Astronomy, the most ancient science, began with the study of the Sun, the Moon, and the visible planets. The modern astronomer is still centrally concerned with recording position, brightness, motion, and other directly observable features of celestial objects and with predicting their motion according to the laws of CELESTIAL MECHANICS. Astrophysics, a 19th- and 20th-century outgrowth of classical astronomy, uses quantum mechanics, relativity theory, and molecular, atomic, nuclear, and elementary-particle physics to explain observed celestial phenomena as the logical result of predictable physical processes. The astrophysicist seeks to characterize the constituents of the universe in terms of temperatures, pressures, densities, and chemical compositions. Although the term astronomer is still used, virtually all astronomers have been trained in astrophysics. The broad aim of modern astronomy is to develop encompassing theories of the origin, evolution, and possible destiny of the universe as a whole, a field of endeavor that is known as COSMOLOGY. NEW WINDOWS ON THE UNIVERSE In the 20th century advances in astronomy have been so rapid that the second half of the century can be considered a golden age. Traditional optical astronomy has been revolutionized by the development of new techniques of FAINT OBJECT DETECTION, including more sensitive photographic emulsions and a plethora of electronic imaging devices. Using a standard TELESCOPE, the optical astronomer can now see fainter and more distant objects than ever before. In addition the astronomer is no longer limited to observing the visible light from celestial bodies. New instruments now allow the study of the heavens in entirely new regions of the SPECTRUM. Radio Astronomy In 1931 Karl G. JANSKY of the Bell Telephone Laboratories discovered extraterrestrial radiation at radio wavelengths and launched the field of RADIO ASTRONOMY. During the 1930s Grote REBER, a radio engineer, further investigated celestial radio radiation, and single-handedly brought radio astronomy to the attention of professional astronomers. As a result of theoretical investigations by astronomers in the Netherlands during World War II, an observable radio line, emitted by neutral hydrogen atoms in space, was predicted at a wavelength of 21 cm. Detection of this line caused radio astronomy to advance rapidly after the war. Today, radio telescopes throughout the world are used to study radio emission from the stars, the planets, the interstellar medium in the Galaxy, and extragalactic sources. Achievements in radio astronomy include the mapping of galactic structure and the discovery of quasars, pulsars, and a large number of complex organic molecules in interstellar space. RADAR ASTRONOMY has also been used within the solar system to determine, for example, the rotational periods of Venus and Mercury. Infrared Astronomy Although scientists have known since the time of William Herschel in the late 18th century that infrared radiation from celestial objects can be detected, it was not until the late 1950s and early 1960s that INFRARED ASTRONOMY became the subject of intensive research. Sensitive detectors were developed that allowed astronomers to explore the infrared region of the spectrum. Infrared astronomy has been helpful in studying the very young or evolved stars that are commonly associated with dense clouds of dust. Ultraviolet, X-Ray, and Gamma-Ray Astronomy In 1957 the USSR launched the first satellite, thus beginning the space age. Few other disciplines have benefited from artificial SATELLITES to the extent that astronomy has. (See SPACE EXPLORATION.) For the astronomer, the atmosphere presents a murky or opaque barrier through which observations of the far infrared, ultraviolet, X-ray, and gamma-ray spectral regions are difficult or impossible. Satellites and, to a limited extent, high-altitude balloons and rockets have become platforms from which to observe these spectral regions. Since 1962, the United States and other nations have launched several orbiting observatories devoted to observing the ultraviolet and X-ray regions (see OAO; OSO; HIGH ENERGY ASTRONOMICAL OBSERVATORY; UHURU). These studies have resulted in better understanding of very hot stars and have produced strong evidence of the existence of black holes (see BLACK HOLE). The impact of extraterrestrial observations on astronomy in all parts of the wavelength spectrum is being extended in the 1990s by a continuing program of space astronomy supported by the SPACE SHUTTLE (see GAMMA-RAY ASTRONOMY; SPACE TELESCOPE; ULTRAVIOLET ASTRONOMY; X-RAY ASTRONOMY). THE SOLAR SYSTEM The achievements of astronomy and astrophysics are evident in the rapidly growing knowledge of the extraterrestrial environment, from the SOLAR SYSTEM to the most remote galaxies. The solar system, as it is known today, comprises the Sun and nine planets, in order of increasing distance from the Sun: MERCURY, VENUS, EARTH, MARS, JUPITER, SATURN, URANUS, NEPTIUNE and PLUTO. The last, Pluto, was discovered in 1930 by Clyde TOMBAUGH, an astronomer at the Lowell Observatory. Planets, Asteroids, and Comets Except for Mercury and Venus, each planet has from 1 to more than 20 natural satellites (see SATELLITE), including Pluto, whose moon was not discovered until 1978. The four gas-giant planets Jupiter, Saturn, Uranus and Neptune all have ring systems made up of vast swarms of small icy and rocky fragments circling those planets in the plane of their equators. By far the most spectacular of these ring systems is that of Saturn; the others are less developed. Between the orbits of Mars and Jupiter lies a belt containing thousands of minor planets, or asteroids (see ASTEROID). The orbits of most of the asteroids restrict them to the region between Mars and Jupiter, but exceptions of various kinds exist--including orbits that cross the Earth's orbit or lie still closer to the Sun. COMETS can attain distances 150,000 times greater than that from the Earth to the Sun. In 1950 the Dutch astronomer Jan Oort speculated that the solar system is surrounded by a cloud of comets, most of which never enter the inner regions of the solar system. The orbits of only a few are disturbed sufficiently to bring them near the Earth. HALLEY'S COMET, known since 240 BC, swings around the sun once every 76 years and was visited by probes in 1985-88, during its most recent swing. Flyby space probes involving most of the planets, and surface landings on the Moon, Venus, and Mars, have transformed planetary astronomy. No longer must observations be made at great distances; on-site measurement of numerous physical properties is now possible. In studying the planets, the astronomer must also enlist the aid of the chemist, the geologist, and the meteorologist. In spite of the great increase in knowledge, however, the probes and landings have raised more questions than they have answered. The origin of the solar system, for example, remains unknown. The VENERA and Magellan missions to Venus and the VIKING landers on Mars indicate that life as it is known on Earth does not exist on either planet, nor is life possible on the Moon. The Sun The Sun is a star with a surface temperature of 5,800 K and an interior temperature of about 15,000,000 K. Because the Sun is the nearest star and is easily observed, its chemical composition and surface activity have been intensely investigated. Among the surface features of the Sun are SUNSPOTS, prominences, and flares. It is now known that the maximum number of sunspots occurs approximately every 11 years, that their temperature is approximately 4,300 K, and that they are related to solar magnetic activity in a cycle taking about 22 years to complete. Studies of historical records have also shown long-scale variations in sunspot numbers. Predictions based on theory indicate that energy-generating processes deep within the Sun and other stars should produce a certain number of chargeless, weightless particles called neutrinos (see NEUTRINO). Efforts to detect solar neutrinos have thus far indicated a far lower rate of neutrino production than current theory seems to require, and revisions of theory may in time prove necessary. On the other hand, physicists and cosmologists are equally interested in the concept that at least some forms of neutrino have mass and undergo transformations inside the Sun. THE STARS The accumulation of precise data on some of the nearer STARS early in the 20th century enabled Ejnar HERTZSPRUNG and Henry Norris RUSSELL, working independently, to plot a graph of brightness and color, two basic stellar properties. When they plotted intrinsic stellar brightness on one axis and stellar color (equivalent to surface temperature) on the other axis, Hertzsprung and Russell found that, instead of being scattered over the graph, the stars fell into distinct regions: a heavily populated, diagonal band, known as the MAIN SEQUENCE, that varies from bright, hot, blue stars to faint, cool, red ones; a horizontal band containing bright, cool, red stars (the giants); and a sparsely populated, horizontal band containing very luminous stars of all colors (the supergiants). In honor of these scientists, graphs of the type they plotted are called HERTZSPRUNG-RUSSELL DIAGRAMS, or simply H-R diagrams. The features found on the H-R diagrams are a key to modern astrophysics, because they are basic to an understanding of STELLAR EVOLUTION. The star's initial mass determines exactly the position of the star on the main sequence. The star gradually changes, however, thus changing its position on the H-R diagrams. As the hydrogen that fuels the star's fusion reaction becomes depleted, the outer layers of the star expand, and it enters the giant phase. Eventually they become unstable and begin to lose mass--some smoothly, others catastrophically, depending on their masses. Most stars pulsate smoothly; some may brighten rapidly in older age, blowing material off into space to form a PLANETARY NEBULA. A few giant, unstable stars explode as supernovas (see SUPERNOVA). In any case, the end is inevitable and evolution proceeds to the stellar graveyard. The most common result of evolution is the WHITE DWARF; A large star would end up as a NEUTRON STAR (PULSAR) and, possibly, a black hole. THE GALAXIES The solar system is located in the outer regions of our GALAXY. From the Earth, the visible part of our Galaxy is seen in the night sky as the Milky Way. The Galaxy is actually a flattened disk about 100,000 light-years wide, surrounded by a spherical halo approximately 200,000 light-years in diameter and by a much larger corona. In all, this system contains stars, gas, and dust in quantities equivalent to more than 1,000 billion solar masses. The Sun takes about 200 million years to orbit the galactic center, approximately 30,000 light-years from the Sun in the direction of the constellation Sagittarius. Before the pioneering work of Harlow SHAPLEY in 1917, astronomers believed that the Sun was near the center of the galaxy. Besides correctly locating the solar system, Shapley showed that globular star clusters (see CLUSTER, star) form a halo about the center of the Galaxy. The structure of the Galaxy has been mapped by now, using the distances of extremely luminous stars as well as radio observations of the 21-cm (8.27-in) line of the hydrogen spectrum, and has been shown to take the form of a typical spiral galaxy. Three basic types of galaxies exist: spirals, such as the Milky Way and the Andromeda galaxy; irregulars, such as the Magellanic clouds; and ellipticals. Representatives of the elliptical galaxies exist at both extremes of galactic size. A dwarf elliptical may contain only a few million stars; a giant elliptical may contain trillions (see EXTRAGALACTIC SYSTEMS). The Galaxy is a member of a gravitationally bound cluster of galaxies known as the Local Group. The ANDROMEDA GALAXY and the MAGELLANIC CLOUDS are conspicuous members of this group, which contains some 20 galaxies. As clusters of galaxies go, this is rather sparse. Other clusters, such as the one in the direction of the constellation Virgo, can contain more than 1,000 galaxies. Gerard de Vaucouleurs indicated the possible existence of a local supercluster of galaxies, a cluster of clusters of which the Local Group is a member. Cosmology The nature of the spiral nebulae, as vast, remote collections of suns, was not understood until 1929 when Edwin P. HUBBLE identified a variable star in the Andromeda galaxy and determined its distance. In 1912 Vesto M. Slipher had discovered that the spiral nebulae are receding from the Earth at high velocities. With a knowledge of the distances of these objects, Hubble was able to demonstrate a relationship between a galaxy's RED SHIFT (a DOPPLER EFFECT observed in the galaxy's spectrum, indicating its recessional velocity) and its distance: the farther away a galaxy is, the greater is its red shift (see HUBBLE's CONSTANT). This is considered good evidence for the BIG BANG THEORY mentioned below. Since Hubble's computations, a continuous effort has been made to extend the boundaries of the observable universe to more remote, higher red-shift objects. Observation of such distant galaxies contributes to greater understanding of the origin and possible fate of the universe. The search for higher red shifts took an unexpected turn when, in 1960, two radio sources were identified with what appeared to be stars. This was surprising, because stars were not expected to be such strong radio sources, and the spectra of these objects could not be associated with any type of star. It was not until 1963 that Maarten Schmidt correctly interpreted the spectra as having enormous red shifts. These objects, known as quasars, are still the subject of great controversy. It is not known whether their red shifts are attributable to their great distance, as is true of normal galaxies, or to other physical phenomena. Other evidence has given astronomers a good idea of the origin of the universe--the concern of COSMOLOGY. In 1964 A. A. Penzias and R. W. Wilson discovered an isotropic microwave BACKGROUND RADIATION characteristic of that emitted by a blackbody at 3 K. This BLACKBODY RADIATION, whose existence has since been confirmed by numerous observations, is believed to be an artifact of the big bang with which most astronomers believe the universe began. According to the big bang theory, a single, cataclysmic event occurred about 20 billion years ago that disrupted the dense mass composed of all matter and radiation in the universe. The matter then dispersed, cooled, and condensed in the form of stars and galaxies. Most cosmologists now accept the big bang theory rather than the rival STEADY-STATE THEORY, and are attempting to account for the exotic physical events that would have been involved in the very first moments of the big bang (see INFLATIONARY THEORY). They have yet to determine, however, whether the universe will expand indefinitely or will ultimately collapse upon itself and perhaps repeat the process indefinitely. Cynthia E. Irvine Bibliography: Abell, G. O., Realm of the Universe (1984); Ferris, T., Coming of Age in the Milky Way (1988); Field, G. B., and Chaisson, E. J., The Invisible Universe (1985); Fredrick, L. W., and Baker, R. H., An Introduction to Astronomy, 9th ed. (1981); Harwit, Martin, Cosmic Discovery (1984); Illingworth, Valerie, The Macmillan Dictionary of Astronomy, 2d ed. (1985); Jastrow, Robert, and Thompson, Malcolm R., Astronomy: Fundamentals and Frontiers, 4th ed. (1984); Lovell, Sir Bernard, and Smith, Sir F. Graham, The Guide to Modern Astronomy (1987); Mitton, Jacqueline, Key Definitions in Astronomy (1982); Pasachoff, Jay M., Astronomy, 2d ed. 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