ultraviolet astronomy, study of celestial objects by means of the ultraviolet radiation they emit, in the wavelength range from about 90 to about 350 nanometers. Ultraviolet (UV) line spectrum measurements are used to discern the chemical composition, densities, and temperatures of interstellar gas and dust, and the temperature and composition of hot young stars. UV observations can also provide essential information about the evolution of galaxies. Because atmospheric interference from the ozone layer, oxygen, and nitrogen makes UV radiation difficult to observe from ground-based telescopes, high-altitude balloons, sounding rockets, and orbiting observatories are employed.

Although attempts to study the sun's UV spectrum from balloons were made during the 1920s, it was not until 1946 that rocket-borne instruments made this possible. Only limited additional progress was made until 1962, when the first Orbiting Solar Observatory (OSO) satellite was launched by the National Aeronautics and Space Administration (NASA). These returned thousands of UV spectra, including the first exteme-ultraviolet (wavelengths below 200 nanometers) observations of the solar corona. Through continuous monitoring of the sun over a 15-year period, this program enhanced our understanding of the solar atmosphere and of the 11-year sunspot cycle.

NASA's Orbiting Astronomical Observatory (OAO) satellites, the first of which was launched in 1966, returned UV data about stars and interstellar gas and dust and the first observations of the powerful UV radiation emitted by certain galaxies. Data from Copernicus (OAO-3), which was launched in 1972, led to the determination of the abundance of deuterium in interstellar matter; it also provided considerable information about the atmospheres of luminous hot stars. The Netherlands Astronomical Satellite (ANS) and the TD-1 satellite performed photometric and spectrophotometric surveys of stars in the UV wavelengths.

The International Ultraviolet Explorer (IUE)—a joint project of the United States, the European Space Agency, and Great Britain—was launched in 1978. In orbit for a decade, it monitored the UV spectrum of Halley's comet during its 1986 approach, provided data about the UV reflectivity of the major planets, and contributed to the understanding of quasars; its large telescope made possible the first UV observations of objects beyond the Milky Way, permitting the determination of temperature and structural changes of cool stars during their starspot cycles. The Extreme Ultraviolet Explorer (EUVE; 1992-2000) was the first orbiting observatory to focus on that part of the spectrum. In addition to data from these satellites, UV observations have also been made from two satellites launched in 1990 primarily for other purposes, the X-ray astronomy satellite ROSAT [ROentgen SATellite] and the Hubble Space Telescope.

radio astronomy, study of celestial bodies by means of the electromagnetic radio frequency waves they emit and absorb naturally.

Radio Telescopes

Radio waves emanating from celestial bodies are received by specially constructed antennas, called radio telescopes, whose use corresponds to that of the optical telescope in observing visible light. In the most common design, a parabolic "dish" replaces the mirror of the reflecting optical telescope. This dish serves to focus the radio waves into a concentrated signal that is then filtered, amplified, and finally analyzed using a computer. The radio signals received from outer space are extremely weak, and long observing times are required to collect a useful amount of energy. Therefore, most radio telescopes are mounted so that they can automatically track a given object as its position changes because of the rotation of the earth.

Galactic Sources of Radio Waves

Naturally occurring radio emission from the sky was accidentally discovered in 1931 by Karl Jansky. An inexplicable source of radio noise was identified in 1940 by Gröte Reber, using a radio telescope in the backyard of his home, as originating from our own galaxy, the Milky Way. This radiation is spread over a wide band of radio frequencies and originates in the ionized interstellar gases surrounding hot, bright stars. In these so-called H II regions, free electrons emit radio waves when they are scattered by collisions with the heavier ions. Other sources of radio waves within our galaxy are the remnants of supernovas, or exploding stars. The most famous example of a supernova remnant is the Crab Nebula in Taurus.

Because there are strong magnetic fields (see magnetism) in the vicinities of supernovas remnants, an additional mechanism is present for producing radio waves. This is the synchrotron radiation emitted by energetic electrons as they rapidly spiral around the magnetic lines of force, instead of simply being deflected by collisions with ions.

A third source of radio waves within our own galaxy consists of the atoms and molecules in the interstellar matter. This radiation is at discrete frequencies instead of over a broad band, or continuum, of frequencies. The first of these "radio lines" to be discovered was the line at a wavelength of 21 cm produced by the hydrogen atom (as opposed to the hydrogen molecule, which is composed of two atoms). The intensity of this line in the radiation from a given region is a direct measure of the amount of hydrogen there. Because hydrogen is a major constituent of the interstellar medium, the 21-cm line has provided astronomers with a means of mapping the spiral structure of the Milky Way. The visible light is blocked off by the same interstellar material in which the hydrogen giving rise to a 21-cm line lies, so that the view of the galaxy is obscured in certain directions, particularly in the direction of the center of the galaxy. Thus, before the advent of radio astronomy, the spiral structure of the Milky Way had not actually been observed but was only inferred from comparison with the Andromeda Galaxy and from other indirect studies. Besides atomic hydrogen, certain simple organic (carbon-based) molecules, including cyanogen (CN) and formaldehyde (H₂CO), have been discovered in the interstellar medium by means of their radio lines.

Extragalactic Sources of Radio Waves

Radio waves also come from outside the Milky Way. These extragalactic radio sources have great implications for cosmology, the theory of the overall structure of the universe. Spiral galaxies like the Milky Way are only weak sources of radio waves, but certain giant elliptical and irregular galaxies emit more than a million times as much radio energy as ordinary galaxies. Such galaxies are usually marked by dust lanes, which are unusual for galaxies lacking spiral arms. Some of these objects can be detected only by their radio emission, but in other cases the position of the radio source has been determined accurately enough to allow astronomers to identify the radio source with a galaxy visible in an image taken with a large optical telescope.

Other radio sources were optically identified with what at first appeared to be faint blue stars. However, it was discovered that these "stars" had enormous red shifts (shifting of the spectral lines toward the red end of the spectrum) that implied, according to Hubble's law, that they were the most remote objects ever detected and that their intrinsic intensities were about 1000 times greater than an entire galaxy. These extraordinary objects were named quasi-stellar radio sources, which was soon shortened to quasars. Their nature is still not completely understood.

Many thousands of extragalactic radio sources are known. Of those optically identified radio sources, roughly one third are quasars, and the remainder are radio galaxies. In addition to these localized radio sources, there is uniform low-level radio noise from every direction in the sky. This cosmic background radiation is believed to be an indication that the universe began with an explosive big bang rather than having always existed in an unchanging steady state. More recently radio astronomy has discovered pulsars, thought to be rapidly spinning neutron stars that radiate bursts of energy on and off regularly between 1 and 30 times a second.

radar astronomy, application of radar to the determination of distances and planetary features within the solar system, such as rotation rates. A short burst of radio waves is transmitted in the direction of the object under study. The object reflects the radio waves back to earth, where they are detected by the same antenna that sent the signal. The time between sending the signal and receiving the "echo" can be precisely measured electronically. Since radio waves travel with the speed of light, the roundtrip distance from the earth to the object and back is then easily computed. This technique differs from radio astronomy in that the celestial object is here merely a passive reflector, rather than the actual source of the emission. The first yield of radar astronomy was a much improved value for the distance from the earth to the moon. Using more powerful transmitters, the distances to Venus and Mercury were also measured, as well as the planets' rotational periods and gross surface properties. Even greater precision is obtained by replacing the radio transmitter with a laser. During the Apollo project, special reflectors were installed on the moon; subsequently, by bouncing laser light off the moon the distance from the earth to the moon could be determined within centimeters. Radar observations are also useful for asteroids and comets whose orbits take them relatively near the earth. Much of the surface of Venus has been mapped by unmanned probes using radar altimeters to penetrate the cloud cover.

neutrino astronomy, study of stars by means of their emission of neutrinos, fundamental particles that result from nuclear reactions and are emitted by stars along with light. Approximately 100 billion neutrinos have raced through your body since you began reading this article. The light received from a star is emitted by the surface layers, which in turn absorb the light coming from the interior. Neutrinos, on the other hand, are absorbed only very weakly by matter and, once created by nuclear reactions in the stellar core, pass directly through the outer parts of the star. Thus neutrinos permit astronomers to look directly into the energy-producing core of a star. Their weak tendency to interact with matter also makes them very difficult to detect. Neutrino "observatories" are located in deep mines, where hundreds of feet of rock shield out the cosmic rays that would completely swamp the tiny effects due to neutrinos. The neutrinos pass as easily through the rock as they pass through the star. They react with chlorine in the detector to produce a radioactive isotope of argon, which is detectable. Because of its proximity, the sun is expected to be by far the most intense source of neutrinos and has been the initial object of study. However, several neutrino detectors observed a rush of neutrinos from Supernova 1987A in a nearby galaxy called the Large Magellanic Cloud. Although their journey from the exploding star began at the moment its core collapsed, they did not move quickly at first since the gravity of the core was so strong. When the shock wave from the explosion reached the neutrinos, it freed them to travel between galaxies, and they arrived on earth about three hours before the first visible light of the explosion appeared.

infrared astronomy, study of celestial objects by means of the infrared radiation they emit, in the wavelength range from about 1 micrometer to about 1 millimeter. All objects, from trees and buildings on the earth to distant galaxies, emit infrared (IR) radiation. The study of such radiation from celestial objects is of particular importance for several reasons. Cosmic dust particles effectively obscure parts of the visible universe, such as the center of our galaxy, the Milky Way, but this dust is transparent in the IR wavelengths. Most of the energy radiated by objects ranging from interstellar matter to planets lies in the IR wavelengths; IR observations are therefore significant in studying asteroids, comets, planetary satellites, and interstellar dust clouds where stars are forming. Finally, because the expansion of the universe shifts energy to longer wavelengths, most of the visible radiation emitted by stars and galaxies during the early stages of the formation of the universe is now shifted to the IR range; studies of the most distant objects in the IR spectrum are necessary if astronomers are to understand how the universe was formed.

The beginnings of IR astronomy can be traced to the discovery of IR radiation in the spectrum of the sun by English astronomer Sir William Herschel about 1800. It is reported that Irish astronomer Lord William Rosse detected IR radiation from the moon about 1845. As early as 1878 the American inventor Thomas Alva Edison observed a solar eclipse from a site in Wyoming using a sensitive IR detector, and during the 1920s the first systematic IR observations of celestial objects were made by Seth B. Nicholson, Edison Pettit, and other American astronomers. However, modern IR astronomy did not begin until the 1950s because of the lack of appropriate instrumentation. Since then, special interference filters and cryogenic systems (to minimize IR interference from the radiation emitted by the equipment itself) have been introduced for ground-based observations, and aircraft, balloons, rockets, and orbiting satellites have been successively employed to carry the equipment above the water vapor in the earth's atmosphere.

The Kuiper Airborne Observatory (KAO), operated by the National Aeronautics and Space Administration (NASA), had its first flight in 1975. Named for the American astronomer Gerard P. Kuiper, the KAO was a C-141 jet transport that carried its 36-inch (91-cm) telescope to altitudes of up to 45,000 ft (13,720 m). Before it flew its last mission in 1995, the KAO was instrumental in the discovery of the rings of Uranus, the atmosphere around Pluto, and the definitive detection of water during the crash of comet Shoemaker-Levy 9 into Jupiter. Also sponsored by NASA is the Infrared Telescope Facility, a 10-ft (3-m) IR telescope located at an altitude of 14,000 ft (4,270 m) on the summit of Mauna Kea in Hawaii; established in 1979, it effectively is the U.S. national IR observatory. Also near the summit of Mauna Kea is the 12.5-ft (3.8-m) United Kingdom Infrared Telescope (UKIRT), the largest telescope in the world used solely for IR observations.

The first IR satellite to be launched (1983) was the Infrared Astronomical Satellite (IRAS), a joint venture of the United States, Great Britain, and the Netherlands. Orbiting the earth for 10 months, IRAS performed an all-sky survey that yielded catalogs of hundreds of thousands of IR sources, more than half of these previously unknown, including asteroids and comets; detected a new class of long-lived "cool" galaxies that are dim in the visible region of the spectrum; located a protoplanetary disk around a nearby star; and showed clearly for the first time the bulge near the center of the Milky Way. In 1989 the second IR satellite, the Cosmic Background Explorer (COBE), was launched by NASA. Operating through 1993, COBE detected small temperature variations in the cosmic microwave background radiation that provided vital clues to the nature of the early universe and its evolution since the "big bang." The European Space Organization launched the Infrared Space Observatory (ISO) in 1995. Operating until May, 1998, ISO monitored nearby planets, asteroids, and comets. It found water vapor in the atmospheres of Saturn, Neptune, Uranus, and Titan, Saturn's largest moon; detected water vapor and fluorides in the interstellar medium; and studied the "cool" galaxies first seen by IRAS. The near-infrared camera multiobject spectrometer (NICMOS) was placed aboard the Hubble Space Telescope in 1997. Consisting of three cameras and three spectrometers, it has been used to study interstellar clouds where stars are being formed, young stars, and the atmospheres of Jupiter and Uranus.

The Spitzer Space Telescope, a cryogenically cooled satellite observatory with a 2.8-ft/0.85-m telescope, was launched in Aug., 2003, and placed in a solar orbit in which it trails the earth by 5.4 million mi (8.7 million km); it is expected to have a two-to-five-year operating lifetime. Future plans for IR astronomy include a KAO replacement, the Stratospheric Observatory for Infrared Astronomy (SOFIA), a joint project of NASA and the German space agency, DLR, that consists of a Boeing 747-SP aircraft modified to accommodate a 8.2-ft/2.5-m reflecting telescope (the largest airborne telescope in the world). SOFIA is expected to go into service in 2004 and have a 20-year lifetime.

gamma-ray astronomy, study of astronomical objects by analysis of the most energetic electromagnetic radiation they emit. Gamma rays are shorter in wavelength and hence more energetic than X rays (see gamma radiation) but much harder to detect and to pinpoint. X rays and some gamma rays are produced throughout the universe by the same catastrophic astrophysical events, such as supernovas and black holes, and gamma-ray astronomy can be considered an extension of X-ray astronomy to the extreme shortwave end of the spectrum.

Gamma rays are difficult to observe from ground-based telescopes due to atmospheric interference, and high-altitude balloons, sounding rockets, and orbiting observatories are therefore used. Some ground-based facilities, including a large 33-ft (10-m) dish with many small mirrors at Mount Hopkins, Ariz., are successful gamma-ray collectors because they record the radiation emitted by very-high-energy gamma rays as they generate high-speed electrons in the upper atmosphere. Another approach to detecting this radiation is the Milagro detector in the Jemez Mountains of New Mexico. It consists of hundreds of phototubes floating within a pond containing 6 million gallons of water; through interactions with the water, the radiation generates weak trails of light that are detected by the phototubes, yielding data about the energy and direction of the gamma rays.

Cygnus X-3 and the Crab and Vela pulsars are well known gamma-ray sources. In addition, gamma rays have been detected as general background radiation concentrated along the plane of the Milky Way. These gamma rays may result from cosmic rays interacting with gaseous matter in the interstellar medium. Gamma rays from outside the Milky Way have been found emanating from radio galaxies (galaxies whose radio emissions constitute an extraordinarily large amount of their total energy output), Seyfert galaxies (galaxies with extremely bright cores—called Active Galactic Nuclei [AGN]—that are strong emitters of radio waves, X rays, and gamma rays), and supernovas.

The first gamma-ray telescope was carried into orbit on the Explorer XI satellite in 1961. Additional gamma-ray experiments flew on the OGO, Vela, and Russian Cosmos series of satellites. The Orbiting Solar Observatory OSO-3 made the first certain detection of celestial gamma rays in 1972, and OSO-7 detected gamma-ray emission lines in the solar spectrum. However, the first satellite designed as a "dedicated" gamma-ray mission was the second Small Astronomy Satellite (SAS-2) in 1972. In 1975 the European Space Agency launched the COS-B satellite to survey the sky for gamma-ray sources. SAS-2 and COS-B confirmed the earlier findings of gamma-ray background radiation and also detected a number of point sources, but the poor resolution of the instruments made it impossible to associate most of these point sources with individual stars or stellar systems. The third High Energy Astronomy Observatory (HEAO-3), launched in 1979, studied both cosmic rays and gamma radiation. A number of satellites launched during the 1980s carried gamma-ray experiments into orbit. The Compton Gamma-Ray Observatory (CGRO), launched in 1991, carried a collection of four instruments that were larger and more sensitive than any gamma-ray telescope previously orbited. In addition to creating a comprehensive map of celestial gamma-ray sources and demonstrating that gamma-ray bursts are evenly distributed across the sky (which suggests that the radiation is coming from the distant reaches of the universe and not just from within the Milky Way), CGRO detected a number of "firsts," such as the first gamma-ray quasar. During the 1990s a number of planetary probes, such as Mars Observer (1983), and earth-orbiting satellites, such as Minisat 1 (1997), carried gamma-ray detection and measurement devices as part of their instrumentation.

The turn of the century saw designs for gamma-ray astronomy satellites that allow for imaging resolution and spectral resolution powers never before possible. Launchings of orbiting gamma-ray observatories include missions such as the High Energy Transient Explorer (HETE-2), launched in 2000, the European Space Agency's International Gamma-Ray Astrophysics Laboratory (INTEGRAL), launched in 2002, and the Swift Gamma Ray Burst Explorer, launched in 2004.

In 1967 a Vela military satellite designed to detect nuclear explosions discovered the first gamma-ray bursts (GRBs). These events are very short-lived, lasting from about 50 milliseconds to, in extreme cases, several minutes, and occur on an almost daily basis. It has been suggested that the formation of black holes is associated with these intense gamma-ray bursts. Beginning with a giant star collapsing on itself or the collision of two neutron stars, waves of radiation and subatomic particles are propelled outward from the nascent black hole and collide with one another, releasing the gamma radiation. Also released is longer-lasting—from a few days to several years—electromagnetic radiation (called the afterglow) in the form of X rays, radio waves, and visible wavelengths that can be used to pinpoint the location of the disturbance.

astronomy, branch of science that studies the motions and natures of celestial bodies, such as planets, stars, and galaxies; more generally, the study of matter and energy in the universe at large.

Ancient Astronomy

Astronomy is the oldest of the physical sciences. In many early civilizations the regularity of celestial motions was recognized, and attempts were made to keep records and predict future events. The first practical function of astronomy was to provide a basis for the calendar, the units of month and year being determined by astronomical observations. Later, astronomy served in navigation and timekeeping. The Chinese had a working calendar as early as the 13th cent. B.C. About 350 B.C., Shih Shen prepared the earliest known star catalog, containing 800 entries. Ancient Chinese astronomy is best known today for its observations of comets and supernovas. The Babylonians, Assyrians, and Egyptians were also active in astronomy. The earliest astronomers were priests, and no attempt was made to separate astronomy from astrology. In fact, an early motivation for the detailed study of planetary positions was the preparation of horoscopes.

Greek Innovations

The highest development of astronomy in the ancient world came with the Greeks in the period from 600 B.C. to A.D. 400. The methods employed by the Greek astronomers were quite distinct from those of earlier civilizations, such as the Babylonian. The Babylonian approach was numerological and best suited for studying the complex lunar motions that were of overwhelming interest to the Mesopotamian peoples. The Greek approach, on the contrary, was geometric and schematic, best suited for complete cosmological models. Thales, an Ionian philosopher of the 6th cent. B.C., is credited with introducing geometrical ideas into astronomy. Pythagoras, about a hundred years later, imagined the universe as a series of concentric spheres in which each of the seven "wanderers" (the sun, the moon, and the five known planets) were embedded. Euxodus developed the idea of rotating spheres by introducing extra spheres for each of the planets to account for the observed complexities of their motions. This was the beginning of the Greek aim of providing a theory that would account for all observed phenomena. Aristotle (384-322 B.C.) summarized much of the Greek work before him and remained an absolute authority until late in the Middle Ages. Although his belief that the earth does not move retarded astronomical progress, he gave the correct explanation of lunar eclipses and a sound argument for the spherical shape of the earth.

The Alexandrian School and the Ptolemaic System

The apex of Greek astronomy was reached in the Hellenistic period by the Alexandrian school. Aristarchus (c.310-c.230 B.C.) determined the sizes and distances of the moon and sun relative to the earth and advocated a heliocentric (sun-centered) cosmology. Although there were errors in his assumptions, his approach was truly scientific; his work was the first serious attempt to make a scale model of the universe. The first accurate measurement of the actual (as opposed to relative) size of the earth was made by Eratosthenes (284-192 B.C.). His method was based on the angular difference in the sun's position at the high noon of the summer solstice in two cities whose distance apart was known.

The greatest astronomer of antiquity was Hipparchus (190-120 B.C.). He developed trigonometry and used it to determine astronomical distances from the observed angular positions of celestial bodies. He recognized that astronomy requires accurate and systematic observations extended over long time periods. He therefore made great use of old observations, comparing them to his own. Many of his observations, particularly of the planets, were intended for future astronomers. He devised a geocentric system of cycles and epicycles (a compounding of circular motions) to account for the movements of the sun and moon.

Ptolemy (A.D. 85-165) applied the scheme of epicycles to the planets as well. The resulting Ptolemaic system was a geometrical representation of the solar system that predicted the motions of the planets with considerable accuracy. Among his other achievements was an accurate measurement of the distance to the moon by a parallax technique. His 13-volume treatise, the Almagest, summarized much of ancient astronomical knowledge and, in many translations, was the definitive authority for the next 14 centuries.

Development of Modern Astronomy

The Copernican Revolution

After the fall of Rome, European astronomy was largely dormant, but significant work was carried out by the Muslims and the Hindus. It was by way of Arabic translations that Greek astronomy reached medieval Europe. One of the great landmarks of the revival of learning in Europe was the publication (1543) by Nicolaus Copernicus (1473-1543) of his De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres). According to the Copernican system, the earth rotates on its axis and, with all the other planets, revolves around the sun. The assertion that the earth is not the center of the universe was to have profound philosophical and religious consequences. Copernicus's principal claim for his new system was that it made calculations easier. He retained the uniform circular motion of the Ptolemaic system, but by placing the sun at the center, he was able to reduce the number of epicycles. Copernicus also determined the sidereal periods (time for one revolution around the sun) of the planets and their distance from the sun relative to the sun-earth distance (see astronomical unit).

Brahe and Kepler

The great astronomer Tycho Brahe (1546-1601) was principally an observer; a conservative in matters of theory, he rejected the notion that the earth moves. Under the patronage of King Frederick II, Tycho established Uraniborg, a superb observatory on the Danish island of Hveen. Over a period of 20 years (1576-97), he and his assistants compiled the most accurate and complete astronomical observations to that time. At his death his records passed to Johannes Kepler (1571-1630), who had been his last assistant. Kepler spent nearly a decade trying to fit Tycho's observations, particularly of Mars, into an improved system of heliocentric circular motion. At last, he conceived the idea that the orbit of Mars was an ellipse with the sun at one focus. This led him to the three laws of planetary motion that bear his name (see Kepler's laws).

Galileo's Telescope

Galileo Galilei (1564-1642) made fundamental discoveries in both astronomy and physics; he is perhaps best described as the founder of modern science. Galileo was the first to make astronomical use of the telescope. His discoveries of the four largest moons of Jupiter and the phases of Venus were persuasive evidence for the Copernican cosmology. His discoveries of craters on the moon and blemishes on the sun (sunspots) discredited the ancient belief in the perfection of the heavens. These findings were announced in The Sidereal Messenger, a small book published in 1610. Galileo's Dialogue on the Two Chief Systems of the World (1632) was an eloquent argument for the Copernican system over the Ptolemaic. However, Galileo was called before the Inquisition and forced to renounce publicly all doctrines considered contrary to Scripture.

Astrophysical Discoveries

Isaac Newton (1642-1727), possibly the greatest scientific genius of all time, succeeded in uniting the sciences of astronomy and physics. His laws of motion and theory of universal gravitation provided a physical, dynamic basis for the merely descriptive laws of Kepler. Until well into the 19th cent., all progress in astronomy was essentially an extension of Newton's work. Edmond Halley's prediction that the comet of 1682 would return in 1758 was refined by A. C. Clairault, who included the perturbing effects of Jupiter and Saturn on the orbit to calculate the nearly exact date of the return of the comet. In 1781, William Herschel accidentally discovered a new planet, eventually named Uranus. Discrepancies between the observed and theoretical orbits of Uranus indicated the existence of a still more distant planet that was affecting Uranus's motion. J. C. Adams and U. J. J. Leverrier independently calculated the position where the new planet, Neptune, was actually discovered (1846). Similar calculations for a large "Planet X" led in 1930 to the discovery of Pluto, now classed as a dwarf planet.

By the early 19th cent., the science of celestial mechanics had reached a highly developed state at the hands of Leonhard Euler, J. L. Lagrange, P. S. Laplace, and others. Powerful new mathematical techniques allowed solution of most of the remaining problems in classical gravitational theory as applied to the solar system. In 1801, Giuseppe Piazzi discovered Ceres, the first of many asteroids. When Ceres was lost to view, C. F. Gauss applied the advanced gravitational techniques to compute the position where the asteroid was subsequently rediscovered. In 1838, F. W. Bessel made the first measurement of the distance to a star; using the method of parallax with the earth's orbit as a baseline, he determined the distance of the star 61 Cygni to be 60 trillion mi (about 10 light-years), a figure later shown to be 40% too large.

Modern Techniques, Discoveries, and Theories

Astronomy was revolutionized in the second half of the 19th cent. by the introduction of techniques based on photography and spectroscopy. Interest shifted from determining the positions and distances of stars to studying their physical composition (see stellar structure and stellar evolution). The dark lines in the solar spectrum that had been observed by W. H. Wollaston and Joseph von Fraunhofer were interpreted in an elementary fashion by G. R. Kirchhoff on the basis of classical physics, although a complete explanation came only with the quantum theory. Between 1911 and 1913, Ejnar Hertzsprung and H. N. Russell studied the relation between the colors and luminosities of typical stars (see Hertzsprung-Russell diagram). With the construction of ever more powerful telescopes (see observatory), the boundaries of the known universe constantly increased. E. P. Hubble's study of the distant galaxies led him to conclude that the universe is expanding (see Hubble's law). Using Cepheid variables as distance indicators, Harlow Shapley determined the size and shape of our galaxy, the Milky Way. During World War II Walter Baade defined two "populations" of stars, and suggested that an examination of these different types might trace the spiral shape of our own galaxy (see stellar populations). In 1951 a Yerkes Observatory group led by William W. Morgan detected evidence of two spiral arms in the Milky Way galaxy.

Various rival theories of the origin and overall structure of the universe, e.g., the big bang and steady state theories, have been formulated (see cosmology). Albert Einstein's theory of relativity plays a central role in all modern cosmological theories. In 1963, the moon passed in front of the radio source 3C-273, allowing Cyril Hazard to calculate the exact position of the source. With this information, Maarten Schmidt photographed the object's spectrum using the 200-in. (5-m) reflector on Palomar Mt., then the world's largest telescope. He interpreted the result as coming from an object, now known as a quasar, at an extreme distance and receding from us at a substantial fraction of the speed of light. In 1967 Antony Hewish and Jocelyn Bell Burnell discovered a radio source a few hundred light years away featuring regular pulses at intervals of about 1 second with an accuracy of repetition of one-millionth of a second. This was the first discovered pulsar, a rapidly spinning neutron star emitting lighthouse-type beams of energy, the end result of the death of a star in a supernova explosion.

The discovery by Karl Jansky in 1931 that radio signals were emitted by celestial bodies initiated the science of radio astronomy. Most recently, the frontiers of astronomy have been expanded by space exploration. Perturbations and interference from the earth's atmosphere make space-based observations necessary for infrared, ultraviolet, gamma-ray, and X-ray astronomy. The Surveyor and Apollo spacecraft of the late 1960s and early 1970s helped launch the new field of astrogeology. A series of interplanetary probes, such as Mariner 2 (1962) and 5 (1967) to Venus, Mariner 4 (1965) and 6 (1969) to Mars, and Voyager 1 (1979) and 2 (1979), provided a wealth of data about Jupiter, Saturn, Uranus, and Neptune; more recently, the Magellan probe to Venus (1990) and the Galileo probe to Jupiter (1995) have continued this line of research (see satellite, artificial; space probe). The Hubble Space Telescope, launched in 1990, has made possible visual observations of a quality far exceeding those of earthbound instruments.

X-ray astronomy, study of celestial objects by means of the X rays they emit, in the wavelength range from 0.01 to 10 nanometers. X-ray astronomy dates to 1949 with the discovery that the sun emits X rays. Since X rays could not be observed from ground-based telescopes, V-2 rockets launched from White Sands, N.Mex., occasionally carried telescopes to study solar X-ray emissions. In 1962 a group led by R. Giacconi launched a small rocket from White Sands to search for celestial sources of X rays with instruments similar to Geiger counters. During the 5-min flight the experiment discovered an X-ray source now called Scorpius X-1, a close binary star in which one star expels gas onto a very dense neighbor, which may be a white dwarf, a neutron star, or a black hole. This mission also found that the earth is bathed in diffuse X rays coming from all directions. Soon afterward X-ray emissions were found coming from the Crab Nebula and the radio galaxies (galaxies whose radio emissions constitute an extraordinarily large amount of their total energy output) Centaurus A and Virgo A. Other types of galaxies, particularly Seyfert galaxies (galaxies with extremely bright cores that are strong emitters of radio waves, X rays, and gamma rays), also emit X rays. The center of our galaxy is a strong X-ray source, which is an indicator of the violent activity taking place there.

In 1970 the Uhuru satellite, one of NASA's small astronomy satellites, began to look specifically for X-ray sources. Uhuru used detectors filled with argon, in which incoming X radiation gives off electrons in amounts proportional to its strength. Uhuru mapped more than 400 sources and discovered a series of X-ray binary stars in which ordinary stars orbit neutron stars that emit X rays. One of these sources, Cygnus X-1, is an object with ten times the mass of the sun. Too massive to be a neutron star, it is possibly a black hole.

Much of the data in X-ray astronomy is now gathered by orbiting satellites. In addition to the United States, Germany and Japan are among the countries having X-ray satellites. In the 1970s the Skylab space station and Orbiting Solar Observatory satellites continued the study, as did the Solar Maximum Mission the following decade. A series of High Energy Astrophysical Observatories (HEAO) were launched during the late 1970s to study X rays, cosmic rays, and gamma rays. HEAO-1, launched in 1977, increased the number of known X-ray sources from 350 to 1,500. HEAO-2—also known as the Einstein Observatory—carried the largest X-ray telescope ever built. It detected several thousand new X-ray sources in our galaxy and beyond, discovered that cataclysmic variable stars in our own galaxy emit X rays when they are in outburst, achieved the first unambiguous detection of X rays from ordinary stars other than the sun, and obtained the first X-ray images of supernova remnants, pulsars, and star clusters. As a result, supernova remnants mapped in X-ray wavelengths can be compared with visible light and radio images. In an example of cooperation between amateur and professional astronomers, the Einstein Observatory was turned toward SS Cygni (see variable star) whenever amateur astronomers with backyard telescopes reported it in outburst. The few days' duration of these outbursts allowed enough time to change the satellite's observing schedule so that it could examine the star, and it discovered the source of the star's X-ray emissions.

During the 1980s the European, Russian, and Japanese space agencies continued to launch successful X-ray astronomy missions, such as the European X-ray Observatory Satellite (EXOSAT), Granat, the Kvant module (of the Mir space station), Tenma, and Ginga. These missions were more modest in scale than the HEAO program in the 1970s and were directed toward in-depth studies of known phenomena.

In 1990, ROSAT [Roentgen Satellite], a joint project of Germany, the United States, and Great Britain, was launched. Operational until 1999, it was instrumental in the discovery of X-ray emissions from comets and conducted an all-sky survey in the X-ray region of the spectrum. Five other satellites launched in the 1990s are still operational. ALEXIS [Array of Low Energy X-ray Imaging Sensors] was launched in 1993; a minisatellite containing six coffee-can-sized wide-angle, ultrasoft-X-ray telescopes, it provided the data for a unique sky map for studying celestial flashes of soft X rays. Also launched in 1993, the Advanced Satellite for Cosmology and Astrophysics is a joint Japanese-American project; containing four X-ray telescopes, its primary purpose is the X-ray spectroscopy of such astrophysical entities as quasars and cosmic background X radiation. In 1995, NASA orbited the Rossi X-ray Timing Explorer (RXTE) to study the variations in the emission of such X-ray sources as black-hole candidates, active galactic nuclei, white dwarf stars, neutron stars, and other high-energy sources. The RXTE played a key role in the discovery in 1996 of a "pulsing burster" located near the center of the Milky Way. Unlike other X-ray sources, this one burst, oscillated, and flickered simultaneously, with bursts lasting from 6 to 100 seconds. Before it burned out, the unexplained object was the brightest source of X rays and gamma rays in the sky, radiating more energy in 10 seconds than the sun does in 24 hours. BeppoSAX, a joint Italian-Dutch satellite, was launched in 1996. When on Dec. 14, 1997, for 1 or 2 seconds the most energetic burst of gamma radiation ever detected was recorded by the Compton Gamma Ray Observatory, BeppoSAX recorded the X-ray afterglow of the burst, thereby providing a relatively accurate location for the source. The Chandra X-ray Observatory was deployed from a shuttle and boosted into a high earth orbit in 1999; it focuses on such objects as black holes, quasars, and high-temperature gases throughout the X-ray portion of the electromagnetic spectrum. Also launched in 1999 was X-ray Multimirror Mission, an ESA satellite that carries an optical-ultraviolet telescope together with three parallel mounted X-ray telescopes, allowing it to simultaneously observe phenomena in two regions of the spectrum.

Stratospheric Observatory for Infrared Astronomy: see infrared astronomy.

Nuffield Radio Astronomy Laboratories: see Jodrell Bank Observatory.

National Radio Astronomy Observatory (NRAO), federal observatory for radio astronomy, founded in 1956 and operated under contract with the National Science Foundation by Associated Universities, Inc., a group of major universities. The headquarters are at Charlottesville, Va.; the original observatory site is in Greenbank, W.Va., where the antennas, or radio telescopes, include a fully steerable 140-ft (43-m) paraboloid; an interferometer consisting of three steerable 85-ft (26-m) paraboloids; a horn-shaped antenna 120 ft (37 m) in length that is fixed in place; and two smaller, steerable paraboloids; a modern 328-ft (100-m) fully steerable telescope is under construction. At Kitt Peak, near Tucson, Ariz., NRAO has a 36-ft (11-m) steerable paraboloid; near Socorro, New Mexico, the NRAO's Very Large Array (VLA) consists of 27 parabolic dishes, each 82 ft (25 m) in diameter, mounted on a Y-shaped track with arms up to 14 mi (21 km) long. Finally, the observatory operates the Very Baseline Array (VLBA) consisting of ten radio telescopes placed around the earth that operate in unison. Principal research programs of the NRAO include the study of galactic structure, extragalactic radio sources, molecules in space, pulsars, quasars, and the evolution of stars and galaxies. Astronomers using the VLA have discovered filaments, jets, and high-temperature features in the center of our own galaxy and in extragalactic radio sources that may help explain the high energy of quasars. The system allows the study of the nuclei of active galaxies and helps determine distances to radio sources more accurately.

Study of astronomical objects and phenomena by observing the ultraviolet radiation (UV radiation) they emit. It has yielded much information about chemical abundances and processes in interstellar matter, the Sun, and other stellar objects, such as hot young stars and white dwarf stars. Ultraviolet astronomy became feasible once rockets could carry instruments above Earth's atmosphere, which absorbs most electromagnetic radiation of UV wavelengths. Since the early 1960s, a number of unmanned space observatories carrying UV telescopes, including the Hubble Space Telescope, have collected UV data on objects such as comets, quasars, nebulae, and distant star clusters. The Extreme Ultraviolet Explorer, launched in 1992, was the first orbiting observatory to map the sky in the shortest UV wavelengths, at the boundary with the X-ray region of the electromagnetic spectrum.

Learn more about ultraviolet astronomy with a free trial on Britannica.com.

Study of celestial bodies by measuring the energy they emit or reflect at radio wavelengths. It began in 1931 with Karl Jansky's discovery of radio waves from an extraterrestrial source. After 1945, huge dish antennas, improved receivers and data-processing methods, and radio interferometers let astronomers study fainter sources and obtain greater detail. Radio waves penetrate much of the gas and dust in space, giving a much clearer picture of the centre and structure of the Milky Way Galaxy than optical observation can. This has allowed detailed studies of the interstellar medium in the Galaxy and the discovery of previously unknown cosmic objects (e.g., pulsars, quasars). In radar astronomy, radio signals are sent to near-Earth bodies or phenomena (e.g., meteor trails, the Moon, asteroids, nearby planets) and the reflections detected, providing precise measurement of the objects' distances and surface structure. Because radar waves can penetrate even dense clouds, they have provided astronomers' only maps of the surface of Venus. Radio and radar studies of the Moon revealed its sandlike surface before landings were made. Radio observations have also contributed greatly to knowledge about the Sun. Seealso radio telescope.

Learn more about radio and radar astronomy with a free trial on Britannica.com.

Study of astronomical objects by observing the infrared radiation they emit. Its techniques enable examination of many celestial objects that give off energy at wavelengths in the infrared region of the electromagnetic spectrum but that cannot otherwise be seen from Earth because they do not emit much visible light or because that light is blocked by dust clouds, which infrared radiation can penetrate. Infrared astronomy originated in the early 19th century with the work of William Herschel (see Herschel family), who discovered infrared radiation while studying sunlight. The first systematic infrared observations of other stars were made in the 1920s; modern techniques, such as the use of interference filters for ground-based telescopes, were introduced in the early 1960s. Because atmospheric water vapour absorbs many infrared wavelengths, observations are carried out with telescopes sited on high mountaintops and from airborne and space-based observatories. Infrared astronomy allows studies of the dust-obscured core of the Milky Way Galaxy and the hearts of star-forming regions and has led to many discoveries including brown dwarf candidates and disks of matter around certain stars.

Learn more about infrared astronomy with a free trial on Britannica.com.

Science dealing with the origin, evolution, composition, distance, and motion of all bodies and scattered matter in the universe. The most ancient of the sciences, it has existed since the dawn of recorded civilization. Much of the earliest knowledge of celestial bodies is often credited to the Babylonians. The ancient Greeks introduced influential cosmological ideas, including theories about the Earth in relation to the rest of the universe. Ptolemy's model of an Earth-centred universe (2nd century AD) influenced astronomical thought for over 1,300 years. In the 16th century, Nicolaus Copernicus assigned the central position to the Sun (see Copernican system), ushering in the age of modern astronomy. The 17th century saw several momentous developments: Johannes Kepler's discovery of the principles of planetary motion, Galileo's application of the telescope to astronomical observation, and Isaac Newton's formulation of the laws of motion and gravitation. In the 19th century, spectroscopy and photography made it possible to study the physical properties of planets, stars, and nebulae, leading to the development of astrophysics. In 1927 Edwin Hubble discovered that the universe, hitherto thought static, was expanding (see expanding universe). In 1937 the first radio telescope was built. The first artificial satellite, Sputnik, was launched in 1957, inaugurating the age of space exploration; spacecraft that could escape Earth's gravitational pull and return data about the solar system were launched beginning in 1959 (see Luna; Pioneer). Seealso big bang; cosmology; gamma-ray astronomy; infrared astronomy; radio and radar astronomy; ultraviolet astronomy; X-ray astronomy.

Learn more about astronomy with a free trial on Britannica.com.