 mi) from Earth, with the coma shining as brightly as Venus and its tail spanning 90 of sky. At the 1910 return, closest approach was 0.15 AU, with the comet reaching mag. 0; Earth actually passed through the comet's tail on May 20 of that year. The most recent return, in 1985-86, however, was relatively unfavourable, with 1P/Halley never closer than 0.42 AU (April 11) and being fairly close to the Sun in the sky when at perihelion. In the early months of 1986, the comet reached mag. +3.0 and showed a 10 tail.

Although unfavourable in many respects for Earth-bound observers, the 1985-86 return was extremely rewarding scientifically. Observations across a range of astronomical disciplines were co-ordinated by an International Halley Watch, with the results being archived centrally at the Jet Propulsion Laboratory, Pasadena, USA. Early in 1986 March a flotilla of spacecraft passed close to the comet, sending back an immense quantity of data on its structure and composition. The nucleus was imaged by the European Giotto spacecraft and found to be an irregular body with an extremely dark (albedo 0.04) outer crust and dimensions of roughly 15 X 8 km (9 X 5mi). An estimated 3.1 tonnes of dust was being ejected from the nucleus each second at around the time of the Giotto close approach on 1986 March 13; even at this rate of loss, 1P/Halley may have an active lifetime of more than 100,000 years.

Large ground-based telescopes were able to follow the comet on its way out from perihelion until it was at the distance of Uranus' orbit in 1994. Surprisingly, 1P/Halley showed an outburst of activity in February 1991, when 14.3 AU from the Sun (beyond Saturn's orbit). The cause of this brightening is uncertain, but it may have been connected with shockwaves from a coronal mass ejection propagated through the solar wind.

The comet's next return, in 2061, will be even less favourable than that of 1985-86, with 1P/Halley at perihelion while close to conjunction on the far side of the Sun from Earth.

Comet 1P/Halley is the parent of the meteor stream that produces the eta aquarids and orionids.

Halley, Edmond (1656-1742) English scientist, the second astronomer royal, famous for the long-period comet named after him, his observation and cataloguing of southern stars, his part in the publication of Newton's Principia, and his discovery of stellar proper motions. Outside astronomy, his accomplishments were numerous. He founded geophysics, charting variations in the Earth's magnetic field and establishing the magnetic character of the aurora borealis. He showed that atmospheric pressure decreases with altitude, and studied monsoons and trade winds. Halley was one of the first to use mortality statistics to cost life assurance policies.

He matriculated at Oxford University in 1673, where he made his earliest published observations, including one of a lunar occultation of Mars in 1676. At that time he also began assisting the first Astronomer Royal, John flam-steed, with observations of star positions at the Greenwich Observatory. Halley never completed his formal studies at Oxford, opting in 1676 November to voyage to the South Atlantic island of St Helena to map the stars of the southern hemisphere. Despite poor weather, he made observations from which he compiled a catalogue of 341 southern stars, observed a transit of Mercury, and carried out comprehensive meteorological and hydrographic studies. Also, he confirmed that the period of a pendulum located closer to the equator was longer than for a similar device back in England, indicating that the Earth is oblate. When Halley returned, King Charles II bestowed upon him a degree from Oxford. Halley then published his southern star catalogue, Catalogus stellarum australium, and devised a method for determining the solar parallax from his observations of the transit of Mercury.

In the 1680s, as an outgrowth of his interest in the INVERSE-SQUARE LAW of gravitation - which he managed to derive from Kepler's third law - and its implication that the planets must move in elliptical orbits, Halley befriended Isaac NEWTON. He encouraged and paid for the publication of Newton's principia, and even corrected the book's proofs. From 1686 to 1693 Halley edited the famous Philosophical Transactions of the Royal Society. In the 1690s, he began a mathematical investigation of the orbits of comets, paying close attention to the bright comets of 1531, 1607 and 1682. He concluded that these three comets, as well as similar objects appearing in 1305, 1380 and 1456, were just different apparitions of the same comet, which had a period of 76 years. Taking into account the perturbations of Jupiter, he calculated an orbit for the comet indicating it would return in 1758. Although he did not live to see his prediction come true, he achieved immortality when it appeared on Christmas Day of that year. Comet 1P/HAL-LEY has since become the best-studied of all comets.

After further scientific voyages (1698-1701), Halley was appointed Savilian Professor of Geometry at Oxford in 1704, where he continued his astronomical work. In 1710, after noting that the observed positions of some stars differed significantly from their coordinates given in Ptolemy's catalogue, he confirmed PROPER MOTION in three bright stars - Sirius, Arcturus and Procyon. In 1720 he published a paper in which he discussed what is now called OLBERS' PARADOX, arguing for the infinity of space. Also in that year, Halley succeeded Flamsteed as Astronomer Royal, a post he held for 21 years. During his tenure he modernized the Greenwich Observatory, installing the first transit instrument and devising a method for determining longitude at sea by using lunar observations.

halo Ring of light seen round a celestial body. A halo is an atmospheric phenomenon produced as a result of refraction by ice crystals in thin cirrus cloud at altitudes of 10-15 km (6-9 mi), near the top of the TROPOSPHERE. The commonest form of halo is a 46 diameter ring of light with the Sun or Moon at its centre. Lunar haloes are usually only seen when the Moon is within five days of full; at other times its light is not sufficiently bright. Solar haloes often show some colour, with a blue outer and red inner edge; the sky inside the halo is noticeably darker than that outside.

halo The most prominent components of spiral galaxies like the Milky Way are the stars, gas and dust populating their nuclear bulge and disk. Many are also surrounded by tenuous halos of hot plasma. This X-ray image of a Milky Way-type galaxy reveals how vast such a halo can be.

halo (galactic halo) Extensive distribution of stars in the outer regions of a GALAXY. The term also refers to the invisible, extended distribution of DARK MATTER extending beyond the visible part of a galaxy.

halo population stars Oldest, most metal-poor stars, found in the halo of the Galaxy and in GLOBULAR CLUSTERS. See also POPULATIONS, STELLAR


Hamal The star a Arietis, visual mag. 2.01, distance 66 l.y., spectral type K2 III. Its name comes from an Arabic word meaning 'the lamb'.

Hamburg Observatory Former national observatory, dating from the beginning of the 19th century. It moved to its present location at Bergedorf, 20 km (12 mi) east of Hamburg, in 1909. The observatory operates several small telescopes, including a 1-m (40-in.) reflector, and has been part of the University of Hamburg since 1968. Today, the observatory's research centres around theoretical astrophysics, together with studies of metal-poor stars, the interstellar medium, quasars and gravitational lenses.

Harriot, Thomas (1560-1621) English mathematician, astronomer and surveyor. He came to move in the circle of John Dee (1527-1608), and under the patronage of Sir Walter Raleigh, whom he had tutored, spent 1585 as a member of a survey expedition to Virginia. He and his Welsh friend William Lower (c.1570-1650) began to use the telescope for astronomy some time before Galileo. By 1609 he had studied Jupiter's satellites, observed sunspots (deriving the Sun's rotation period) and mapped the Moon, though he never published his results.

Harrison, John (1693-1776) English instrument-maker who invented the MARINE CHRONOMETER to solve the problem of finding longitude at sea. When in 1714 the British Government offered a 20,000 prize for the solution of the problem, Harrison devoted his life to the perfection of a sea-going 'chronometer'. Between 1735 and 1761, he produced four chronometers, each of which was an improvement on its predecessor, and in due course received the prize money. Contrary to popular mythology, Harrison was a highly respected scientist, not an outsider at odds with the British scientific establishment.

Hartebeesthoek Radio Astronomy Observatory (HartRAO) South African national research facility in the Magaliesburg Hills 65 km (40 mi) north-west of Johannesburg. Built as a NASA tracking station in 1961 and converted for radio astronomy in 1975, the observatory became a national facility in 1988. It has a 26-m (85-ft) fully steerable dish - the only major radio telescope in Africa.

Hartmann, Johann Franz (1865-1936) German astronomer who made many important breakthroughs in astrophysics while working at the observatories at Potsdam (1896-1909), Gottingen (1909-21) and La Plata (1921-34), often with instruments he designed and built himself. One such device was a microphotometer, which used photoelectric cells to measure the density of light passing through photographic plates of starfields; this intensity was then compared to images of stars to be photometrically measured. He also devised a method for testing lenses that is named after him. Hartmann discovered the presence of interstellar matter by studying the spectrum of the spectroscopic binary 8 Orionis in 1904.

Harvard classification Classification system for stellar spectra developed at Harvard College Observatory by Edward PICKERING, Williamina FLEMING, Antonia MAURY and Annie CANNON between 1890 and 1901. Stars were lettered from A to Q primarily on the basis of the strength of the HYDROGEN SPECTRUM. Several letters were then dropped as unnecessary, while B was placed in front of A, and O in front of B, such that other absorption lines (principally helium) formed a continuous sequence. The result was the spectral sequence OBAFGKM, which correlates with both colour and temperature. See also SPECTRAL CLASSIFICATION

Harvard College Observatory (HCO) One of the first observatories established in the United States, founded in 1839; W.C. BOND was its first director. Located at Cambridge, Massachusetts, the observatory is the home of a 15-inch (38-cm) refractor dating from 1847 that was for twenty years the largest telescope in the United States. It established itself as a major astronomy research centre under its fourth director, Edward C. PICKERING. Today, the HCO carries out a broad programme of research in collaboration with the Smithsonian Astrophysical Observatory (see HARVARD-SMITHSONIAN CENTER FOR ASTROPHYSICS).

Harvard-Smithsonian Center for Astrophysics (CfA) Major US institution that combines the resources and research facilities of the HARVARD COLLEGE OBSERVATORY and the SMITHSONIAN ASTROPHYSICAL OBSERVATORY (SAO) to study the basic physical processes that determine the nature and evolution of the Universe. The symbiosis between these institutions began when the SAO moved its headquarters to Cambridge, Massachusetts, in 1955, and was formalized by the establishment of the Center in 1973. The CfA has pioneered many new areas of astronomical instrumentation, and participated in the landmark CfA Redshift Survey of the distances to some 20,000 galaxies.

harvest moon Full moon nearest to the time of the autumnal equinox in the northern hemisphere (around September 23). At this time of year, the inclination of the Moon's path to the horizon is very low, causing it to rise no more than 15 minutes later each evening, as opposed to the more usual half-hour or more, providing a succession of moonlit evenings. The name derives from the fact that this was of great benefit to farm workers bringing in the harvest. In the southern hemisphere, the harvest moon is the full moon nearest to the vernal equinox, around March 21. See also HUNTER'S MOON

Hat Creek Observatory Radio observatory near Redding, California, some 400 km (250 mi) north of Berkeley. Its principal instrument is the BIMA (BERKELEY ILLINOIS MARYLAND ASSOCIATION) Millimeter Array, an aperture synthesis telescope that operates at wavelengths of 3 mm and 1 mm. It has ten antennae 6.1 m (20 ft) in diameter that can be located at various stations along a T-shaped track.

Haute Province, Observatoire de (OHP) French national facility, established in 1937 on a 645-m (2100-ft) high plateau at Saint-Michel, some 100 km (60 mi) north of Marseilles. The OHP's telescopes include a 1.93-m (76-in.) reflector, operating since 1958, and a 1.52-m (60-in.) reflector built in 1967. Both these instruments are used for spectroscopy, while two smaller reflectors and a Schmidt camera are used for imaging. A search for extrasolar planets is being undertaken with the 1.93-m telescope.

Hawking, Stephen William (1942- ) English theoretical physicist who has applied general relativity and quantum theory to cosmology, greatly advancing the understanding of BLACK HOLES and SPACETIME. While still a graduate student at Cambridge University, where he has spent his entire career, Hawking was diagnosed in 1963 with a degenerative neuromuscular disease and told he had only 18 months to live; against these odds, he has survived to become the foremost researcher of theoretical relativistic cosmology.

In the late 1960s, Hawking demonstrated that a singularity must have characterized the beginning of the Universe in the BIG BANG. In the early 1970s, he developed mathematical proofs of the 'no hair theorem' originated by physicist John Archibald Wheeler (1911- ), which states that a black hole's gravitational field is defined by three quantities - its mass, angular momentum and electric charge.

In 1971 Hawking showed that the Big Bang could have given rise to numerous 'mini black holes', exotic objects the size of a proton but with a mass of about a billion tonnes. In 1974 he discovered that the physical temperature of a black hole is not necessarily absolute zero. Hawk-ing's model of black holes showed that they can emit a kind of thermal radiation by the simultaneous creation of pairs of matter and anti-matter particles. This process, called HAWKING RADIATION, implies that the black hole shrinks and may eventually evaporate, since it is gradually converting its own mass into thermal energy.

Cambridge University appointed Hawking Lucasian Professor of Mathematics in 1979. In 1983 he put forth his 'no-boundary proposal': that although both space and time are infinite, these quantities have no physical boundaries. Hawking has popularized many of his theories through a series of magazine articles, books and television programmes; his book A BriefHistory of Time (1988) spent 4 years on the bestseller lists.

Hawking radiation Radiation that is emitted from a BLACK HOLE; the process was proposed by Stephen HAWKING. From quantum field theory (the theory developed by Max PLANK in 1900, which proposes that various properties of a system, including energy, can only change by discrete amounts), it can be deduced that vacuum fluctuations occur throughout space whereby pairs of virtual particles are created. If such a pair is created near the EVENT HORIZON of a black hole, one particle may lie just on the inside, one on the outside, and the outside particle would be free to radiate away. A black hole of one solar mass will evaporate over 1067 years. A black hole at the centre of a galaxy will take around 1097 years to evaporate. Primordial black holes created in the Big Bang will have already disappeared.

Hayashi track Path on a HERTZSPRUNG-RUSSELL DIAGRAM that a pre-MAIN-SEQUENCE star follows as it evolves on to the main sequence. It is named after the Japanese astrophysicist Chushiro Hayashi (1920- ), who studied stellar evolution during the 1950s and 1960s.

PROTOSTARS lie to the right of the main sequence. An initial rapid collapse of the protostar, which is poorly understood, moves the star leftwards out of the so-called Hayashi forbidden zone. This collapse is followed by a large decrease in LUMINOSITY, caused by the contracting radius. This line on the diagram is called the Hayashi track. During this phase the star is almost entirely convective, with only a small radiative core. Finally, the star becomes radiative and approaches the main sequence along the HENYEY TRACK.

Haystack Observatory Multidisciplinary research facility of the Massachusetts Institute of Technology (MIT) at Westford, Massachusetts, engaged in radio astronomy, geodesy, atmospheric science, radio interferometry and radar imaging, established in 1964 originally as a military facility. The principal instrument is a 37-m (120-ft) fully steerable antenna, enclosed in the world's largest space-frame radome. The telescope can be used either for single-dish observing or as part of a VERY LONG BASELINE INTERFEROMETRY network.

HD Abbreviation of henry draper catalogue HDF Abbreviation of hubble deep field heat death of the Universe Eventual fate of the Universe, perceived in the 19th century to be the logical outcome of applying the second law of thermodynamics to the whole Universe. This law states that in a closed system, entropy can only increase or remain constant -or that heat is transferred only from warmer masses to cooler ones. The Universe 'dies' once when all matter has reached the same temperature, for then the Universe's entropy will have reached a maximum, and there will be no energy available. This 'running down' was pointed out by Lord kelvin, who preferred to avoid such a fate by invoking an omnipotent force that created living beings. In terms of modern cosmology, the heat death would not occur in a closed finite universe in which the density parameter is greater than 1, but it would occur in a universe that expanded for ever.

Heaviside layer See kennelly-heaviside layer heavy elements All the chemical elements apart from hydrogen and helium. Within astronomy, the term 'metals' is sometimes used as a synonym for heavy elements. The concept of the heavy elements is valuable because in terms of relative abundance (see astrochemistry) hydrogen and helium account for around 98% of the mass of the observable Universe, while the remaining heavier elements amount to just 2%. Hence it is often useful to consider them all under one heading.

Hebe Large main-belt asteroid discovered in 1847; number 6. Hebe is about 186 km (116 mi) in diameter.

HED Abbreviation for howardite-eucrite-diogenite association

Heinrich Hertz Telescope Joint facility of steward observatory and the max-planck-institut fur radioastronomie run by the Submillimeter Telescope Observatory. The enclosed 10-m (33-ft) telescope is located on Mount Graham, Arizona, and operates in the 0.3-2-mm wavelength range. It is named after the German physicist Heinrich Rudolf Hertz (1857-94), pioneer investigator of electromagnetic waves.

Hektor Third trojan asteroid to be discovered, in 1907; number 624. It is the largest of all Trojans, at approximately 225 km (140 mi) in size. Variations in Hektor's brightness indicate that it has an elongated shape, perhaps like a dumbbell.

Helene Small satellite of saturn; it is co-orbital with dione. It was discovered by Pierre Laques (1934- ) and Jean Lecacheux (1944- ) in 1980. It is spheroidal in shape, measuring about 32 km (20 mi). Helene has a near-circular, near-equatorial orbit, remaining close to the L4 lagrangian point associated with Dione's orbit around Saturn. It takes 2.74 days to make a circuit of Saturn at a distance of 377,400 km (234,500 mi).

heliacal rising Strictly, the rising of a celestial body simultaneously with the Sun. More commonly, it refers to the date when the body first becomes visible in the dawn sky, rising just before the Sun. The heliacal rising of sirius, the brightest star in the sky, was used by the ancient Egyptians to predict the annual flooding of the River Nile, and to denote the beginning of the new year. The Nile flood was an important event in the Egyptian calendar, heralding the first of three seasons; after the river subsided, planting of crops could commence, followed by the harvest.

heliocentric parallax Alternative name for annual parallax. See also diurnal parallax; trigonometric

parallax heliocentric theory Model of the Solar System in which the Sun is at the centre and the planets revolve around it. A heliocentric theory was proposed by the Greek astronomer aristarchus in the 3rd century bc, but it seemed counter-intuitive at the time and was not widely adopted. geocentric theory, championed by Aristotle and Ptolemy and by later scholars and theologians, was to remain supreme for nearly 1500 years.

It was Nicholas Copernicus who re-invigorated heliocentric theory with his copernican system, published the year of his death. Opposed by both Roman Catholic and Protestant ecclesiastics, its influence spread only very slowly. However, the great improvements in observational accuracy introduced by Tycho brahe showed up the inadequacy of geocentric theory. Brahe himself favoured a hybrid, now referred to as the tychonic system, in which the Earth remained at rest and was orbited by the Sun, but the other planets orbited the Sun. His observations showed that the stars were much more distant than the planets and he also realized that comets moved in orbits that would have taken them through the 'crystalline spheres' of Aristotle's geocentric theory. Thus the spheres could not be real.

The telescopic discoveries by galileo - that a body other than the Earth, namely Jupiter, had moons in orbit around it, that the planet Venus exhibited phases like the Earth's Moon and that the Sun was covered in spots which made it less than 'perfect' - was evidence enough to put an end to the geocentric theory. Galileo's open advocacy of the Copernican position placed him in conflict with elements within the Catholic Church. His main publication in support of heliocentrism, the dialogues of 1632, led to his trial before the Inquisition. When Johannes Kepler, using data accumulated by Brahe, discovered his three laws (see kepler'slaws), the need for the complicated epicyclic motions was swept aside by the simplicity of elliptical orbits.

In the decades following Kepler's and Galileo's discoveries, heliocentric theory gained wide acceptance. Isaac Newton, with the publication of his principia in 1686-87, showed that the assumption of an inverse-square law of gravitational attraction would account for Kepler's laws. Following his work, the heliocentric theory of the Solar System could no longer be questioned.

heliometer See divided telescope

Helios Either of two probes launched in 1974 and 1976 to study the Sun and interplanetary space. They both approached the Sun to within 43 million km (26.7 million miles).

helioseismology Study of the structure and dynamics of the solar interior by the analysis of sound waves that propagate through the Sun and manifest themselves as oscillations of the photosphere. Helioseismology is a hybrid name combining the Greek words helios (for Sun), seismos (meaning quake or tremor) and logos (for reasoning or discourse). The periods of the vertical oscillations are usually around five minutes, but they can range to a few hours long. They are most often detected by the doppler effect of a solar absorption line formed in the photosphere, but they can also be detected by light variations of the photosphere. There are millions of distinct, resonating, sound waves whose periods depend on their propagation speeds and the depths of their resonant cavities. Examination of the photosphere oscillations that the sound waves produce enables helioseismologists to infer the temperature, chemical composition and motions at different depths within the Sun.

helioseismology This series of SOHO images shows shockwaves propagating away from a flare associated with an active region on the Sun.

heliosphere Vast region of space surrounding the Sun where the interplanetary magnetic field and solar wind have a dominant influence on the movements of electrons and ions. The heliosphere, immersed in the local interstellar medium, defines the extent of the Sun's influence. It extends well beyond the planets to the heliopause, which is located at a distance from the Sun of about 100 AU. This outer boundary marks the place where the solar wind pressure balances that in interstellar space.

heliostat Flat, equatorially mounted mirror that is driven to follow the Sun's apparent motion across the sky and to direct its light into a fixed solar telescope. Because of the long focal length of a solar telescope, and the fact that the heat of the Sun creates a layer of warm, turbulent air close to the ground, the heliostat is usually mounted on the top of a tall tower. As it follows the Sun across the sky, the image it forms slowly rotates during the course of the day and, because of this, the more sophisticated coelo-stat is sometimes preferred. See also siderostat

helium (symbol He) Second lightest chemical element; it is the second most abundant element in the Universe (see astrochemistry). Helium-4 forms almost 100% of the naturally occurring element and has an atomic number of 2 and an atomic mass of 4.0026 amu. Helium is the lightest of the 'noble' gases - those elements that are virtually inert chemically. Its boiling point is 4.2 K. There are five known isotopes, three of which (helium-3, helium-4 and helium-5) are stable. The nucleus of helium-4 is also called the a-particle (alpha particle).

Helium is thought to be generated by nucleosynthesis in stars. The basic reaction, which powers main-sequence stars, involves the fusion of hydrogen to form helium via the proton-proton reaction and the carbon-nitrogen-oxygen cycle. However, the present abundance of helium in the Universe cannot be explained solely by nucleosynthesis in stars. It is now generally accepted that most of the helium observed today was synthesized during the first few minutes of the big bang. The sequence of nucleosynthetic reactions continues after the formation of helium with the triple-a process, in which three helium-4 nuclei collide almost simultaneously to form carbon-12. The reaction requires a temperature of around 100 million K for its initiation.

Helium was first identified in the solar spectrum by Jules janssen and Joseph lockyer in 1868 as a set of lines that did not correspond with those for any element then known. It was named after Helios, the Greek for 'Sun'. It was found on Earth in 1895 as a gas released by radioactivity from the mineral clevite.

Helium is used to pressurize the propellant tanks of some liquid-fuelled rockets and, in liquid form, to cool many infrared and radio detectors.

helium flash Theoretically predicted event in the evolution of a lower-mass star (around 1 to 2 solar masses) whereby helium fusion occurs explosively, with changes in the central regions of the star occurring over timescales of minutes.

After the hydrogen in the core of a main-sequence star has been exhausted, the star becomes a red giant, with hydrogen shell burning occurring. The core collapses until temperatures of 108 K are reached, when helium burning can start. In stars of low mass, this event occurs explosively and is known as the helium flash. In stars of higher mass, helium fusion will commence gradually because the temperature in the core can reach this point before the core becomes degenerate. See also degenerate matter

Helix Nebula One of the closest planetary nebulae, NGC 7293 is visually faint because its light is spread over a fairly large area of sky.

helium star Non-white dwarf star, the outer layers of which contain more helium than hydrogen. A helium star is an evolved star that has lost its hydrogen-rich envelope, possibly by the effect of a companion in a close binary system, thereby exposing its helium core. The r coronae borealis stars are examples of helium stars. The term 'helium star' was also originally used for B-type stars.

Helix Nebula (NGC 7293) planetary nebula in southern Aquarius (RA 22h 29m.6 dec. -2048'). Lying less than 300 l.y. away, the Helix is one of the closest planetary nebulae and, as a result, subtends a large angular diameter of 13'. Ostensibly bright, at mag. +7.3, the nebula is spread over such a wide area that it has low surface brightness and is consequently rather difficult to observe visually. The central star has mag. + 13.6.

Hellas Planitia Largest impact basin on mars (43.0S 290.0W); it has an average diameter of 1800 km (1100 mi). Hellas Planitia appears on early maps of Mars as a roughly circular region of ochre hue. It is situated south of the Martian equator, in a hemisphere predominantly well above datum (an arbitrary height at which the pressure is 6.2 millibars). Hellas Planitia itself lies well below this level, and at its deepest point, near to the western rim, it descends to 5 km (3 mi).

Hellas Planitia The largest impact structure on Mars, Hellas Planitia is one of the most obvious features in the planets southern hemisphere. The roughly circular basin has a diameter of 1800 km (1100 mi).

Hellas' rugged hilly rim is between 50 and 400 km (30-250 mi) in width and is associated with a concentric pattern of fracturing that extends as far as 1600 km (1000 mi) from the centre of the floor. The floor of the basin is occupied by plains materials, which are believed to be volcanic in origin, and is mantled by windblown debris. Large sections of the rim are either missing or difficult to trace due either to erosion or to later burial by younger deposits. The most continuous section lies on the western side, where it passes into the blocky mountains of Hellespontus Montes. Spacecraft imagery reveals there to be several annular, inward-facing scarps outside of the main rim, together with a small number of large ancient volcanic structures, such as Amphitrites Patera and Hadriaca Patera. The floor of Hellas Planitia is occupied by rather complex plains units, on the surface of which are small impact craters and some short channel systems.

Helwan Observatory Egyptian observatory at Helwan, about 30 km (20 mi) south of Cairo. Dating from the turn of the 20th century, the observatory was equipped with a 0.76-m (30-in.) reflector in 1905. A new observing station was set up at Kottamia, 70 km (45 mi) north-east of Helwan in the 1950s, and a 1.9-m (74-in.) telescope began work there in 1964; it was upgraded in the late 1990s.

Henderson, Thomas (1798-1844) Scottish astronomer, second Astronomer at the Cape (1831-33) and first Astronomer Royal for Scotland (183414). His observations from South Africa in 1832 provided the first parallax for a Centauri, though he did not announce his result (0".93, corresponding to a distance of 3.5 l.y.) until 1839, the year after Friedrich Wilhelm BESSEL's measurement of the parallax of Sixty-one Cygni. Henderson made over 60,000 observations of star positions, leading to the compilation of several important star catalogues.

Henry, Paul Pierre (1848-1905) and Henry, Prosper Mathieu (1849-1903) French brothers whose achievements in ASTROPHOTOGRAPHY helped to stimulate the carte du ciel sky survey. The Henrys discovered 14 asteroids, beginning with (175) Liberatrix in 1872. Their searches required the preparation by hand of detailed star charts for the ecliptic, so that asteroids would not be confused with stars - a labour-intensive and error-prone procedure. Their successful photography of star clusters, including the Double Cluster in Perseus, a -hour exposure that recorded stars as faint as 12th magnitude, convinced them that good charts could be prepared photographically.

The Henrys designed and built a 13.5-inch (340-mm) refractor for Paris Observatory specifically for astrophotog-raphy, completed in 1885. At the Astrographic Congress on 1887 April 18, observatories round the world agreed to undertake an all-sky photographic survey, the Carte du Ciel, using 'astrographs' - telescopes identical to the 13.5-inch at Paris - about half of which were made by the Henrys.

Henry Draper Catalogue (HD) Catalogue of stellar spectra compiled by Annie CANNON at Harvard College Observatory. It was named after American pioneer of astrophotography Henry DRAPER, whose widow supported the work financially. The catalogue, completed in 1924, classified about 225,000 stars to 10th magnitude according to the Harvard system: O, B, A, F, G, K, M, in order of decreasing surface temperature. The Henry Draper Extension (HDE), with spectral classifications of 47,000 11-th magnitude stars, appeared in 1936. Stars are still widely known by their HD or HDE numbers. See also SPECTRAL CLASSIFICATION

Henyey track Evolutionary path on a HERTZSPRUNG-RUSSELL DIAGRAM that a pre-MAIN-SEQUENCE star follows from the base of the HAYASHI TRACK to the main sequence. It is named after the American astrophysicist Louis Henyey (1910-70), who studied stellar evolution during the 1950s.

Along the Henyey track both EFFECTIVE TEMPERATURE and LUMINOSITY increase such that luminosity, L, is proportional to effective temperature, Te (L is proportional to Te4/5). During this phase, energy transfer within the star is largely radiative rather than convective, in contrast to the HAYASHI TRACK. The length of the Henyey track (the increase in Te) is greater for high-mass stars than low-mass stars, although the time spent on this track is shorter for the high-mass stars. Stars of mass less than 0.5 solar mass do not have a Henyey track, instead they evolve directly on to the main sequence from the Hayashi track. Before reaching the main sequence, the star's luminosity dips slightly due to the expansion of the star caused by the onset of nuclear burning.

Hephaistos APOLLO ASTEROID discovered in 1978; number 2212. It is one of the largest Apollos, with a size of about 8 km (5 mi). See table at NEAR-EARTH ASTEROID

Heraclides, Promontorium Southern termination of the Moon's Sinus IRIDUM; it rises to 1220 m (9000 ft) and appears as a cape-like structure jutting out into Mare IMBRIUM. With the northern termination, Promontorium Laplace, it forms the outer boundaries of a much larger impact structure. This structure was inundated with lava, covering the eastern rim. Multiple mare (wrinkle) ridges lie between the two promontories.

Herbig-Haro object (HH object) Any of a class of small, faintly luminous nebulae discovered, independently, by George Herbig (1920- ) and Guillermo Haro (1900-1990) in the 1950s. HH objects are irregular in outline and contain bright knots; they are found in regions rich in interstellar material. Their spectra reveal a weak continuum dominated by emission lines from hydrogen, oxygen, nitrogen and iron. HH objects are produced when high-velocity material, ejected from young stars, interacts with surrounding material.

HerbigHaro object Highspeed jets from the young object HH 34, near right centre, impact on the surrounding interstellar material to produce V-shaped shockwaves. HH 34 lies in Orion, in a region of ongoing star formation.

The stars that produce HH objects are thought to be young, pre-main-sequence stars that are accreting material via an ACCRETION DISK. The whole system is surrounded by the material from which the star is forming. High-velocity ejections are then produced, although the mechanism by which this occurs is not yet fully understood, and they interact with the surrounding material producing shock waves. The shock waves heat and ionize the surrounding gas, producing the emission lines observed in HH objects.

HH objects are roughly split into low and high-excitation objects, according to their spectra. The difference is thought to be caused by the velocities of the shock waves. High-excitation HH objects, with strong emission lines of highly ionized material, suggest shock wave velocities of 200 km/s (120 mi/s), while low-excitation HH objects have shock wave velocities of only around 20 km/s (12 mi/s). Many HH objects show bright emission knots located in two diametrically opposing lobes, moving in opposite directions away from the source. This is consistent with a high-velocity jet of material being ejected as a bipolar outflow. See also TTAURI STAR

Hercules See feature article

Hercules X-1 Strong X-ray source and an ECLIPSING BINARY system; it comprises a visible star, HZ Herculis, and an accreting neutron star that is an X-ray pulsar. The pulsar has a period of 1.24 seconds, emitting in X-rays and visible light. The orbital period of the binary is 1.7 days, and there is also a 35-day modulation of the X-ray emission possibly caused by precession of the accretion disk. Since Hercules X-1 is an eclipsing binary, the stellar masses can be determined - 0.98 solar mass for the neutron star and 1.99 solar mass for the companion.

Hermes Third APOLLO ASTEROID to be discovered, in 1937, when it was passing within about 800,000 km (500,000 mi) of the Earth. This close approach led to the popular assumption of a name despite the orbit not being sufficiently well determined to allow Hermes to be added to the list of numbered asteroids. Hermes was observed for only a few days and then lost. It was catalogued as 1937 UB.

Herodotus Lunar crater (23N 50W), 37 km (23 mi) in diameter, with rim components reaching 1220 m (4000 ft) above its floor. Herodotus' floor has been resurfaced with lava, rendering it nearly featureless. Its northern wall appears to be degraded, with a gap pointing towards Vallis SCHROTERI. The Aristarchus Plateau, surrounding Herodotus, has many dark patchy areas, representing volcanic fire fountaining.

Herschel family Dynasty of English astronomers, originally from Hanover, Germany. Its founder was Friedrich Wilhelm Herschel (1738-1822), who settled in England in 1757 and assumed the name William HERSCHEL when he became a naturalized Englishman. In 1772 he wasjoined by his youngest sister, Karoline Lucretia Herschel (1750-1848), who became his astronomical assistant (see Caroline HERSCHEL). William was also aided by his son, John Frederick William HERSCHEL (1792-1871), who went on to become one of the foremost English scientists of his day. John's son Alexander Stewart Herschel (1836-1907), born during his father's sojourn at the Cape of Good Hope, determined a number of meteor radiants and their association with cometary orbits. The male line of the British Herschels is now extinct.

Herschel, Caroline Lucretia (1750-1848) German astronomer, sister of and assistant to William HERSCHEL, and aunt of John HERSCHEL. Karoline Herschel (the German form of her name) was born in Hanover and worked as a family drudge until 1772, when William brought her to England, where she became first his domestic companion, then his musical protegee, and finally his astronomical assistant. When they moved to Datchet and Slough she started sweeping the sky herself, discovering several nebulae (including the Sculptor Galaxy, NGC 253, in 1783) and eight comets. When William died in 1822, she returned to Hanover, where she received many honours.

Herschel, John Frederick William (1792-1871) English scientist and astronomer, the only child of William HERSCHEL's marriage to Mary Pitt in 1788. After studying mathematics, and an abandoned attempt at training for the legal profession, he became his father's protege and successor in astronomy, discovering several hundred new nebulae and clusters from England. John Herschel's main goal was to extend his father's sky surveys, undertaken for the purpose of fathoming the 'construction of the heavens', what we would call cosmology. In 1834 he took one of William's telescopes to the Cape of Good Hope (modern South Africa) to survey the southern skies, and found 1200 new double stars and 1700 new nebulae and clusters.

By 1840 Herschel's observing career was effectively over. The rest of his life was devoted to interpreting his, and his father's, cosmological work, and acting as an international consultant for all things astronomical and physical. He compiled the General Catalogue of Nebulae and Clusters from his and his father's observations; this would form the basis for J.L.E. Dreyer's new general catalogue. Her-schel's Outlines of Astronomy (1849) remained a standard textbook for many decades. He was also a pioneer of early photographic chemistry and astrophotography.

Herschel, (Frederick) William (1738-1822) German-born English astronomer and musician. He took up astronomy in the 1770s, making his own telescopes and mirrors, and won fame in 1781 for his discovery of the planet Uranus. He discovered two satellites of Uranus

A Herbig-Haro object Highspeed jets from the young object HH 34, near right centre, impact on the surrounding interstellar material to produce V-shaped shockwaves. HH 34 lies in Orion, in a region of ongoing star formation (1787) and two of Saturn (1789). He observed and catalogued many double stars, nebulae and clusters. Herschel realized that the Milky Way is the plane of a disk-shaped stellar universe, the form of which he calculated by counting the numbers of stars visible in different directions, thus establishing for the first time what he called the 'construction of the heavens'.

Wilhelm Friedrich Herschel, as he was christened, was born in Hanover, the son of an army band musician. As a boy he received a surprisingly good education at the garrison school, especially in music, and visited England in 1756 with his regiment. Finding that England offered abundant well-paid opportunities for skilled musicians, he returned in 1757, resolved to seek his fortune. After several years of hard work, his reputation won him the prestigious post of organist at Bath's fashionable Octagon Chapel in 1766. In 1772 he brought his sister Caroline Lucretia HERSCHEL from Germany to become his domestic companion and to allow her to pursue her own musical career.

By 1771 William Frederick (as he now called himself) was earning 400 a year from music - a very handsome income for the time - and, as Caroline recorded, William's interest in the mathematical harmonics of music aroused in him a curiosity about the mathematical basis of light and astronomy. In 1773 he started to make reflecting telescopes with mirrors made from speculum (Latin for 'mirror') metal, and used them to observe the heavens from the Herschel's house at 19 New King Street, Bath (now the Herschel Museum). He soon showed himself to be a brilliantly gifted practical optician, for it was later declared by Nevil MASKELYNE, the Astronomer Royal, that his reflecting telescopes were superior to any professionally made instruments then on sale in London. Herschel was thrust into international celebrity in 1781 when he discovered a 'comet' that soon turned out to be the planet Uranus - the first new planet in the Solar System to be discovered in recorded history.

This find, however, was the unexpected by-product of one of Herschel's surveys of the sky with a 6-inch (150-mm) reflecting telescope. He had begun systematically to 'sweep' the skies on a zonal basis, primarily as part of a strategy aimed at discovering star clusters, binary stars and nebulae, and trying to understand their distribution on the sky. By the mid-1780s, indeed, this technique had led him to conclude that the Milky Way consisted of a flat plane of stars running through space, so that when one looked through the long axis of the plane one saw dense starfields all around, whereas when one looked at right angles out of the plane, one saw relatively few stars. This observation still holds good for the general structure of our Galaxy, though not for the Universe in general.

Herschel's discovery of Uranus fundamentally changed his career. The international fame that it brought won him (and his sister Caroline) royal patronage, William's fellowship of the Royal Society, and an annual pension of 200, which enabled him to give up music and devote himself full-time to astronomy. This drop in income was more than made up, however, by commercial demand for his telescopes. Herschel produced the optics, and employed joiners to make the elegant mahogany tubes and stands. A Herschel 6-inch refractor of 7 ft (2.1 m) focal length sold for 105, while bigger instruments, built for European royalty, could cost as much as 3150.

Herschel's truly creative decade was the 1780s, for in the wake of the MESSIER CATALOGUE of 1784, his sky surveys increased the number of known deep-sky objects to over 2000. He was the first scientist to realize that the cosmos is not unchanging, but dynamic, as stars were now understood to form clusters under gravity, disintegrate into nebulous matter, and then re-condense into incandescent stars. Herschel also realized that the light from dim objects gathered by his 18|-inch (0.48-m, 1783) and 48-inch (1.2-m, 1788) aperture telescopes of 20 and 40 ft (6 and 12 m) focal length had been travelling through space for so long that when observing them, one was looking into the past. By 1795 Herschel had pushed contemporary optical technology as far as it could go, and he spent the remaining 27 years of his life trying to obtain new data to make further breakthroughs. But not until the invention of ASTROPHOTOGRAPHY and astronomical spec-troscopy in the next century would this be possible. In 1788 Herschel married Mary Pitt, and in 1792 John Frederick William HERSCHEL was born to them.

Herschelian telescope Type of telescope developed by William HERSCHEL in the late 18th century. It uses a PARABOLOIDAL primary, as in the NEWTONIAN TELESCOPE, but the primary is tilted so that the image is formed to one side of the incoming light, obviating the need for a secondary mirror: there is thus no central obstruction. The EYEPIECE is aimed down the tube, with the observer facing the main mirror. This optical arrangement, also known as the front-view system, introduced image distortions, and was soon superseded by the achromatic refractor and the silver-on-glass Newtonian reflector.

Herschel-Rigollet, Comet 38P/ Comet found by Caroline Herschel on 1788 December 21 and unexpectedly recovered in 1939 July by Roger Rigollet (1909-81). At each return, the comet attained peak mag. + 7.5. Initially thought to be parabolic, the comet's elliptical orbit has a current period of 155 years.

Herschel Space Observatory (Far Infrared and Submillimetre Telescope, FIRST) EUROPEAN SPACE AGENCY (ESA) observatory to be launched in 2007. It will orbit at the L2 Lagrangian point between the Earth and the Sun, 1.5 million km (0.9 million mi) from the Earth. Named after William HERSCHEL, who discovered infrared light, the craft is equipped with a 3.5-m (11-ft) diameter primary mirror. It will cover the full FAR-INFRARED and SUBMILLIMETRE waveband with the prime objective of studying the formation and evolution of stars. Herschel will be working in tandem with another ESA astronomical observatory, PLANCK.

Hertzsprung, Ejnar (1873-1967) Danish astronomer after whom the HERTZSPRUNG-RUSSELL DIAGRAM is named. Hertzsprung spent most of his career (1919-44) at the Leiden Observatory, which he directed for nine years. He was a prolific gatherer of stellar data, making more than a million photographic observations of binary stars and finding the brightnesses, spectral types and proper motions of thousands of stars in the Pleiades. He began to establish the mathematical form of Henrietta Leavitt's period-luminosity relation for Cepheid variable stars, and used it to determine accurately the distance to the Small Magellanic Cloud. Hertzsprung's work spanned the extremes of stellar types: the Cepheids are highly luminous yellow supergiants; and, in contrast, he was the first to classify, in 1911, the giant and dwarf subdivisions of late-type stars.

The culmination of this work, based in part on the work of pioneering astronomer Antonia MAURY, was his invention of the Hertzsprung-Russell diagram, so called because the American astronomer Henry Norris RUSSELL later rediscovered the relation. Hertzsprung found, by plotting luminosity against temperature for many individual stars, that most 'normal' stars, including the Sun, fall along a well-defined curve called the main sequence. His results were published in 1905 and 1907 in an obscure journal, and were unknown to Russell.

Hertzsprung gap Region to the right of the MAIN SEQUENCE on the HERTZSPRUNGRUSSELL DIAGRAM where very few stars are seen. The gap exists because stars move very rapidly through this region. In the gap, the stars are evolving off the main sequence, burning hydrogen in shells before helium burning starts in the core. They travel on near-horizontal tracks with near-constant luminosity as the radius expands with a corresponding drop in effective temperature.

Hertzsprung-Russell diagram (HR diagram) Plot of the ABSOLUTE MAGNITUDE of stars against their spectral class. It is equivalent to a plot of the luminosity of stars against their surface temperature. The Hertzsprung-Russell diagram is an extremely valuable tool wherein observational and theoretical information are blended to produce a deep understanding of STELLAR EVOLUTION. It is a very powerful diagram, able to represent almost the entire evolution of any star. There are a finite number of equations defining a star's structure at every point in its life. These equations are interpreted on an HR diagram, so that determining the position of any star on an HR diagram will give information on its structure and evolutionary phase.

HertzsprungRussell diagram A plot of stars luminosity against temperature. Most stars lie on the main sequence running from top left to bottom right. Part B shows the principal regions of the HertzsprungRussell diagram, with giant stars above the main sequence to the top right, and dwarfs to the lower left. Part C illustrates some of the evolutionary tracks that stars may take as they branch away from the main sequence. For example, hot, massive O-class stars (top left) evolve to become red supergiants (top right).

In the early 1900s Ejnar HERTZSPRUNG in Denmark and Henry Norris RUSSELL in the United States independently began to consider how the brightnesses of stars might be related to their spectra. In 1911 Hertzsprung plotted the APPARENT MAGNITUDES of stars in several clusters against their spectral class. In 1913 Russell plotted the absolute magnitudes of stars in the solar neighbourhood against their spectral class. Both astronomers emphasized the non-random patterns they saw in the arrangement of the data on these graphs. Such graphs have become extremely important in modern astronomy and are today called Hertzsprung-Russell diagrams.

A Hertzsprung-Russell diagram, or HR diagram, is any graph on which a parameter measuring stellar brightness is plotted against a parameter related to a star's surface temperature. For example, apparent magnitude, absolute magnitude, absolute BOLOMETRIC MAGNITUDE or LUMINOSITY may be plotted along the horizontal axis, while spectral class, COLOUR INDEX or EFFECTIVE TEMPERATURE may be plotted along the vertical axis.

Three HR diagrams are shown with this article, with diagram A being a schematic showing all the different regions. Diagram B resembles Russell's original graph. Each dot represents a star whose absolute magnitude and spectral class have been determined from observations. Most of the data lie along a broad line called the MAIN SEQUENCE, which extends diagonally across the diagram. The Sun (absolute magnitude around +5, spectral type G2) is a typical main-sequence star. A second prominent grouping of data points represents very large, cool stars called RED GIANTS. In the lower left corner, a third grouping identifies compact, hot stars called WHITE DWARFS.

Diagram C shows an HR diagram with luminosity of stars plotted against their surface temperature. Notice that temperature increases towards the left. The spectral classes OBAFGKM constitute a temperature sequence, and the first HR diagrams were drawn with the hot O stars on the left and the cool M stars on the right because the astronomers did not realise exactly what they were plotting. Although this means the temperature scale increases to the left, HR diagrams are still plotted this way today.

These HR diagrams show that the total range in stellar brightness is around 27 magnitudes (corresponding to a factor of 1011 in luminosity), and the range in the surface temperature of stars is from 2200 K to 50,000 K. The size of a star is related to both its luminosity and its surface temperature, as indicated by the dashed lines. Most main-sequence stars are roughly the same size as the Sun. White dwarfs are about the same size as the Earth, while red giants can be as big as the Earth's orbit.

The statistical distribution of data on an HR diagram demonstrates that stars fainter than the Sun are far more numerous than those brighter than the Sun. About 90% of stars are main-sequence stars, about 10% are white dwarfs and about 1% are red giants or SUPERGIANTS. The three main groupings on the HR diagram correspond to the three very different stages through which a typical star passes during its life. Computer models reveal how the luminosity and surface temperature of a star change as it evolves. With this information, the path followed by an evolving star can be plotted on an HR diagram PROTOSTARS are formed in fragmenting molecular clouds. As they evolve, their pre-main-sequence phase is represented on the Hertzsprung-Russell diagram as HAYASHI and HENYEY TRACKS. Contracting, pre-main-sequence stars change rapidly as gravitational contraction compresses and heats their cores. When the central temperature reaches several million degrees, nuclear FUSION begins, in the form of hydrogen burning, and the evolutionary track stops at the main sequence. Main-sequence stars are stable stars, burning hydrogen at their cores. A star will spend most of its life on the main sequence.

Pre-main-sequence evolutionary tracks deposit newborn stars on the main sequence in a position consistent with the MASS-LUMINOSITY RELATION, so that the more massive a star is, the brighter it is. Thus, the main sequence is a sequence in mass, as well as temperature and luminosity. These tracks also show that the more massive a star is, the more rapidly it evolves.

As a star consumes hydrogen, it gradually becomes slightly brighter and cooler. When the hydrogen supply is exhausted, the star's core contracts, its atmosphere expands, and it becomes a red giant or supergiant. It evolves off the main sequence at a point called the TURNOFF POINT. As it burns the hydrogen in its shell, it is on the red giant branch. If it is massive enough, the temperature of the collapsing core can become hot enough for helium burning to start. When a star is burning helium in its core, it is a HORIZONTAL BRANCH STAR.

Between the horizontal branch stars and the red giant branch is a gap. When the evolutionary track of a low-mass star takes it across this region, the star pulsates as an RR LYRAE VARIABLE. This gap is at the lower end of the so-called INSTABILITY STRIP, a vertical region in the middle of the HR diagram where stars are unstable against radial oscillations. When a massive star passes through this region, it pulsates and is called a CEPHEID VARIABLE.

The asymptotic giant branch has stars with both hydrogen and helium shell burning. PLANETARY NEBULAE are ejected at this stage. A star less massive than about 3 solar masses can eject as much as half its mass at the end of its life, producing such a planetary nebula. The exposed, burned-out core of the star then contracts to become a white dwarf. Detailed calculations of this process do indeed produce evolutionary tracks that rapidly take low-mass stars from the red giant region in the upper right of the HR diagram to the white dwarf region in the lower left of the diagram. Massive stars can end their lives in SUPERNOVA explosions, which leave behind NEUTRON STARS or BLACK HOLES. As these are detected at non-visible wavelengths, they are not represented on an HR diagram.

Stellar evolution calculations relate the HR diagram to time, and thus these graphs can provide information about the ages of stars. The HR diagram is particularly useful for estimating the age of star clusters. A very young cluster, such as NGC2362, consists primarily of main-sequence stars. However, the more massive a star is, the more rapidly it consumes its core supply of hydrogen to become a red giant. Since the main sequence is a progression in stellar mass, the stars at the upper end of the main sequence are the first to leave. As the cluster ages, the turnoff point for the evolving stars moves back along the main sequence. M67, for example, is an old cluster because only those stars less massive and less luminous than the Sun still remain on the main sequence. See also SPECTRAL CLASSIFICATION

Hess, Victor Francis (1883-1964) Austrian-American physicist (born Viktor Franz Hess), one of the discoverers of cosmic rays, for which he shared the 1936 Nobel Prize for Physics. Hess began investigating the phenomenon of atmospheric ionization by launching electroscopes in high-altitude balloons. Instead of this ionization decreasing with altitude, he found (1912) that it actually increased by a factor of eight in the upper levels of the atmosphere, suggesting an extraterrestrial source for this radiation, later known as cosmic rays.


Hevelius, Johannes (1611-87) German astronomer and instrument-maker, also a wealthy brewer and city magistrate, born in Danzig (modern Gdansk, Poland). In 1641 he built at Danzig what for a time was the world's leading observatory; called Sternenburg, it was destroyed by fire in 1679. His second wife, Elizabeth Margarethe Hevelius (1646/7-93), assisted him in much of his observational work, and she edited and published his magnum opus, Prodromus astronomiae, published posthumously in 1690. Hevelius was the first astronomer to describe the bright features of the solar photosphere known as faculae, which he found were connected with the formation of sunspots. He also derived an accurate value for the Sun's rotation period. In 1644 he observed the phases of Mercury that Nicholas Copernicus had predicted the planet would show.

Hevelius discovered four new comets, and suggested that these objects, which he called 'pseudo-planetae', moved about the Sun in parabolic orbits. He was also a skilled observer of the Moon and planets. In Selenographia (1647), the first truly detailed lunar atlas depicting how the appearance of lunar features changed with the Moon's phases, Hevelius coined the term 'mare' to describe the Moon's dark areas. He measured the heights of lunar mountains, obtaining results more accurate than Galileo's. His catalogue of 1564 stars and the accompanying Ura-nographia star atlas was based on many years of naked-eye observations with sighting instruments similar to those used by Tycho BRAHE; Hevelius was the last major astronomer not to use a telescope.

Hewish, Antony (1924- ) English radio astronomer and recipient, with Martin RYLE, of the 1974 Nobel Prize for Physics for their role in discovering pulsars, and also for their development of the radio-astronomy technique of APERTURE SYNTHESIS. Hewish was part of the team at Cavendish Laboratory, Cambridge, that mapped the radio sky during the 1950s and 1960s, compiling the Cambridge Catalogues of radio sources. In the 1960s he made extensive studies of radio scintillation, the variation of radio waves' intensities, quantifying how Earth's atmosphere, solar radiation and even the interstellar medium all contribute to this effect. Although he received credit for discovering pulsars, it was actually his graduate student Jocelyn BELL BURNELL who first noticed the pulsed emissions.

hexahedrite See IRON METEORITE

Hey, (James) Stanley (1909-2000) British physicist and radar pioneer whose work during World War II led to the development of modern radio astronomy techniques. In 1942 he investigated interference with England's defence radar, which many experts thought was 'jamming' by the enemy; instead, he discovered that solar radiation was to blame. In later years Hey studied radio emission from sunspots, as well as variations in the Sun's overall radio emission. After the war, Hey turned his attention to radio sources outside the Solar System, and was the first to identify the exact location of the Cygnus A radio galaxy.

HH object Abbreviation of HERBIG-HARO OBJECT

Hidalgo Asteroid number 944; it is unusual in that it occupies a comet-like orbit stretching from perihelion near 2 AU out to aphelion at 9.6 AU. Hidalgo was discovered in 1920. It is about 30 km (20 mi) in size. See also CENTAUR; DAMOCLES; JUPITER-CROSSING ASTEROID


Higgs field Field due to the Higgs boson. The standard model of elementary particles leaves room for one more particle, possibly the spin-0 Higgs boson. This particle has never been detected. However, Alan Guth postulated in the early universe, the high temperature left spacetime in a false vacuum state. As it cooled, the symmetry of the field associated with the Higgs boson spontaneously broke, dumping energy into spacetime and causing the inflation of the universe. See also big bang theory

highlands, lunar Ancient elevated crust of the Moon. Because of their elevation, the highlands have not been covered by later lava flows. The highlands have a different composition from the maria (see mare) and appear lighter due to a higher reflectance of sunlight. As the highlands are much older than the maria, they are more deeply cratered.

highlands, lunar As it passed Earth, picking up speed for its long trip to Jupiter, the Galileo spacecraft obtained this false-colour image of the lunar highlands. The light highlands contrast with the dark maria, and are heavily cratered.

high-redshift quasar Most distant quasars observed, as determined from their redshifts and the hubble law. Quasars are intrinsically so bright that some can be seen out to extremely large distances. Although the distribution of quasars peaks around z = 2.5, many quasars have been detected as far as z -4.1. These extremely luminous, distant sources are seen as they were several billions of years ago.

high-velocity cloud (HVC) Cloud of mostly neutral hydrogen that is moving 100 to 200 km/s (60-120 mi/s) faster than the expected circular orbital velocity for its distance from the centre of the Galaxy. The clouds are normally only detectable from their radio emissions. The best-known example is an enormous bridge of material between our Galaxy and the magellanic clouds. Known as the Magellanic Stream, it probably resulted from a close passage of the Magellanic clouds with the Galaxy at some time in the past. Other HVCs may be the result of supernova explosions or may be infalling intergalactic material still being accreted by the Galaxy. The origin of most HVCs is not known.

high-velocity star Star whose space motion exceeds about 100 km/s (60 mi/s). There are several quite distinct classes. Many nearby Population II stars have high velocities by virtue of their galactic orbits, which may be neither circular nor close to the galactic plane; the nearby red dwarf binary 61 Cygni is an example. runaway stars are young massive O or B stars moving at very high velocities, thought to have been ejected from binary systems or clusters. Somewhat older A stars of high velocity include many binary systems, making them less likely to have been ejected from clusters, and have orbits that inhabit a thick disk. They may trace the remnants of smaller galaxies that have merged with the Milky Way and not yet settled into its thin gas-rich disk. High-velocity neutron stars originate in the disruption of a binary during a supernova explosion; asymmetric explosions can impart a velocity 'kick'.

Hill, George William (1838-1914) American mathematician who applied his skills to celestial mechanics. He is most famous for his work on lunar theory, a working out of the Moon's complex motion and the perturbations exerted upon it. Hill solved many complicated problems involving the orbits of Jupiter and Saturn and made innovative strides towards solving the three-body problem.

Hilda asteroid Any of a group of asteroids with orbital periods close to two-thirds that of jupiter. The archetype is (153) Hilda. Unlike the resonances represented by the kirkwood gaps, this 2:3 resonant position attracts objects and provides for long-term dynamical stability. See also hirayama family centre. It has a substantial inclination (near 27) and moderate eccentricity (0.162). See also elara

Himalia One of the small satellites in jupiter's intermediate group, discovered by Charles Perrine (1867-1951) in 1904. Himalia is c.186 km (c.116 mi) in size, making it the largest of Jupiter's family of satellites apart from the galilean satellites and amalthea. It takes 250.6 days to circuit the planet at an average distance of 11.46 million km (7.12 million mi) from its

Hind's Crimson Star Name given to the deep-red variable star R Leporis, the colour of which was likened by the English astronomer John Russell Hind (1823-95) to 'a drop of blood'. R Leporis is a mira star, ranging from 12th magnitude at minimum to a peak of mag. 5.5 with a period of 14 or 15 months. It is a carbon star of spectral type C6 II.

Hipparchus Lunar crater (6S 5E), 135 km (84 mi) in diameter, with rim components reaching 1220 m (4000 ft) above its floor. An ancient crater, Hipparchus has been severely modified by meteorite erosion. Indeed, this erosion has removed all remnants of its ejecta blanket and has brought its walls to near level with the surrounding surface. The floor of Hipparchus is relatively smooth because of lava flooding.

Hipparchus of Nicea (c.190-120 bc) Greek astronomer, geographer and mathematician who made observations from Nicea (in what is now modern Turkey), Alexandria, Egypt and the Greek island of Rhodes. Little is known of his life, and the only book of his to survive is a commentary on the work of aratus. Most information about him comes from Ptolemy's Almagest, from which it is clear that Hipparchus was the greatest observational astronomer of antiquity. He is best known for his discovery of the precession of the equinoxes. Hipparchus measured the length of the tropical year to within 6 minutes; this allowed him to compare the dates of the equinoxes with the dates given by aristarchus of Samos and other earlier astronomers, and he found that the equinoxes had moved westwards around the ecliptic. His catalogue of some 850 stars -believed by some scholars to form the basis of the catalogue in the Almagest - was the first to classify stars by their apparent visual magnitudes.

Hipparchus developed mathematical models describing the apparent motion of the Sun and Moon, introducing epicycles, basing his work on his own and older Babylonian observations. He determined the sizes and distances of the Sun and Moon from observations of solar and lunar eclipses. Finding the Sun's parallax too small to measure, he assumed a value based on the accuracy of his observing equipment. This was almost five times too large, giving a value for the Sun's distance almost five times too small, and a figure for the Sun's diameter less than one-sixteenth of the true value, but these results were a vast improvement on previous estimates. Hipparchus' calculations of the Moon's average distance as 59-67 Earth radii (modern value 60.4) and its diameter as 0.33 times the Earth's (modern value 0.27) were more accurate.

Hipparcos (High Precision Parallax Collecting Satellite) European space agency satellite, the first satellite to be designed for astrometry. The launch in 1989 was only partially successful, placing Hipparcos in a highly elliptical orbit instead of the intended geostationary orbit. However, the satellite was able to meet its scientific objectives before operations ended in 1993 August.

Hipparcos Astrometric data from the Hipparcos satellite revealed warping of the disk of the Galaxy. The scale of the warping is exaggerated in this artists rendition.

During systematic scanning of the sky, Hipparcos used direct triangulation to measure the relative positions, annual proper motions and trigonometric parallaxes of some 120,000 selected stars down to 11th magnitude - nearly every star within 250 l.y. of Earth.

The satellite's payload was a telescope that could view simultaneously in two directions 58 apart. The satellite rotated at approximately 12 revolutions per day about an axis perpendicular to the two fields of view. The direction of this axis of rotation was changed slowly so that the whole sky was scanned many times during the mission.

The results were eventually published in 1997 as the hipparcos catalogue. The Hipparcos survey more than doubled the number of known variable stars and discovered over 10,000 new binary or multiple star systems. It also led to the creation of the first accurate three-dimensional picture of the bright stars in our stellar neighbourhood. Distance measurements of cepheid variable stars indicated that the Universe must be 10% older than previously thought. Other measurements showed that the Milky Way is slightly warped and in the process of changing shape.

Hipparcos Catalogue (HIP) Catalogue, published in both paper and electronic forms, compiled from data gathered by the hipparcos astrometry satellite. It lists 118,218 selected stars down to mag. 12.4 with highly accurate positions (to around 0".001), parallaxes, proper motions and photometry. The Tycho Catalogue (TYC) gives less accurate positions (to around 0".05) but lists 1,058,332 stars to mag. 11.5 with two-colour photometric measurements. The catalogue was published in 1997; it was followed in 2000 by the Tycho-2 Catalogue, which made use of data from the Astrographic Catalogue (see carte du ciel) to derive very accurate proper motions.

Hirayama family Group of asteroids characterized by having closely similar orbital elements. The existence of such families was pointed out by Japanese astronomer Kiyotsugu Hirayama (1874-1943) in 1928. About a hundred such families are now recognized, each being named usually after its largest member. Some of the more populous families may have hundreds of constituents. About half of all main-belt asteroids are members of one Hirayama family or another.

Four of the main families are the flora, Eos, Themis and Koronis groups. The fact that in many such families the members have similar spectral reflectivities suggests that they originated through the collisional break up of a larger progenitor. gaspra, for example, is a member of the Flora family, and images of it are indicative of its liberation as an independent body in the astronomically recent past (during the last hundreds of millions of years as opposed to billions). Such disintegrations are known to have occurred from other lines of evidence; for example iron meteorites must once have been part of much larger bodies, which were sufficiently massive for internal heating to have occurred and caused the liquefaction and chemical fractionation that is obvious from the meteorites' appearance. Similarly, the family associated with vesta appears to be the source of several distinct meteorite types, in that the reflectivities of these meteorites are matched by the surface regions of that asteroid and its cohorts. Bands of interplanetary dust believed to be associated with certain families may be debris produced by collisions in relatively recent times (within the past few million years).

Other types of asteroid family may be recognized on the basis of orbital similarity, but with that similarity having been produced by dynamical segregation and accumulation around certain sets of orbital parameters, rather than by a common genesis. The trojan asteroids represent an obvious example, another being the hilda asteroids; other groupings include the hungaria asteroids.

HI region Interstellar gas cloud in which the hydrogen is in the form of neutral atoms. The name derives from the convention whereby ionized atoms are labelled with a Roman numeral one larger than the number of electrons lost: ionized hydrogen is HII (see hii region) and neutral hydrogen is therefore HI. The sizes of HI regions can vary considerably, but typically they are 10 to 20 l.y. across and have a mass of 10 to 100 solar masses. Their temperature is around 100 K, and the particle density around 107 mT3 (ten times denser than the average for interstellar matter). The density is too low for molecular hydrogen to form in significant quantities, and any that did form would soon be dissociated by stellar radiation.

HI regions do not emit visible radiation, but they can be detected from their twenty-one centimetre radio emissions. Large radio telescopes can thus detect HI regions even in distant galaxies. Different kinds of galaxy contain different proportions of HI. Our own Galaxy and other spiral or irregular galaxies contain an appreciable fraction of their total mass in HI. Elliptical galaxies contain no (or extremely little) detectable HI.

In our Galaxy, HI clouds are heavily concentrated to a flattened disk in which a spiral pattern occurs. Spiral arms are made conspicuous, even in very distant galaxies, by the groups of hot, high mass, blue stars lying along the arms, like pearls on a string. These groups contain stars that have formed recently from the HI clouds. Not all HI clouds are capable of forming stars; only the densest ones that also contain molecular cores can do so (see giant molecular clouds). A number of HI clouds lie out of the galactic plane and are very diffuse. Even dense hydrogen clouds are much more tenuous than the best laboratory vacuum achievable on Earth.

HII region Interstellar gas cloud in which the hydrogen is in the form of ions (see stromgren sphere). The name derives from the notation, HII, used to denote ionized hydrogen (see hi region). The hydrogen is ionized by ultraviolet radiation from hot young stars embedded within the region, and the gas is heated to betweeen 8000 and 10,000 K. HII regions are often embedded within HI regions, but have densities 10 to 100,000 times greater than those of the latter. The high temperatures and densities of HII regions mean that the gas pressure inside these regions is far higher than that of the surrounding HI region or interstellar medium. All HII regions are thus increasing in size at expansion velocities of around 10 km/s (6 mi/s). Visible radiation from an HII region results from the recombination of ions and electrons, and hot dust causes infrared emission; free-free transitions lead to emission by the region at radio wavelengths. On images, HII regions often appear red because of the hydrogen-alpha line at 656 nm; however, they can look greenish when seen visually because of a prominent pair of FORBIDDEN LINES at 496 and 501 nm emitted by oxygen.

Three main classes of HII regions are recognized: ultra-compact HII Regions (UCHIIRs), which are less than about 0.1 l.y. in size; compact HII regions, which are about 1 l.y. across; and classical HII regions, which may be several tens of light-years in size. Giant and supergiant HII regions, with sizes up to several hundred light-years, are found in some external galaxies. UCHIIRs and compact HII regions occur around single stars, but the larger classes usually have from a few to 100,000 stars within them. The amount of ionized gas in the cloud ranges from a small fraction of a solar mass to millions of solar masses. The gas density similarly ranges from 107 to 1011 particles mT3, with the largest clouds being the least dense. The smaller HII regions are often, but not always, roughly spherical in shape. The larger regions can have very complex morphologies arising from the distribution of gas and stars within them. The ORION NEBULA is a good example of a classical HII region.

Regions of ionized hydrogen can occur for many reasons, such as ultraviolet emission from old hot stars or from nova or supernova explosions, or they may form as a result of high velocity jets impacting the material of the interstellar medium. Conventionally, however, these are not included in the term HII region, but are instead given their own specific names (see PLANETARY NEBULA, SUPERNOVA REMNANT and HERBIGHARO OBJECT).

HII regions are stellar nurseries. The hot stars that ionize the gas have recently been formed through gravitational collapse within a GIANT MOLECULAR CLOUD. The life of an HII region is only a few million years. The presence of such regions, along with the hot stars, marks out the arms in spiral galaxies. HII complexes also characterize STAR-BURST galaxies, in which high-mass star-forming activity is occurring strongly.

HM Nautical Almanac Office Principal organization in the UK for providing astronomical almanacs and other numerical data. With the US NAVAL OBSERVATORY it is responsible for producing The astronomical almanac, The Nautical Almanac and The UK Air Almanac. The Office dates back to the foundation of the ROYAL GREENWICH OBSERVATORY (RGO) in 1675; with the closing of the RGO in 1998, it moved to its present location at the Rutherford Appleton Laboratory.

The mirror consists of 91 identical segments forming a hexagon 10 X 11m (33 X 36 ft) in size. The working aperture of 9.2 m (30 ft), determined by the optics of the SAC, is smaller than the primary mirror to allow the beam to track across the mirror during observation. An imager and a spectrograph are mounted at the prime focus, while larger spectrographs are mounted below the telescope and connected by fibre-optic feeds.

Hoffleit, (Ellen) Dorrit (1907 ) Pioneering American woman variable-star astronomer and compiler of the Yale bright star catalogue. Working at Harvard College Observatory, she discovered over a thousand variable stars, mostly in the southern sky. Between 1948 and 1956 Hoffleit determined very accurate spectroscopic absolute magnitudes for many southern stars, which allowed other astronomers to map the Milky Way's large-scale structure. In 1956 shejoined Yale University and undertook a thorough revision of the Bright Star Catalogue, first compiled by Frank SCHLESINGER. She has produced three editions of this catalogue, which contains positional, spectral and other basic data for over 9000 stars.

Hoffmeister, Cuno (18921968) German astronomer, founder, and for 42 years the director, of the Sonneberg Observatory, who specialized in variable stars. Using a battery of fourteen small cameras, he secured over 100,000 photographic plates of the night sky. His careful examination of these plates resulted in the discovery of over 10,000 new variables. Hoffmeister also conducted long-term visual meteor counts to refine the positions of shower radiants and the parameters of meteor streams. He made detailed studies of the zodiacal light and gegenschein.

Hogg, Helen (Battles) Sawyer (190593) American astronomer who spent her entire professional career in Canada, investigating variable stars in globular clusters and popularizing astronomy. In 1930 she married Canadian Frank Scott Hogg (190451), and they both joined the Dominion Astrophysical Observatory. From 1931 to 1935, she used DAO's 72-inch (1.82-m) reflector to photograph variable stars, work that would lead to the compilation of an important catalogue of those objects. In 1935 the Hoggs moved to David Dunlap Observatory, where Helen used the 74-inch (1.88-m) telescope to discover hundreds of new variable stars.

Hoba Largest single METEORITE known. Hoba is approximately 3 X 3 m (10 X 10 ft) across, and at least 1m (3.3 ft) deep; it is estimated to weigh c.60 tonnes. Hoba is an ataxite IRON METEORITE. It was found in 1920, near Grootfontein, Namibia, where it still lies in the ground, preserved as a national monument.

Hobby-Eberly Telescope (HET) Large optical telescope of unusual design at an elevation of 2025 m (6640 ft) on Mount Fowlkes at MCDONALD OBSERVATORY. It is run by a consortium consisting of Penn State University, Stanford University, Ludwig-Maximilians Universitat Munchen, and Georg-August Universitat Gottingen. With its innovative simple mounting, it was only a fraction of the cost of a conventional 8-metre class telescope. It entered operation in 1997. It is named after William P. Hobby and Robert E. Eberly, American supporters of public education.

The HET utilizes a 'tilted arecibo' concept: the telescope tube is fixed at 55 from the horizontal; the telescope itself remains stationary during observations, though the whole structure can rotate through 360 of azimuth on air bearings. This single rotational axis, combined with the apparent rotation of the heavens, gives the HET access to 70% of the sky visible from McDonald Observatory over the course of the year. To follow the stars, there is a tracker mounted in the focal plane of the spherical primary mirror; this carries a Spherical Aberration Corrector (SAC).

Holden, Edward Singleton (18461914) American astronomer, director of Washburn (University of Wisconsin) Observatory (188185) and first director of Lick Observatory (188897). At Washburn, Holden studied Saturn's rings, prepared catalogues of red stars and southern stars, and encouraged Sherburne Wesley BURNHAM to search for new double stars using

Hoba The world's largest single meteorite, Hoba lies where it fell in Namibia. This iron meteorite is now preserved as a national monument.

HOROLOGIUM (gen. horologii, abbr. hor) Obscure southern constellation, next to the southern end of Eridanus, introduced by Lacaille in the 18th century. It represents a pendulum clock. a Hor, mag. 3.85, spectral type K1 III, is its only star above 4th magnitude. R Hor is a mira star, ranging from 5th to 14th magnitude every 13 months or so.

Horsehead Nebula Silhouetted against the emission nebula IC 434, the Horsehead Nebula (B 33) in Orion is a very difficult object visually. Photographs such as this show the striking profile of the intruding dark nebula in the foreground.

Washburn's 15-inch (380-mm) Clark refractor. He also made some of the earliest statistical studies of stellar distribution that would later show the Milky Way to be a disk-shaped spiral galaxy with a central bulge. Holden was the major force behind the founding, in 1889, of the astronomical society of the pacific.

Holmes, Comet 17P/ Short-period comet discovered in 1892 during what seems to have been an unusually bright and active apparition, when it became a naked-eye object and may have undergone nuclear fragmentation. It was seen again in 1899 and 1906 but was then lost until 1964, when it was below mag. +18. Comet 17P/Holmes has since been recovered at every apparition, most recently in 2000, but remains very faint. It has an orbital period of 7.07 years.

Homestake Mine Site of an underground astronomical observatory, at Lead in the Black Hills of South Dakota, opened in 1965. At its depth, 1500 m (4850 ft), cosmic rays are blocked and experiments can be conducted to observe solar neutrinos, which were discovered here in 1968. In 2001 it ceased to be an operating mine (Homestake Gold Mine had been the largest US producer of gold), putting the future of the facility in doubt, given the existence of deeper sites such as Canada's Sudbury Neutrino Observatory, some 2070 m (6800 ft) below the surface.

homogeneity Principle that in any particular volume of spacetime the properties of the Universe on average are identical to the same volume anywhere else in the Universe. On small scales, this is obviously not correct, but on scales larger than galactic clusters this idea holds.

Homunculus Nebula Bright dust-cloud (reflection nebula) produced by material ejected during outbursts from Eta Carinae beginning in 1843. Located within the eta carinae nebula, the Homunculus (RA 10h 45m.1 dec. 5941') is named after its resemblance to a human outline. It spans an area of 17"X 12".

Hooker Telescope Name of the 100-inch (2.5-m) telescope at mount wilson observatory, immortalized in the 1920s as the instrument with which Edwin hubble discovered the distance scale of the Universe. It is named after the financier John D. Hooker (1837-1910), who was persuaded by George Ellery hale to fund its construction. It was completed in 1917, and was the largest reflector in the world until the hale telescope became operational in 1948. The telescope can be used in various optical configurations.

The Hooker Telescope is the work of George W. ritchey, the American optician who pioneered the development of large telescope optics in the early 20th century. The 4.5-tonne mirror blank was cast in 1908, and two years later Ritchey began the five-year task of polishing and figuring it. The completed instrument was first turned on the sky in 1917. It soon made history when, in 1920, Francis G. pease and John A. anderson used the stellar interferometer built and fitted to the 100-inch by Albert Abraham Michelson (1852-1931) to make the first measurements of star diameters.

After a long and illustrious career the telescope was declared inactive in 1986, but in 1994 it was reinstated as a research instrument, and underwent major upgrades to its control systems, including a state-of-the-art adaptive optics system.

Hooke, Robert (1635-1703) England's first great experimental physicist, best known for his law of elasticity. In 1662 he became curator of instruments at the Royal Society, and the same year tried to measure the gravitational constant using a pair of weights, and in 1665 was observing Jupiter's belts and the craters of the Moon with a telescope of focal length 36 ft (11 m). He realized that the nucleus was the active part of a comet, and in 1669 made the first serious attempts to measure the parallax of a star, y Draconis. He pioneered the use of William gascoigne's micrometer, and designed instruments for the Royal Observatory, Greenwich.

horizon great circle On the celestial sphere 90 away from the zenith, or overhead point, of an observer located on the Earth's surface. Any point on the celestial sphere greater than 90 from the observer's zenith is below their horizon and therefore invisible to them. The term is also used to describe a boundary in spacetime beyond which events cannot be observed (an event horizon) or particles cannot yet have travelled (a particle horizon). The term is also used to describe the farthest distance from which light could possibly reach us given the age of the Universe and the finite speed of light.

horizontal branch star Star that is burning helium in its core. Such stars lie on a horizontal strip of the hertzsprung-russell diagram to the right of the main sequence and to the left of the red giant branch.

When a star has finished burning hydrogen in its core, it starts hydrogen shell burning. The star expands off the main sequence to become a red giant. If the star has sufficient mass, helium burning can occur in the core. When a star is burning helium in its core, it is on the horizontal branch. Low-mass stars start helium core burning with a helium flash, and the star moves rapidly to the horizontal branch. If the mass of the star is small, it lands on the left extension of the horizontal branch; if the mass of the star is larger, it will fall farther to the right on the horizontal branch.

Hertzsprung-Russell diagrams of globular clusters show differences in the form of their horizontal branches from cluster to cluster. As yet there is no consensus of opinion as to what causes the differences, although the metal abundance of the stars and the age of the cluster appear to be two contributing factors. rr lyrae variables are formed where the instability strip crosses the horizontal branch. On some Hertzsprung-Russell diagrams, variable stars are not shown and thus there is a gap in the horizontal branch.

horizontal coordinates System of locating points or objects on the celestial sphere using the horizon as the plane of reference and the measures of altitude and azimuth. Altitude is the angular distance of a celestial body above an observer's horizon. It is measured vertically from 0 at the horizon, along the great circle passing through the object, to a maximum of 90 at the zenith. Azimuth is the angular distance of an object measured westwards along the horizon, with north as the zero point, to the vertical circle (meridian) running through the object.

Horologium See feature article

Horrocks, Jeremiah (161942) English astronomer, the first to build significantly on the work of Galileo and Johannes Kepler. Born at Toxteth, Liverpool, into a farming family, he studied at Emmanuel College, Cambridge, between 1632 and 1635, where he discovered the errors in existing planetary tables, and set about correcting them from instruments of his own devising, including an 'astronomical radius', a solar projector and a small telescope. In 1636 he became acquainted with William crabtree of Salford, and soon afterwards William gascoigne of Leeds and Christopher Towneley (1604-74). Horrocks' surviving writings abound with references to the importance of primary observation. In 1637 he and Crabtree observed the sudden occultation of the stars of the Pleiades by the Moon, which led him to believe that the Moon must be airless. By 1638 he had demonstrated that the lunar orbit is a Keplerian ellipse, and had measured the changing angular diameters of the Sun and Moon, matching them with elliptical orbits. His measurement of the solar parallax gave a Sun-Earth distance of around 100 million km (about 60 million mi), which would remain the best estimate for many years. His successful prediction and observation of the transit of Venus on 1639 November 24 won him posthumous immortality. From it, Horrocks calculated the angular diameter and parallax of Venus and suggested that they were consistent with a Keplerian orbit.

Horsehead Nebula (B33) Dark nebula seen in silhouette against the emission nebula IC 434 near Orionis (RA 05h 41m.0 dec. -0224'). It is part of the extensive gas and dust complex in the Orion region, lying at a distance of 1600 l.y. Covering an area of 6'X 4', the Horsehead is named after its resemblance in long-exposure images to a chess knight. Visually, the Horsehead shows little contrast with IC 434, and it is very difficult to see even in large telescopes.

horseshoe mounting Variation on the equatorial mounting, used for large telescopes.

Horseshoe Nebula See omega nebula

hot dark matter Type of matter that has been proposed as the missing mass component of galaxies (see missing mass problem). Dark matter was postulated to be responsible for closing the universe (see closed universe) and for explaining galaxy formation. Hot dark matter could consist of massive neutrinos moving near the speed of light (hence the term hot). However, predictions for galaxy formation assuming the presence of hot dark matter do not seem to agree with current observations.

hour angle (HA) Measure of the elapsed sidereal time since a given celestial object last crossed the observer's meridian. Hour angle is also a measure of the angle between the hour circle passing through a celestial body and the observer's local celestial meridian, measured westwards from the meridian.

hour circle Any circle on the celestial sphere that passes through both celestial poles, perpendicular to the celestial equator. Each half of an hour circle, taken from pole to pole, is a line of constant right ascension, and the declination of a celestial object is measured along its hour circle. The zero hour circle coincides with the observer's meridian. The term is also used to describe the scale on the mounting of an equatorial telescope that indicates the hour angle at which the telescope is pointing.

Hourglass Nebula (MyCn 18) Young planetary nebula in the southern constellation of Musca (RA 13h 40m dec. - 6723'). The Hourglass lies 8000 l.y. away and has a apparent diameter of 4". Images from the Hubble Space Telescope in 1996 showed loops of ejected material surrounding the central star.

Hourglass Nebula A Hubble Space Telescope image from 1996, showing delicate loops in the twin lobes from which the Hourglass Nebula takes its name.

howardite See howardite-eucrite-diogenite association

howardite-eucrite-diogenite association (HED) One of the achondrite meteorite subgroups. Howardite-eucrite-diogenite association is a suite of generally brecciated igneous rocks formed by volcanism on their parent body. Diogenites are coarse-grained orthopyroxenites. Eucrites are cumulates and finegrained basalts, composed of pyroxene and plagioclase. Howardites are regolith breccias (asteroidal soils), rich in both solar wind gases and clasts of carbonaceous material. The HEDs all have similar oxygen isotopic compositions. Candidates for the HED parent body are the asteroid vesta or its hirayama family.

Hoyle, Fred (1915-2001) English astrophysicist and cosmologist known for his origination and advocacy of the steady-state theory and for his fundamental explanation of how stars synthesize elements heavier than helium. In 1957 he co-authored, with Margaret and Geoffrey BURBIDGE and William FOWLER, the seminal paper 'Synthesis of the Elements in Stars'. Dubbed 'B2FH' for the initials of its authors' surnames, this paper describes how stars synthesize chemical elements heavier than hydrogen and helium. Hoyle's major contribution to this theory of stellar nucleosynthesis was his prediction of an excited state of the 12C nucleus. In 1948, with Hermann BONDI and Thomas GOLD, Hoyle conceived a cosmological theory describing the Universe as flat space expanding at a constant rate, the expansion being balanced by the continuous creation of matter so that its mean density remains unchanged. This steady-state theory was proposed as an alternative to the BIG BANG, a term Hoyle coined to poke fun at the model accepted by most cosmologists.

Hoyle later helped Geoffrey Burbidge to develop a 'quasi-steady-state theory', which, unlike the original version, accounts for the COSMIC MICROWAVE BACKGROUND radiation. This theory suggests that the isotope 4He is produced, not in the Big Bang, but by stellar hydrogen burning, as are the isotopes 2H (deuterium), 3He, 6Li. 7Li, 9Be, 10B and 11B.

Hoyle modelled the collapse of interstellar dust clouds to form stars, and in the 1940s he was the first to describe mathematically the process of accretion, by which stars accumulate INTERSTELLAR MATTER. He suggested that the dust clouds are composed of inorganic carbon graphite grains or organic forms of carbon, the latter forms possibly imported to Earth with other organic molecules where they formed the building blocks of life, thus reviving the PANSPERMIA theory. With this and other non-mainstream theories, Hoyle often courted controversy.

For many years Hoyle was Plumian Professor of Astronomy at Cambridge University, founding its Institute of Theoretical Astronomy (1966).



H2 Booster by which Japan achieved its own independent launch into geostationary orbit. The H2 booster was first launched, two years late, in 1994. It flew five successful missions but there were two failures, and high costs forced a cancellation of the programme. The new H2A was intended to launch in 1999, but the programme was delayed by technical problems until 2001. The standard H2A will replicate the capability of the original booster, carrying 4 tonnes to geostationary transfer orbit; larger versions will increase this capability to 7.5 tonnes in 2005.

Hubble, Edwin Powell (1889-1953) American astronomer who proved that the spiral 'nebulae' were galaxies lying far beyond our own Milky Way, established a widely used scheme for the classification of galaxies, and who derived the velocity-distance relation for these objects, thus allowing the scale of the Universe to be reckoned for the first time. It is difficult to overstate Hubble's impact on observational cosmology; it has been said that what Nicholas Copernicus did for the Solar System, and William Herschel did for the Galaxy, so Hubble did for the Universe.

A native of Missouri, Hubble worked briefly (1914-17) as a research assistant at Yerkes Observatory, then, following a stint in the United States Infantry from 1917 to 1919, he joined the staff of MOUNT WILSON OBSERVATORY, where he remained for the rest of his career. He was also involved with the construction of the 200-inch (5-m) HALE TELESCOPE at Palomar Observatory, which he used in his latter researches (1948-53). For most of his career, Hubble used Mount Wilson's 60-inch (1.5-m) and 100-inch (2.5-m) reflecting telescopes to study CEPHEID VARIABLES in the Andromeda Galaxy (M31), M33 and NGC 6822, the distance and nature of which were the subjects of the so-called GREAT DEBATE in 1920. From 1925 to 1929 he published three important papers showing that these nebulae were galaxies lying at distances much greater than any object in the Milky Way, confirming the 'island universe' model of Heber CURTIS. In 1932 he discovered the first globular clusters outside the Milky Way, in M31.

Extending the earlier work of Vesto M. SLIPHER, Hubble measured the spectral REDSHIFTS of 46 galaxies, attributing the redshifts to a recession of the galaxies and using the results to find their recessional velocities. Comparing these velocities to the distances of the galaxies, in 1929 he announced what is now called HUBBLE'SLAW: the farther away a galaxy is, the faster it is receding. When these two quantities, velocity and distance, were plotted against each other, the result was an almost perfectly linear fit - the slope of the line is the HUBBLE CONSTANT (H0), and its reciprocal, the Hubble time, is the age of the Universe since the Big Bang. These discoveries provided powerful evidence that the real Universe resembled the kind of expanding universe derived theoretically by Albert EINSTEIN and Willem DE SITTER.

From 1929 to 1936, Hubble and his assistant, Milton HUMASON, published a series of studies showing conclusively that for velocities of recession up to 40,000 km/s (25,000 mi/s), the velocity-distance relation holds true. The two astronomers also invented the first (1925) systematic scheme for the classification of galaxies (now called the HUBBLE CLASSIFICATION) based on their morphologies, labelling these objects as elliptical, 'normal' spiral or barred spiral galaxies, which they further divided into subtypes. Hubble believed that his scheme indicated an evolutionary sequence for galaxies, from ellipticals to spirals, though this is now known not to be the case.

Hubble's surveys of bright galaxies, carried out in the 1930s and 1940s, showed them to be distributed isotropi-cally on the largest scales of the Universe. On more local scales, galaxies were often found in clusters composed mostly of elliptical galaxies - Hubble was the first to measure accurately the surface brightness profiles of ellipticals. His counts of galaxies confirmed that they are comparatively scarce in the ZONE OF AVOIDANCE along the plane of the Milky Way, where they are obscured by our Galaxy's disk of dust and gas. Hubble's studies of the Milky Way's spiral arms (1935-13) provided the first strong evidence that they 'trail' as the Galaxy rotates, instead of opening outwards 'ahead' of the galactic rotation, as some astronomers had hypothesized. He was the first (1922) to divide the bright diffuse nebulae of the Milky Way into reflection and emission nebulae, giving the proper astro-physical explanations for their different spectra. Three decades after his death, the hubble space telescope (HST) was named to honour him.

Hubble classification System for classifying galaxies according to their shape on photographs; it was introduced by Edwin hubble in 1925, with extensions and revisions by several later workers, including Allan sandage, Gerard de vaucouleurs, and Sidney Van den Bergh (1929-).

Hubble classification A useful way of describing galaxy shapes, the Hubble classification splits these into three main classes. Ellipticals (E) are subdivided on the basis of how far they are from spherical. The classification branches (and is hence sometimes known as the tuning-fork diagram) to afford description of spiral (S) and barred spiral (SB) galaxies.

While the Hubble classification was originally designed to be descriptive, and based on blue-light photographs of a particular exposure range, it has retained enormous utility because these designations correlate well with physically interesting galaxy properties. Stellar and gas content and star formation rate, for example, change systematically between Hubble types.

Hubble constant Parameter in the hubble law that relates the velocity of a galaxy to its redshift. In general relativity, it is the rate at which the scale size (R) of the Universe is changing with time. It is also the rate at which distances between all 'co-moving' objects - that is, those objects with no individual motion relative to the fabric of space - are increasing. The units for the Hubble constant are such that its inverse is a time, which can very loosely be thought of as an 'age' for the Universe. More usually, however, the units employed are kilometres per second per megaparsec since, for small redshifts, galaxy recession velocities increase by this amount for each megaparsec of distance.

The Hubble constant is normally designated H0, the zero subscript specifying that it is the expansion rate at the present epoch that is meant. The relative expansion rate changes with time and the Hubble parameter and its rate of change are related to the deceleration parameter, q0, by

H0 = R/R

qo = R/(RHo2) both values being calculated for the present epoch.

The search for accurate values for H0 and q0 has been the driving force behind much of observational cosmology in the last 80 years. Many different methods of obtaining values for H0 have been devised. But they all have in common redshift and distance measurements of a set of similar objects carefully chosen to eliminate selection and measurement bias to the greatest extent possible. A key project for the Hubble Space Telescope was the task of deriving an accurate value for the Hubble constant. After many years of work, the best value of H0 available at present from the Space Telescope Key Project is 68 6.0 km per second per megaparsec.

Hubble Deep Field (HDF) Long exposure of a small area of sky in the north of Ursa Major obtained by the hubble space telescope in 1995. Just 2'.5 across, the HDF is a composite of 342 exposures accumulated over 100 hours in four wavelength ranges in the visible, near-ultraviolet and near-infrared. In 1998 the Hubble Deep Field South (HDF-S) was imaged: a 2' sample of a region in the constellation Tucana. Both fields show over 2000 galaxies, including some of the most distant - and thus the youngest - ever imaged.

Hubble diagram Graph in which the apparent magnitude of galaxies is plotted against the redshift of their spectral lines. It is a straight line, demonstrating the linear relation between redshift and distance, as embodied in the hubble law.

Hubble diagram Plotting the apparent magnitude of galaxies relative to redshift produces a straight line, indicating that objects with higher redshifts are more distant.

Hubble law Law proposed by Edwin hubble in his landmark paper of 1929 claiming a linear relation between the distance of galaxies from us and their velocity of recession, deduced from the redshift in their spectra. The law can be stated as:

v = H0d

where v is the radial velocity of a galaxy, d is the distance to the galaxy and H0 is a constant now known as the hubble constant. This linear velocity-distance relation could be readily explained if the Universe as described by general relativity were expanding.

Hubble-Sandage variable See s doradus star

Hubble Space Telescope (HST) Telescope put into orbit by the Space Shuttle Discovery STS 31 in 1990 April. The Hubble Space Telescope (HST) is in an orbit inclined to the equator by 28.5, which is almost circular at an altitude of about 607 km (380 mi). HST was designed to be serviced in orbit by later Space Shuttle crews and to have instruments removed and replaced, including its twin 12.19-m-long (40-ft-long) solar arrays. The 11,600 kg telescope is 13 m (43 ft) long and, at its widest, 4.2 m (14 ft) in diameter. It is equipped with two high-gain antennae, which enable it to transmit direct to the ground using a tracking and data relay satellite system, and two low-gain antennae. HST has a data management system and a fine-pointing system, which allows it to be aimed and remain locked on to any specific target to within 0".01. The optical telescope assembly is configured in such a way that the telescope is the equivalent of 57.6 m (189 ft) long, but compacted to 6.4 m (21 ft). Light entering the aperture door travels down the tube on to a 2.4-m (94-in) primary mirror and is reflected on to a secondary mirror 0.3 m (12 in) in diameter, where it is reflected through a hole in the centre of the primary mirror on to the focal plane. HST's various science instruments then receive the light.

Hubble Space Telescope Launched in 1990, the Hubble Space Telescope has provided astronomers with many results of fundamental importance. Here it is seen soon after release following the second servicing mission, carried out by Space Shuttle astronauts in 1997.

HST was originally equipped with a Faint Object Camera, Wide-Field/Planetary Camera (WFPC), God-dard High-Resolution Spectrograph (GHRS), Faint Object Spectrograph (FOS), High Speed Photometer (HSP) and Fine Guidance Sensors (FGS). Once the telescope had reached orbit, and despite the fact that some good images of the Universe were taken, it soon became clear that the primary mirror was suffering from spherical aberration, which apparently arose during manufacture. The Corrective Optics Space Telescope Axial Replacement (COSTAR) unit was flown to the HST aboard Space Shuttle STS 61 in 1993 December and fixed inside the telescope during one of a series of five spacewalks. The dual teams of astronauts also replaced a solar panel, repaired electronics, installed a new computer processor, replaced the WFPC with a new unit, installed magnetometers, removed the HSP and installed a redundancy kit for the GHRS. The results of installing COSTAR were startling: HST images were now truly spectacular and the telescope immediately caught the imagination of the public worldwide.

Another planned HST servicing mission by Shuttle astronauts was launched in 1997 February. The STS 82 Discovery crew made comprehensive changes to Hubble's suite of instruments and conducted routine spacewalk servicing. The GHRS and FOS were removed and replaced with a Space Telescope Imaging Spectrograph and a combined Near-Infrared Camera and Multi-Object Spectrometer. The astronauts replaced an FGS and other equipment, installed an optical electronics enhancement kit, and changed the HST stabilization and fine-pointing reaction wheel assemblies. Other work included the laying of new thermal insulation blankets.

Hubble worked on, entering its tenth year of service when it was time to launch a new Shuttle servicing mission. This was planned originally for 2000 but when gyroscopes on the telescope failed to a critical level, the mission was brought forward to 1999 December and split into two, with the second half of the mission being shifted to 2001. STS 103 Discovery was launched in 1999 December, and the mission installed six new gyros and voltage/temperature kits, a new computer 20 times faster and with six times the memory, a new digital tape recorder and a replacement FGS and a radio transmitter. The spacewalking astronauts also placed some new insulation over part of the HST. STS 109 Columbia in 2002 March installed new solar arrays and an Advanced Camera for Surveys, a Cosmic Origins Spectrograph and a third WFPC.

Hubble's Variable Nebula reflection nebula in northern Monoceros (RA 06h 39m.2 dec. + 0844'); it covers a V-shaped area of3'.5 X 1'.5. The nebula's brightness and appearance change in response to variations in the light output of the illuminating star, R Monocerotis. Hubble's Variable Nebula has the distinction of being the first object photographed with the 200-inch (5-m) Hale Telescope at Mount Palomar, California, in 1949. It lies at a distance of 2600 l.y.

Huchra, John Peter (1948- ) American astrophysicist and observational astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA) who has collaborated with Margaret geller and others to map the three-dimensional large-scale distribution of galaxies in the Universe. He has taken part in the CfA Redshift Survey, which showed that galaxies are arranged in sheets and clumps, separated by large soap bubble-like voids tens of millions of light-years wide. Huchra is also involved with the two micron all sky survey (2MASS). His other major research interest is the extragalactic distance scale.

Huggins, William (1824-1910) English amateur astrophysicist and pioneer of astronomical spectroscopy. He used an inheritance to build an observatory at Tulse Hill, south London with a 12-ft (3.7-m) dome, equipped with a 5-inch (130-mm) equatorial by John dollond, and a separate transit telescope room. In 1858 he acquired an 8-inch (200-mm) refractor with an objective by alvan clark & sons. Huggins became interested in astronomical spectroscopy after learning of Gustav kirchhoff's spectroscopic investigations of the Sun. With his neighbour, William Allen Miller (1817-70), professor of chemistry at King's College, Huggins used two dense flint glass prisms to construct a new instrument, the stellar spectroscope, which they attached to the Clark refractor. To compare stellar spectra with the spectral lines produced by known chemical elements, Huggins equipped his observatory with batteries, Bunsen burners and other chemical apparatus, making it unlike any other astronomical observatory then in existence.

By 1863 Huggins and Miller had published their first observations of stellar spectra. The next year, Huggins found that some nebulae displayed monochromatic spectra typical of gases, whereas the 'spiral nebulae' showed continuum, stellar-type spectra. He concluded that, contrary to what was thought at the time, not all nebulae were aggregations of stars too faint to be resolved. In 1866 he was the first to examine the spectrum of a nova, which displayed a series of bright hydrogen lines produced by the very hot gaseous emissions typical of these objects. In 1868 he made the first detailed spectroscopic investigation of a comet, confirming the bright C2 'Swan bands'. That same year, Huggins and Miller made the first stellar radial velocity measurement, obtaining a value of 47 km/s (29 mi/s) for Sirius.

In 1875 Huggins married Margaret Murray, herself an accomplished amateur astronomer and telescope-maker. Lady Margaret Lindsay Huggins (1848-1915) collaborated with her husband for 35 years. As early as 1863, Huggins had photographed the spectra of Sirius and Capella, but was frustrated by the impracticalities of the 'wet collodion' plates then available. In 1876 he photographed the spectrum of Vega on a gelatine dry plate, the first such application by an astronomer. The dry plates allowed longer exposures and thus fainter details could be captured, including features in the near-ultraviolet part of the spectrum. Over the next 30 years, Huggins obtained numerous photographic spectra proving that many of the chemical elements known on Earth (for example, hydrogen, calcium, sodium and iron) are also present in stars and nebulae.

Hulse, Russell Alan (1950- ) and Taylor, Joseph

Hooton, Jr (1941- ) American physicists whose 1974 discovery of the first binary pulsar provided strong evidence for the gravitational waves predicted by Einstein's general theory of relativity. Using Arecibo Observatory's 305-m (1000-ft) radio telescope, they found a pulsar designated PSR 1913+16, a neutron star of 1 solar mass and diameter 10 km (6 mi), with a close, equally massive companion. Observations showed that this exotic stellar system has a decaying orbit, and Hulse and Taylor deduced that the binary pulsar is losing energy by emitting gravity waves. For their discovery they received the 1993 Nobel Prize for Physics.

Hulst, Hendrik Christoffel van de (1918-2000) Dutch physicist and pioneer radio astronomer who in 1944 predicted that interstellar HI regions (gas clouds composed of neutral hydrogen) emit radio waves at a wavelength of 21 cm. Radio studies of the Milky Way (and other galaxies) by observing the 21-cm emission, first done by Edward Mills Purcell (1912-97) and Harold Irving Ewen (1922- ) in 1951, have allowed galactic structure to be mapped more accurately than by using optical methods alone. Van de Hulst also researched the nature of the solar corona and the interstellar medium.

Humason, Milton Lasalle (1891-1972) American astronomer who made many of the earliest redshift measurements of galaxies. He had no formal training, and was at first a mule driver andjanitor at mount wilson observatory, but in 1917 became a night assistant on the 60-inch (1.5-m) telescope there, working under George Ellery hale. In early work, Humason became an expert observer and was Edwin hubble's chief assistant, measuring the redshifts for many 'nebulae' and helping to show that they were actually galaxies similar to but far beyond the Milky Way. This work, begun in 1928, led to the discovery that the Universe is expanding. Over the next three decades, Humason collaborated with astronomers such as Allan sandage, using the largest telescopes in the world - the 100-inch (2.5-m) at Mount Wilson and the 200-inch (5-m) at Mount Palomar - to measure the redshifts of hundreds of fainter, more distant galaxies. The results of this work, published in 1956, showed that Hubble's law was valid out to a recessional velocity of 60,000 km/s (37,000 mi/s).

Humorum, Mare (Sea of Humours) Lunar lava plain located in the south-west quadrant of the Moon. The geology of this region reveals that it was struck by an enormous object, producing a multi-ring impact basin. Lava later flooded the inner parts, while impact erosion deeply degraded the outer rings, though these are still visible in the western region. An inner mare ridge forms a circle, revealing the presence of an inner ring. Prominent arcuate rilles lie in both the eastern and western regions of Humorum.

Hungaria asteroid Any member of a group of asteroids with orbits similar to (434) Hungaria. Found close to the inner edge of the main belt, these asteroids are objects with low eccentricity orbits and relatively high inclinations. When viewed from the Earth, Hungaria asteroids display large declination motions but lower right ascension movements than other main-belt asteroids.

hunter's moon First full moon following the harvest moon. This usually occurs in October in the northern hemisphere, providing a succession of moonlit evenings. In the southern hemisphere, it occurs in April or May.

Huygenian eyepiece Basic eyepiece consisting of two simple plano-convex elements, commonly used on small refractors. The eye relief is good, the angular field of view is large and the eyepiece is relatively free from chromatic aberration. It is named after Christiaan and Constantijn huygens, who invented it in 1703.

Huygens, Christiaan (1629-95) Dutch physicist and astronomer, born into a prominent family in The Hague. After studying mathematics and law at the universities of Leiden and Breda, Huygens worked at the Bibliotheque Royale in Paris (1666-81), before returning to The Hague. He and his brother Constantijn Huygens (1628-97) became skilled opticians and telescope-makers, designing the achromatic huygenian eyepiece. Christiaan Huygens' first telescope, which he made himself, had a focal length of 3.5 m (11.5 ft). He used it in 1655 to discover titan, determining that this satellite revolved about Saturn in 16 days.

Huygens The conical Huygens probe (lower centre) is seen here attached to the Cassini spacecraft during prelaunch testing. Huygens is scheduled to make a descent to the surface of Saturns moon Titan in 2004.

With a better instrument of 7-m (23-ft) focal length, Huygens made observations of Saturn's rings that were detailed enough for him to deduce their true nature. galileo had seen these changing features as lobes or 'arms', but Huygens correctly described them as a single, thin ring system, detached from Saturn's globe, and published his results in Systema saturnium (1659).

Huygens also made many fundamental contributions to physics that are relevant to astronomy. He explained the optical phenomena of refraction and reflection, and was the first to describe the wave nature of light in his magnum opus, Traits de la lumiere (1678). He derived the formula F = mv2/r to describe centrifugal force. In 1656 he patented the first pendulum clock, which would prove useful in astronomical timekeeping and for finding longitude at sea.

Huygens European spacecraft scheduled to land on titan, Saturn's largest moon, in 2004. Huygens will ride to the Saturn system aboard cassini, landing on the surface of Titan in 2004 November. During its parachute descent, Huygens will study Titan's atmosphere, currently understood as an orange-red smog that rains liquid methane. Huygens may still be operating when it lands on the surface - becoming the first probe to do so on a moon other than our own - the nature of which remains uncertain.

Hyades Second nearest open cluster to the Sun. The Hyades has more than 200 member stars, most of which lie within a distance of 20 l.y. It covers an area of about 6 in the constellation of taurus. The brightest stars, together with Aldebaran, which is a foreground star and not a member of the cluster, form a distinctive V shape. Recognized since antiquity, and even mentioned in Homer's Iliad, the Hyades derived its name from a group of daughters of Atlas and Aethra, half-sisters of the pleiades. The cluster is about 600 million years old.

In the 19th century, from studies of stellar proper motions, it was realized that the Hyades cluster moves in a general south-easterly direction, together with numerous other stars in the surrounding parts of the sky, in what is called the Taurus stream. Refined proper motion measurements subsequently revealed that the Hyades shares a common motion, to within very narrow limits, with an appreciable number of stars in its vicinity. This cluster was designated the Taurus moving cluster; the name Hyades was often used to denote the central denser clustering, which includes the original stars. Today the name Hyades is generally used for the whole cluster, and individual members are sometimes termed Hyads.

The distance to the Hyades has been determined in several ways, most recently by the Hipparcos satellite. It lies at a distance of around 151 l.y. This distance can be used to derive absolute magnitudes for cluster members of different kinds, and these values are in turn used as calibrators for determinations of distances to other stellar aggregates, also containing intrinsically brighter objects. In this way the distance to the Hyades forms one crucial step on the distance ladder of the Universe.

The majority of the Hyades cluster member stars belong to the main sequence, the brightest of these being of spectral type A2. It is from this fact that the age estimate is derived, since more massive stars have had time to evolve away from the main sequence. A few such stars are known in the Hyades; they are now yellow giant stars.

Also belonging to the cluster are a score of white dwarf stars. A considerable share of the cluster members is believed to consist of binary star systems.

The central concentration, primarily of the more massive stars, indicates that the cluster is nearly relaxed, meaning that the stars have had time to distribute the available kinetic energy equally amongst themselves. The relaxation is, however, not complete, as is seen from the somewhat ragged distribution of stars in the outskirts of the cluster.

Due to the close distance of the Hyades, there is no apparent extinction of light by cosmic dust in front of the stars, and also no dust has been found within the cluster. See also ursa major moving cluster

Hyakutake, Comet (C/1996 B2) Long-period comet discovered on 1996 January 31 by Japanese amateur astronomer Yuji Hyakutake (1950-2002). The comet made a remarkably close passage (0.10 AU) to Earth on March 25, becoming a prominent naked-eye object. Around the time of closest approach, C/1996 B2 Hyakutake was particularly well placed for observers in the northern hemisphere, lying close to Polaris, with a bluish ion tail stretching for up to 70 through the Plough and beyond. Several disconnection events were observed in the ion tail. The coma reached a peak magnitude of 1. Comet Hyakutake faded somewhat after its near-Earth approach, but brightened again in late April as it neared its May 1 perihelion, 0.23 AU from the Sun. The comet, possibly making its first visit from the oort cloud to the inner Solar System, has an orbital period of about 14,000 years.

Around 1996 May 1, the ulysses spacecraft recorded anomalies in the solar wind, later determined to be due to an encounter with the ion tail of Comet Hyakutake. Stretching some 3.8 AU downstream from the nucleus, the comet tail is by far the longest yet found.

Hydra See feature article

hydrocarbons See interstellar molecules

hydrogen (symbol H) First and lightest of the chemical elements; it is the most abundant element in the Universe (see astrochemistry). Hydrogen-1 (that is, an atom with a single proton as its nucleus with a single electron attached to it) forms almost 100% of the naturally occurring element and has an atomic number of 1 and an atomic mass of 1.007825 amu. Its boiling point is 20.29 K and its melting point is 14.02 K. Hydrogen-2, which is also known as deuterium, is present on Earth in the proportion of 0.015%. A third isotope, hydrogen-3, also known as tritium, is radioactive, with a half-life of 12.26 years; it is produced in nuclear reactors. The name hydrogen derives from the Greek hydro (water) and genes (forming), and was first used by the French chemist Antoine Lavoisier (1743-94). Hydrogen was recognised as a distinct substance, which he called 'inflammable air', in 1766 by the English chemist Henry Cavendish (1731-1810). In the visible region, hydrogen produces the familiar balmer line spectrum, which arises from transitions to and from hydrogen's first excited level. Transitions to and from the ground state produce the ultraviolet lyman series; from higher excited levels, a number of sets of lines in the infrared are produced, starting with the paschen series.

Our most familiar contact with hydrogen is in the form of one of its two oxides, water (H2O, the other is hydrogen peroxide, H2O2), or more rarely as the gas that appears when an acid and a metal react. It is widely used in industry, to make metals and margarine, to fix nitrogen via the Haber process and, in liquid form, as a convenient, if hazardous, rocket fuel. Its presence in the molecules of carbohydrates also makes it fundamental to the existence of life.

Sun, for example, the immense pressures and temperatures of the interior ionize the atom into free protons and electrons and so turn hydrogen into a nuclear fuel (see NUCLEOSYNTHESIS). At pressures in excess of 1011 Pa, solid molecular hydrogen starts to behave like a metal, and in this form it is thought to make up the central regions of the Jovian planets. The Milky Way contains huge amounts of non-luminous material that is mostly atomic and molecular hydrogen. About half of the mass of our Galaxy is hydrogen gas and most of the rest is in the form of stars that are again largely hydrogen. Hydrogen between the stars is difficult to detect because it is usually at a low temperature, and the lowest energy spectrum lines from both atomic and molecular hydrogen are FORBIDDEN LINES. Nonetheless, atomic hydrogen is so abundant that the forbidden emission line at 21 cm (the TWENTY-ONE CENTIMETRE LINE), which results from the electron switching its direction of spin, is easily picked up by radio telescopes. The presence of hydrogen can also be inferred through its association with INTERSTELLAR DUST particles, which absorb light very strongly and are therefore easily detectable. If the hydrogen gas is in the vicinity of hot stars, ultraviolet radiation from those stars will ionize the hydrogen, producing an HII REGION embedded within the surrounding cold HI REGION. See also HYDROGEN SPECTRUM; STROMGREN SPHERE

hydrogen-alpha line (Ha line) Spectrum line at 656.3 nm caused by the transition between orbits (energy levels) 2 and 3 of the hydrogen atom. In emission, the electron jumps from orbit 3 to orbit 2, in absorption it jumps from 2 to 3. See also BALMER LINES; HYDROGEN SPECTRUM

hydrogen emission region Cloud of hot gas in interstellar space that is principally visible because of the emission of the Balmer series of spectrum lines from hydrogen. The strongest BALMER LINE is the HYDROGEN-ALPHA LINE (H-a), which produces the red glow observed from many EMISSION NEBULAE. See also HII REGION

hydrogen spectrum Set of EMISSION LINES or ABSORPTION LINES produced by the neutral hydrogen atom. Hydrogen is surrounded by an infinite number of potential quantized orbits, each with different energy. While the orbital radii increase in size from the ground state orbit (n = 1) according to n2, their energies approach a limit at 13.6 eV above the ground state.

Upwards or downwards transitions between the energy states form different series of absorption or emission lines, which are named and lettered according to the lowest state's number. The lines that arise from, or land on, the ground state (n = 1) belong to the LYMAN SERIES. The line connecting level 1 to 2 is Lyman a (at 121.6 nm), 1 to 3 Lyman p, and so on, with the series converging to the Lyman limit at 91.2 nm. Photons short of the Lyman limit ionize planetary and diffuse nebulae.

Transitions underlain by level 2 are called the BALMER LINES; they begin at Ha (656.3 nm) and go through Hp, Hy and H8 at 486.1, 434.0 and 410.1 nm, ending at the Balmer limit at 364.6 nm. The infrared PASCHEN SERIES begins and ends on level 3 (Paschen a at 1875.1 nm, with the limit at 820.3 nm), the Brackett series on level 4 (from 4.0512 jm to 1.4584 jm), the Pfund series on level 5, and the Humphries series on level 6. Radio transitions between high states arise from diffuse and planetary nebulae; the first found was the H109a line (level 110-109) at 5007 MHz. Radio lines have been observed nearly to level 300.

Any single-electron ion will have a hydrogen-like spectrum, except that the wavelengths will be shifted downward by Z2, where Z is the charge on the nucleus. The same nomenclature is used for such ions. The a line of the Paschen series of ionized helium falls at 468.6 nm, while its Brackett series (the PICKERING SERIES) overlays the hydrogen Balmer series.

Hydrus See feature article

Largest of the 88 constellations, extending over a quarter of the way around the sky from its head, which adjoins Canis Minor, to its tail, next to Libra. Hydra represents the many-headed water-snake killed by Hercules. For all its size, its only star above mag. 2 is a Hya, known as Alphard. Hya is a close binary for apertures over 75 mm (3 in.), mags. 3.4 and 6.7. R Hya is one of the brightest MIRA STARS, reaching mag. 3.5 at maximum, with a minimum of 10th magnitude and a period of 390 days, while U Hya is a semiregular variable ranging from mag. 4.2 to 6.6 in just under 4 months. M48 is a large open cluster well seen through binoculars, and NGC 3242 is a 9th-magnitude planetary nebula known as the GHOST OF JUPITER. Perhaps the best-known object in Hydra is M83, a face-on spiral galaxy some 15 million l.y. away.

Hyginus, Rima Lunar feature composed of a series of volcanic collapse pits set in a generally V-shaped rille that intersects the crater Hyginus. The rille is beset with many craterlets, which formed where the magma withdrew.

hyperbola CONIC SECTION obtained when a right circular cone is cut by a plane that makes an angle with its base greater than that made by the side of the cone. It is an open curve (not closed like a circle or ellipse), with an ECCENTRICITY greater than 1. If an object is in a hyperbolic orbit then it will escape from the primary body. Long-period comets enter the inner Solar System in highly elliptical or parabolic orbits, but never hyperbolic. For a few of them the perturbations by the planets change the departing orbit into a hyperbola, and thus these comets are ejected from the Solar System. When a spacecraft has a close flyby to a planet it enters a temporary hyperbolic orbit around the planet.

hyperboloid Shape that is obtained when a HYPERBOLA is rotated about its axis. Convex hyperboloid mirrors are conventionally used as the SECONDARY MIRROR in a reflecting CASSEGRAIN telescope in conjunction with a parabolic PRIMARY MIRROR.

hyperfine structure Splitting of atomic energy levels by the magnetic interaction between the spins of the electron and nucleus. The best-known example is the splitting of the ground state of hydrogen into two closely spaced levels. In the upper level, the electron and proton

HYDRUS (gen. hydri, abbr. hyi)

Small, far-southern constellation, between the two Magellanic Clouds, introduced by Keyser and de Houtman at the end of the 16th century. It represents a small water-snake. There is little for the casual observer apart from t Hyi, a binocular double of mags. 5.6 and 5.7. spin in the same direction; in the lower, they spin oppositely. The transition between the two levels creates the TWENTY-ONE CENTIMETRE LINE of interstellar neutral hydrogen. Hyperfine splitting in hydroxyl (OH) underlies the interstellar OH lines at 1667, 1712, 1612 and 1665 MHz, the last two being powerful MASERS associated with star-forming regions.

Hyperion Largest, irregular-shaped SATELLITE of SATURN; it measures 370 X 280 X 225 km (230 X 174 X 140 mi). Hyperion was discovered in 1848 jointly by W.C. and G.P. BOND in the United States and William LASSELL in England. It is dark and reddish and may be a fragment of the interior of a larger satellite destroyed by a collision. Hyperion's reddish colour and irregular shape could also be consistent with it being perhaps a captured cometary planetesimal or asteroid. Hyperion is one of the few satellites to have an orbital period that is not synchronous with its orbit (see SYNCHRONOUS ROTATION). In fact, as Hyperion progresses round its orbit it tumbles in a seemingly random way, with both its rotation period and the axis of rotation itself changing in a chaotic fashion. This random movement is probably caused by Hyperion's eccentric orbit and elongated shape, which lead to a strongly variable tidal pull from Saturn. See data at SATURN

hypersensitization Technique in which film for long exposures in ASTROPHOTOGRAPHY is heated or treated chemically to overcome RECIPROCITY FAILURE. One way to increase the performance of film is to bake it for several hours, but this has now been superseded by chemical hypersensitization (known as hypering), in which the film is soaked for several hours in hydrogen gas, or a gas that contains a proportion of hydrogen.

hypervelocity impact Impact with velocity high enough to generate a stress of an order of magnitude larger than the compressive strength of the target material. Velocities at which impacts attain hypervelocity are different for different materials but are typically in the range of 1 to 10 km/s (0.6-6 mi/s). Hypervelocity impact into solid surfaces of planets and satellites results in the formation of an impact CRATER. Accompanying phenomena include impact melting and vaporization of the impactor and target material, and formation of high-density mineral phases, such as diamond at the expense of graphite