/euh stron"euh mee/, n.
the science that deals with the material universe beyond the earth's atmosphere.
[1175-1225; ME astronomie ( < AF) < L astronomia < Gk. See ASTRO-, -NOMY]

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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). See also big bang; cosmology; gamma-ray astronomy; infrared astronomy; radio and radar astronomy; ultraviolet astronomy; X-ray astronomy.
(as used in expressions)
gamma ray astronomy
X ray astronomy

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▪ 1995

      For astronomy 1994 was a particularly exciting year as astronomers and the general public thrilled to one of the most dramatic solar system encounters in memory, the crash of Comet Shoemaker-Levy 9 into the atmosphere of the giant planet Jupiter. (See Sidebar (Comet Shoemaker-Levy 9: A Spectacular Good-bye ).) Sharp new images of a variety of astronomical objects were taken by the repaired Hubble Space Telescope (HST). The National Aeronautics and Space Administration's Extreme Ultraviolet Explorer (EUVE) satellite, launched in 1992, began making substantial contributions; with its sensitivity to the ultraviolet radiation normally absorbed by Earth's atmosphere, it, too, produced many new views of the cosmos. Japan's ASCA X-ray satellite kept unique observations of the sky pouring in at X-ray wavelengths. Astronomers had a field day using several large Earth-based telescopes (such as the Keck telescope in Hawaii) to provide fresh insights into objects ranging from the nearest asteroids to the most distant quasars.

Solar System.
      Without doubt the most exciting event in astronomy was the impact of Comet Shoemaker-Levy 9 with Jupiter, but studies of other small bodies in the solar system provided their own delights and surprises. Although the solar system is traditionally viewed as comprising the Sun, nine planets, their moons, and the asteroid belt between Mars and Jupiter, the discovery in the past few years of increasing numbers of small cometary or asteroid-like objects beyond the orbit of the planet Neptune was beginning to change that picture. In 1994 Jane X. Luu of Stanford University and David Jewitt of the University of Hawaii reported several more such trans-Neptunian bodies. The 17 objects found as of the end of 1994 orbit the Sun with periods of about 300 years, compared with the planet Pluto's 248-year orbital period. According to Brian Marsden of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., several of these distant objects, like Pluto, are locked into a so-called 3:2 resonance with the much more massive planet Neptune, meaning that they revolve twice about the Sun in stable orbits for each three revolutions of Neptune.

      The Galileo spacecraft, launched in October 1989, continued to beam images to Earth of a variety of solar system objects as it moved closer to its rendezvous with Jupiter. Unfortunately, because its main radio antenna was not working, data had to be relayed to Earth very slowly through a smaller secondary antenna. Nonetheless, by the start of 1994 Galileo had already sent back a number of spectacular observations. In 1991, as the spacecraft passed near the asteroid Gaspra, it snapped the first close-up picture of an asteroid. Two years later it obtained a spectacular image of a second asteroid, 243 Ida, revealing it to be a heavily cratered, elongated body about 52 km across (1 km is about 0.62 mi). Then in early 1994 Galileo sent back an image that showed the presence of another asteroid, only about 1.5 km across, within 100 km of Ida. The chances that two asteroids would be this close together yet independent of each other were estimated to be less than one in a trillion. Therefore, the scientists from the Jet Propulsion Laboratory, Pasadena, Calif., who reported the observation concluded that Ida has a moon of its own, the first known asteroid-moon pair. The small moon, named Dactyl, has about a dozen craters more than 50 m (165 ft) in diameter, implying that it is at least 100 million years old but not as old as the solar system, since it would have been obliterated by repeated hits in less than a billion years. This information suggested that both Ida and Dactyl originated from a much larger asteroid, which itself broke up into a collection of pieces called the Koronis asteroid family.

      In February 1987 observers on Earth witnessed the explosion of a star in the nearby Large Magellanic Cloud galaxy—the brightest supernova seen in more than three centuries. As Supernova 1987A became dimmer, astronomers detected an encircling ring of glowing gas about a light-year in radius. It was believed that the ring was composed of gas that had been ejected previously by the dying massive progenitor star and that was then stimulated to emit visible light by radiation from the supernova explosion. In 1994 Christopher Burrows of the Space Telescope Science Institute (STScI), Baltimore, Md., reported that sharp HST images showed two additional rings several light-years in diameter that appeared to intersect the central ring, producing a double-hoop pattern. The large rings were thought to lie in front of and behind the central ring, forming an hourglasslike arrangement in which the hoops outlined the end caps of the hourglass and the central ring outlined the neck. The new rings had not been predicted and were unique in all of astronomy. Scientists offered several possible explanations for the giant hoops. The most intriguing one involved the illumination of interstellar material by a neutron-star or black-hole remnant of the initial explosion. Such an object might emit fast-moving particle beams or jets that could hit the surrounding gas and cause it to glow.

      The first well-established example of an extrasolar planetary system was reported during the year. Several years earlier astronomers had described two separate instances of a pulsar with one or more planets possibly in orbit around it. One of those reports proved erroneous, leaving the other also open to question. In 1994 Alexander Wolszczan of Pennsylvania State University presented data that confirmed the earlier evidence for at least two planets, and perhaps more, around the pulsar PSR B1257+12. A pulsar is a rapidly rotating neutron star whose spin period, as reflected in its pulse period, is normally extremely regular. The pulse period of PSR B1257+12, however, was observed to increase and decrease periodically above and below its average pulse period of 6.2 milliseconds. The variation was interpreted as due to motion of the pulsar toward and away from the Earth as one or more planet-sized objects orbit the pulsar, gravitationally tugging it to and fro. By measuring the increase and decrease in the pulsar arrival times, Wolszczan showed that at least two planets, each about three times the mass of the Earth, are revolving around the pulsar with orbital periods of roughly 67 and 98 days.

Galaxies and Cosmology.
      The Milky Way Galaxy, in which the solar system resides, was known to be surrounded by at least 10 small satellite galaxies. The nearest had been thought to be the Large Magellanic Cloud, which lies about 150,000 light-years from the Sun. During the year Rodrigo A. Ibata and Gerry Gilmore of the University of Cambridge and Mike Irwin of the Royal Greenwich Observatory, Hailsham, England, discovered a dwarf spheroidal galaxy only about 50,000 light-years from the Sun. The faint galaxy, which lies in the direction of the constellation Sagittarius, had remained undetected because of obscuration caused by stars and dust lying in the disk of the Milky Way. By starting with an image of the region under study and digitally subtracting the light from known foreground stars, the researchers were left with an image of the dwarf galaxy. It probably contains no more than 50 million stars, compared with some 200 billion for the Milky Way. From its elongated appearance, scientists speculated that the "Sagittarius dwarf" is destined to fall into the Milky Way within the next few hundred million years.

      In an independent search for previously undetected galaxies, a Dutch group used a radio telescope to penetrate the Milky Way's obscuring disk of gas and dust. Using the Dwingeloo radio telescope in The Netherlands, Renee Kraan-Korteweg of the University of Groningen and collaborators from The Netherlands, the U.K., and the U.S. reported finding a spiral galaxy some 10 million light-years away. It is thus about five times farther than Andromeda, or M31, the nearest large galaxy. From its apparent size and rotational velocity, the galaxy was estimated to have about a quarter of the mass of the Milky Way.

      New evidence was reported for a massive black hole at the heart of the giant elliptical galaxy M87. The galaxy is close to the Milky Way by cosmic standards, located about 50 million light-years away in the constellation Virgo, making it one of the best active galaxies for detailed study. Images of the centre of M87 captured by the repaired HST showed what seemed to be a tilted disk of hot, ionized gas only about 60 light-years in diameter. The study team, headed by Richard J. Harms of Applied Research Corp., Landover, Md., and Holland C. Ford of STScI, determined the velocity of the gas to be about 500 km per second. If the gas is orbiting a central object, the mass of the object must be about three billion times the mass of the Sun. Because the deduced mass occupies such a small region, it is possible that the central object is a massive black hole. The HST also obtained clear images of the bright jet that emanates from the centre of M87. This feature was thought to be radiation from a beam of electrons accelerated to nearly the speed of light as a result of processes occurring in or near the disk of material spiraling into the purported black hole. Many astronomers believed that the observational evidence, although still circumstantial, provided the best argument to date for the existence of black holes.

      In some sense the study of cosmology is a search for two numbers: the age of the universe and its mass density. The first number is sought by means of attempts to determine the distances to certain types of stellar objects located in moderately distant galaxies. This can be done if one knows the absolute brightness, or luminosity, of these classes of stellar objects from their study within the Milky Way or relatively nearby galaxies. By finding what are believed to be the same types of objects in other galaxies and measuring their luminosities, astronomers can calculate galactic distances. Because galaxies appear to be receding from one another at velocities that vary with their distance from the point from which they are observed, by correlating the distances to galaxies with their measured velocities, astronomers can derive a relation, called the Hubble law, for determining the current rate of expansion of the universe. The resulting number, called Hubble's constant (H0), then can be used to find the age of the universe. Actually, the age also depends on the mass density of the universe, which is not well known, so a range of ages results in which the value being sought is somewhere between 2/3 and 1 times the reciprocal of Hubble's constant (1/H0).

      In 1994 the controversy over the age of the universe gained new force. A group from the Harvard-Smithsonian Center for Astrophysics, headed by Robert Kirshner, reported an age for the universe of 9 billion to 14 billion years. Their work depended on calibrations of the brightness of exploding stars called type II supernovas. A group headed by Michael J. Pierce of Indiana University, along with five Canadian colleagues, used different types of stellar objects, Cepheid variable stars, to determine the distance to the Virgo cluster of galaxies. Their study led to an age estimate for the universe of 7 billion to 11 billion years. Finally, a group of astronomers using the HST and headed by Wendy Freedman of the Carnegie Observatories of California reported its findings for the distance to the galaxy M100, also using studies of Cepheid variable stars. The age of the universe according to their calculations was 8 billion to 12 billion years.

       astronomyAll this consistency may sound like good news; scientists at last know the age of the universe. Unfortunately, nearly half a century of studies of stars indicates that the oldest stars in the Milky Way are at least 16 billion years old. Therefore, (1) the recent determinations of the Hubble constant are in error, (2) the ages of the oldest stars are wrong, or (3) current cosmological models of the expanding universe need revision. Which of those options is correct was not known. Some astronomers, such as Alan Sandage of the Observatories of the Carnegie Institution of Washington, D.C., continued to report a Hubble constant (based on observations of type I supernovas) and an age of the universe consistent with that of the oldest stars. Given all the uncertainties involved in trying to determine the Hubble constant, at year's end the standard picture of an expanding universe still provided a satisfactory description of the history and age of the universe.(For information on eclipses and other standard astronomical events due to take place in 1995, see Table (

      See also Space Exploration .

      This updates the articles The Cosmos; galaxy (Milky Way Galaxy); astronomy solar system; star.

▪ 1994

      For astronomy 1993 was a year of discovery but also one of bitter disappointment. The U.S. Mars Observer spacecraft, eagerly anticipated for its ability to make the first close-up observations of Mars in 17 years, suddenly fell silent on August 21, three days before it was to go into orbit around the planet. The Hubble Space Telescope (HST) produced many new optical images of astronomical objects but was plagued by problems with pointing, power, and a flawed primary mirror. At year's end space shuttle astronauts successfully completed the most elaborate repair mission in the history of the U.S. space program to fix the telescope, although the results of their work would take weeks to evaluate. On the positive side, observations from several spacecraft provided insights into a variety of phenomena, and to cap the year two American astronomers from Princeton University, Russell Hulse and Joseph Taylor (see Nobel Prizes ), were awarded the Nobel Prize for Physics for their discovery and subsequent study of a binary pulsar, a rapidly spinning neutron star in orbit with another star around a common centre of gravity.

Solar System.
      Although the solar system is dominated by the Sun and major planets, some of the more exciting revelations of 1993 involved comets and asteroids. In March a spectacular comet was discovered by Carolyn and Eugene Shoemaker of the U.S. Geological Survey, Flagstaff, Ariz., and David H. Levy of the University of Arizona. The most unusual feature about Comet Shoemaker-Levy 9 was that it looked like a string of glowing pearls. An HST photograph revealed about 20 cometary chunks spread out in a line. Calculations suggested that the comet's nucleus broke up after a near collision with the giant planet Jupiter in July 1992 and predicted that the pieces would plunge into Jupiter's atmosphere about July 20, 1994, unleashing an energy equivalent to roughly 100 million megatons of TNT.

      In 1991, as the Galileo spacecraft passed near the asteroid Gaspra en route to Jupiter, it snapped the first close-up picture of an asteroid. In August 1993 Galileo passed and imaged a second asteroid, 243 Ida. An elongated object about 52 km (32 mi) across, Ida is heavily cratered, suggesting it is at least a billion years old. While passing Ida, Galileo's onboard magnetometer detected shifts in the direction of the magnetic field of the local solar wind. Since the solar wind consists of electrically charged particles blowing away from the Sun and dragging the magnetic field along with it, the measurements suggested that Ida possesses its own magnetic field, which distorts the solar wind field.

      Where do comets come from? For years astronomers have postulated a comet storehouse beyond the orbit of Pluto. According to theory, objects lying in this so-called Kuiper belt would occasionally be perturbed by encounters with nearby stars, thereby hurtling fresh comets into the inner solar system. In 1992 David Jewitt of the University of Hawaii and Jane X. Luu of the University of California at Berkeley discovered an object, designated 1992 QB1, that seemed to be part of this belt. In early 1993 the two astronomers reported a second body, dubbed 1993 FW, lying at what may be the belt's inner edge. By October four more objects had been spotted, although these appeared to lie somewhat closer in, just outside Neptune's orbit. It may be that the latter objects are comets that have left the belt and are moving inward toward the Sun; alternately, they may be asteroids having permanent residence near Neptune.

      In the early 1970s the first gamma-ray observatory satellite, SAS-2, detected a bright gamma-ray source with no obvious optical counterpart. Its discoverers called the object Geminga (Milanese Italian dialect for "it's not there"), but its nature remained a mystery until 1992 when detection of periodic X-ray and gamma-ray emission suggested that Geminga is a pulsar. In February, Italian astronomer Giovanni Bignami and co-workers reported that they had measured the proper motion of the object, from which they concluded that Geminga was the nearest pulsar to Earth detected to date. Their observations also supported the optical identification of Geminga with a very dim (25th-magnitude) star. Because of its rather young age of about 350,000 years and its close distance of about 300 light-years, astronomers speculated that Geminga may have had an effect on Earth when the pulsar formed in a supernova explosion. The solar system lies in a hot, rarefied region of interstellar space called the Local Bubble. The supernova that produced Geminga may have heated and thinned out matter in Earth's local region to form the bubble.

      The enigmatic events called gamma-ray bursts were also first detected in the early 1970s. Unlike most astronomical phenomena, these bursts, which last a few seconds or less, have never been associated with any known type of object. Nonetheless, it was widely hypothesized that the events are somehow produced by neutron stars in the Milky Way. During the year the Burst and Transient Source Experiment (BATSE) aboard the Earth-orbiting Compton Gamma Ray Observatory (GRO) steadily detected such events, with more than 700 bursts reported by late 1993. From GRO data it appeared that there are really two classes of burst: those lasting only tenths of a second and those lasting tens or hundreds of seconds. One event on January 31 (dubbed the Super Bowl burst for its coincidence with the football event) was, while it lasted, 100 times brighter than Geminga. Although there was still no definitive identification of any gamma-ray burst with a star, quasar, or other known object, the distribution of the events over the sky is telling. Because their arrival directions are spread evenly over the sky, unlike the distribution of stars in the galaxy in which the Earth is immersed, gamma-ray bursts seem likely to come from outside the Milky Way.

Galaxies and Cosmology.
      Whether the observed expansion of the universe may someday stop, to be followed by a collapse, depends on the mass density of the universe. With a sufficiently high density the "closed" universe has enough gravitational pull to overcome the expansion. But the amount of matter seen in the form of visible stars, gas, and galaxies is insufficient to close the universe. Nonetheless, many astronomers believe that the universe is closed and have been searching for the so-called dark matter that would confirm their belief. The year saw its share of proposed "sightings" of dark matter. Early on came the announcement of the detection of dark matter in a nearby group of galaxies called the NGC 2300 group. The result was derived from the detection of X-rays from this galaxy cluster by the Röntgensatellit (ROSAT) orbiting observatory. What ROSAT saw was an X-ray glow from the region around NGC 2300, presumably emitted by hot gas filling the local intergalactic space. The ROSAT team concluded that to hold the detected hot gas within the cluster, more mass than is present in the visible galaxies is required. The team reported that if the inferred dark matter also exists in other similar groups of galaxies, it would provide enough mass to close the universe.

      Several groups reported the detection of MACHOs (massive compact halo objects) lying within the outer reaches of the Milky Way. Astronomers believe that a halo consisting mainly of dark matter surrounds the Milky Way. It was proposed that if the halo consists of numerous small starlike objects, each too dim to be seen directly, their presence could be detected indirectly by their effects on the light from more distant visible stars. According to Einstein's general theory of relativity, a mass will act as a lens and bend light that passes through its gravitational field. Thus, light from a more distant star would brighten and dim if a dim foreground MACHO were to pass in front of it. In 1993 a U.S.-Australian team reported detecting the predicted telltale stellar light variations. After monitoring roughly two million stars in the nearby Large Magellanic Cloud Galaxy, the team found a star that became brighter and then dimmer over a period of about a month. By year's end three more reports of such MACHO events had appeared. The nature of the unseen objects remained elusive, although candidates included brown dwarfs, red dwarf stars, and white dwarf stars. Even though the amount of matter represented by the reported MACHOs, if extrapolated to other galaxies, was insufficient to close the universe, the observational technique did open a new channel for detecting dark matter in the universe. (See Physics .) (KENNETH BRECHER)

      See also Space Exploration .

      This updates the articles Cosmos; galaxy (Milky Way Galaxy); astronomy; solar system; star.

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 science that encompasses the study of all extraterrestrial objects and phenomena. Until the invention of the telescope and the discovery of the laws of motion and gravity in the 17th century, astronomy was primarily concerned with noting and predicting the positions of the Sun, Moon, and planets, originally for calendrical and astrological purposes and later for navigational uses and scientific interest. The catalog of objects now studied is much broader and includes, in order of increasing distance, the solar system, the stars that make up the Milky Way Galaxy, and other, more distant galaxies. With the advent of scientific space probes, Earth also has come to be studied as one of the planets, though its more detailed investigation remains the domain of the geologic sciences.

The scope of astronomy
      Since the late 19th century astronomy has expanded to include astrophysics, the application of physical and chemical knowledge to an understanding of the nature of celestial objects and the physical processes that control their formation, evolution, and emission of radiation. In addition, the gases and dust particles around and between the stars have become the subjects of much research. Study of the nuclear reactions that provide the energy radiated by stars has shown how the diversity of atoms found in nature can be derived from a universe that, following the first few minutes of its existence, consisted only of hydrogen, helium, and a trace of lithium. Concerned with phenomena on the largest scale is cosmology, the study of the evolution of the universe. Astrophysics has transformed cosmology from a purely speculative activity to a modern science capable of predictions that can be tested.

      Its great advances notwithstanding, astronomy is still subject to a major constraint: it is inherently an observational rather than an experimental science. Almost all measurements must be performed at great distances from the objects of interest, with no control over such quantities as their temperature, pressure, or chemical composition. There are a few exceptions to this limitation—namely, meteorites, rock and soil samples brought back from the Moon, samples of comet dust returned by robotic spacecraft, and interplanetary dust particles collected in or above the stratosphere. These can be examined with laboratory techniques to provide information that cannot be obtained in any other way. In the future, space missions may return surface materials from Mars, asteroids, or other objects, but much of astronomy appears otherwise confined to Earth-based observations augmented by observations from orbiting satellites and long-range space probes and supplemented by theory.

Determining astronomical distances
      A central undertaking in astronomy is the determination of distances. Without a knowledge of its distance, the size of an observed object in space would remain nothing more than an angular diameter, and the brightness of a star could not be converted into its true radiated power, or luminosity. Astronomical distance measurement began with a knowledge of Earth's diameter, which provided a base for triangulation. Within the inner solar system, some distances can now be better determined through the timing of radar reflections or, in the case of the Moon, through laser ranging. For the outer planets, triangulation is still used. Beyond the solar system, distances to the closest stars (star) are determined through triangulation, with the diameter of Earth's orbit serving as the baseline and shifts in stellar parallax being the measured quantities. Stellar distances are commonly expressed by astronomers in parsecs (pc), kiloparsecs, or megaparsecs. (1 pc = 3.086 × 1018 cm, or about 3.26 light-years [1.92 × 1013 miles].) Distances can be measured out to around a kiloparsec by trigonometric parallax (see star: Determining stellar distances (star)). The accuracy of measurements made from Earth's surface is limited by atmospheric effects, but measurements made from the Hipparcos satellite (Hipparcos) in the 1990s have extended the scale to stars as far as 650 parsecs, with an accuracy of about a thousandth of an arc second. Less-direct measurements must be used for more-distant stars and for galaxies.

      Two general methods for determining galactic (galaxy) distances are described here. In the first, a clearly identifiable type of star is used as a reference standard because its luminosity has been well determined. This requires observation of such stars that are close enough to Earth that their distances and luminosities have been reliably measured. Such a star is termed a “standard candle.” Examples are Cepheid variables, whose brightness varies periodically in well-documented ways, and certain types of supernova explosions that have enormous brilliance and can thus be seen out to very great distances. Once the luminosities of such nearer standard candles have been calibrated, the distance to a farther standard candle can be calculated from its calibrated luminosity and its actual measured intensity. (The measured intensity [I] is related to the luminosity [L] and distance [d] by the formula I = L/4πd2). A standard candle can be identified by means of its spectrum or the pattern of regular variations in brightness. (Corrections may have to be made for the absorption of starlight by interstellar gas and dust over great distances.) This method forms the basis of measurements of distances to the closest galaxies.

      The second method for galactic distance measurements makes use of the observation that the distances to galaxies generally correlate with the speeds with which those galaxies are receding from Earth (as determined from the Doppler shift in the wavelengths of their emitted light (see redshift (red shift)). This correlation is expressed in the Hubble law: velocity = H × distance, in which H denotes Hubble's constant, which must be determined from observations of the rate at which the galaxies are receding. The value of H has been the subject of intense dispute and is still not resolved to the satisfaction of all parties. There is widespread agreement that H lies between 50 and 100 kilometres per second per megaparsec (km/sec/Mpc), with leading research groups offering estimates that have an average value of about 71 km/sec/Mpc. H has been used to determine distances to remote galaxies in which standard candles have not been found. Application of the Hubble law, however, has been questioned as it relates to some quasars (energetic nuclei of galaxies). (For additional discussion of the recession of galaxies, the Hubble law, and galactic distance determination, see physical science: Astronomy (physical science) and cosmos: The extragalactic distance scale and Hubble's constant (Cosmos).)

Study of the solar system
 The solar system took shape 4.57 billion years ago, when it condensed within a large cloud of gas and dust. Gravitational attraction holds the planets in their elliptical orbits around the Sun. In addition to Earth, five major planets ( Mercury, Venus, Mars, Jupiter, and Saturn) have been known from ancient times. Since then only two more have been discovered: Uranus by accident in 1781 and Neptune in 1846 after a deliberate search following a theoretical prediction based on observed irregularities in the orbit of Uranus. Pluto, discovered in 1930 after a search for a planet predicted to lie beyond Neptune, was considered a major planet until 2006, when it was redesignated a dwarf planet by the International Astronomical Union.

      The average Earth-Sun distance, which originally defined the astronomical unit (AU), provides a convenient measure for distances within the solar system. The astronomical unit is now defined dynamically (using Kepler's third law; see Kepler's laws of planetary motion) and has the value 1.49597870691 × 1013 cm (about 93 million miles), with an uncertainty of about 2,000 cm. The mean radius of Earth's orbit is 1 + (3.1 × 10−8) AU. Mercury, at 0.4 AU, is the closest planet to the Sun, while Neptune, at 30.1 AU, is the farthest. Pluto's orbit, with a mean radius of 39.5, is sufficiently eccentric that at times it is closer to the Sun than is Neptune. The planes of the planetary orbits are all within a few degrees of the ecliptic, the plane that contains Earth's orbit around the Sun. As viewed from far above Earth's North Pole, all planets move in the same (counterclockwise) direction in their orbits.

      All of the planets apart from the two closest to the Sun (Mercury and Venus) have natural satellites (moons) that are very diverse in appearance, size, and structure, as revealed in close-up observations from long-range space probes. Pluto has at least three moons, including one fully half the size of Pluto itself. Four planets—Jupiter, Saturn, Uranus, and Neptune—have rings, disklike systems of small rocks and particles that orbit their parent planets.

      Most of the mass of the solar system is concentrated in the Sun, with its 1.99 × 1033 grams. Together, all of the planets amount to 2.7 × 1030 grams (i.e., about one-thousandth of the Sun's mass), with Jupiter alone accounting for 71 percent of this amount. The solar system also contains a few known objects of intermediate size classified as dwarf planets and a very large number of much smaller objects collectively called small bodies (small body). The small bodies, roughly in order of decreasing size, are the asteroids (asteroid), or minor planets; comets (comet), including Kuiper belt and Oort cloud objects; meteoroids (see meteor and meteoroid); and interplanetary dust particles (interplanetary dust particle). Because of their starlike appearance when discovered, the largest of these bodies were termed asteroids, and that name is widely used, but, now that the rocky nature of these bodies is understood, their more descriptive name is minor planets.

      The four inner, terrestrial planets—Mercury, Venus, Earth, and Mars—along with the Moon have average densities in the range of 3.9–5.5 grams per cubic cm, setting them apart from the four outer, giant planets—Jupiter, Saturn, Uranus, and Neptune—whose densities are all close to 1 gram per cubic cm, the density of water. The compositions of these two groups of planets must therefore be significantly different. This dissimilarity is thought to be attributable to conditions that prevailed during the early development of the solar system (see below Theories of origin (astronomy)). Planetary temperatures now range from around 170 °C (330 °F, 440 K) on Mercury's surface through the typical 15 °C (60 °F, 290 K) on Earth to −135 °C (−210 °F, 140 K) on Jupiter near its cloud tops and down to −210 °C (−350 °F, 60 K) near Neptune's cloud tops. These are average temperatures; large variations exist between dayside and nightside for planets closest to the Sun, except for Venus with its thick atmosphere.

      The surfaces of the terrestrial planets and many satellites show extensive cratering, produced by high-speed impacts (see meteorite crater). On Earth, with its large quantities of water and an active atmosphere, many of these cosmic footprints have eroded, but remnants of very large craters can be seen in aerial and spacecraft photographs of the terrestrial surface. On Mercury, Mars, and the Moon, the absence of water and any significant atmosphere has left the craters unchanged for billions of years, apart from disturbances produced by infrequent later impacts. Volcanic activity has been an important force in the shaping of the surfaces of the Moon and the terrestrial planets. Seismic activity on the Moon has been monitored by means of seismometers left on its surface by Apollo astronauts and by Lunokhod robotic rovers. Cratering on the largest scale seems to have ceased about three billion years ago, although on the Moon there is clear evidence for a continued cosmic drizzle of small particles, with the larger objects churning (“gardening”) the lunar surface and the smallest producing microscopic impact pits in crystals in the lunar rocks.

Lunar exploration
 During the U.S. Apollo (Apollo program) missions a total weight of 381.7 kg (841.5 pounds) of lunar material was collected; an additional 300 grams (0.66 pounds) was brought back by unmanned Soviet Luna vehicles. About 15 percent of the Apollo samples have been distributed for analysis, with the remainder stored at the NASA Johnson Space Center, Houston, Texas. The opportunity to employ a wide range of laboratory techniques on these lunar samples has revolutionized planetary science. The results of the analyses have enabled investigators to determine the composition and age of the lunar surface. Seismic observations have made it possible to probe the lunar interior. In addition, retroreflectors left on the Moon's surface by Apollo astronauts have allowed high-power laser beams to be sent from Earth to the Moon and back, permitting scientists to monitor the Earth-Moon distance to an accuracy of a few centimetres. This experiment, which has provided data used in calculations of the dynamics of the Earth-Moon system, has shown that the separation of the two bodies is increasing by 4.4 cm (1.7 inches) each year. (For additional information on lunar studies, see moon.)

Planetary studies
  Mercury is too hot to retain an atmosphere, but Venus's brilliant white appearance is the result of its being completely enveloped in thick clouds of carbon dioxide, impenetrable at visible wavelengths. Below the upper clouds, Venus has a hostile atmosphere containing clouds of sulfuric acid droplets. The cloud cover shields the planet's surface from direct sunlight, but the energy that does filter through warms the surface, which then radiates at infrared wavelengths. The long-wavelength infrared radiation is trapped by the dense clouds such that an efficient greenhouse effect keeps the surface temperature near 465 °C (870 °F, 740 K). Radar, which can penetrate the thick Venusian clouds, has been used to map the planet's surface. In contrast, the atmosphere of Mars is very thin and is composed mostly of carbon dioxide (95 percent), with very little water vapour; the planet's surface pressure is only about 0.006 that of Earth. The outer planets have atmospheres composed largely of light gases, mainly hydrogen and helium.

      Each planet rotates on its axis, and nearly all of them rotate in the same direction—counterclockwise as viewed from above the ecliptic. The two exceptions are Venus, which rotates in the clockwise direction beneath its cloud cover, and Uranus, which has its rotation axis very nearly in the plane of the ecliptic.

      Some of the planets have magnetic fields (magnetic field). Earth's field extends outward until it is disturbed by the solar wind—an outward flow of protons and electrons from the Sun—which carries a magnetic field along with it. Through processes not yet fully understood, particles from the solar wind and galactic cosmic rays (high-speed particles from outside the solar system) populate two doughnut-shaped regions called the Van Allen radiation belts. The inner belt extends from about 1,000 to 5,000 km (600 to 3,000 miles) above Earth's surface, and the outer from roughly 15,000 to 25,000 km (9,300 to 15,500 miles). In these belts, trapped particles spiral along paths that take them around Earth while bouncing back and forth between the Northern and Southern hemispheres, with their orbits controlled by Earth's magnetic field. During periods of increased solar activity, these regions of trapped particles are disturbed, and some of the particles move down into Earth's atmosphere, where they collide with atoms and molecules to produce auroras.

       Jupiter has a magnetic field far stronger than Earth's and many more trapped electrons, whose synchrotron radiation (electromagnetic radiation emitted by high-speed charged particles that are forced to move in curved paths, as under the influence of a magnetic field) is detectable from Earth. Bursts of increased radio emission are correlated with the position of Io, the innermost of the four Galilean moons of Jupiter. Saturn has a magnetic field that is much weaker than Jupiter's, but it too has a region of trapped particles. Mercury has a weak magnetic field that is only about 1 percent as strong as Earth's and shows no evidence of trapped particles. Uranus and Neptune have fields that are less than one-tenth the strength of Saturn's and appear much more complex than that of Earth. No field has been detected around Venus or Mars.

Investigations of the smaller bodies
 More than 125,000 asteroids with well-established orbits are known, and several hundred additional objects are discovered each year. Hundreds of thousands more have been seen, but their orbits have not been as well-determined. It is estimated that several million asteroids exist, but most are small, and their combined mass is estimated to be less than a thousandth that of Earth. Most of the asteroids have orbits close to the ecliptic and move in the asteroid belt, between 2.3 and 3.3 AU from the Sun. Because some asteroids travel in orbits that can bring them close to Earth, there is a possibility of a collision that could have devastating results (see Earth impact hazard).

       comets are considered to come from a vast reservoir, the Oort cloud, orbiting the Sun at distances of 20,000–50,000 AU or more and containing trillions of icy objects—latent comet nuclei—with the potential to become active comets. Many comets have been observed over the centuries. Most make only a single pass through the inner solar system, but some are deflected by Jupiter or Saturn into orbits that allow them to return at predictable times. Halley's Comet is the best-known of these periodic comets, with its next return into the inner solar system predicted for AD 2061. Many short-period comets are thought to come from the Kuiper belt, a region lying mainly between 30 AU and 50 AU from the Sun—beyond Neptune's orbit but including part of Pluto's—and housing perhaps hundreds of millions of comet nuclei. Comet masses have not been well determined, but most are probably less than 1018 grams, one billionth the mass of Earth.

      Since the 1990s hundreds of comet nuclei in the Kuiper belt have been observed with large telescopes; a few are about half the size of Pluto, and at least one, Eris, is estimated to be slightly larger. Pluto's orbital and physical characteristics had long caused it to be regarded as an anomaly among the planets, and, after the discovery of numerous other Pluto-like objects beyond Neptune, Pluto was seen to be no longer unique in its “neighbourhood” but rather a giant member of the local population. Consequently, in 2006 astronomers at the general assembly of the International Astronomical Union elected to create the new category of dwarf planets for objects with such qualifications. Pluto, Eris, and Ceres, the latter being the largest member of the asteroid belt, were given this distinction.

      Smaller than the observed asteroids and comets are the meteoroids (see meteor and meteoroid), lumps of stony or metallic material believed to be mostly fragments of asteroids and comets. Meteoroids vary from small rocks to boulders weighing a ton or more. A relative few have orbits that bring them into Earth's atmosphere and down to the surface as meteorites. Most if not all meteorites that have been collected on Earth are probably from asteroids.

      Meteorites are classified into three broad groups: stony (chondrites and achondrites; about 94 percent), iron (5 percent), and stony-iron (1 percent). Most meteoroids that enter the atmosphere heat up sufficiently to glow and appear as meteors (see meteor and meteoroid), and the great majority of these vaporize completely or break up before they reach the surface. Many, perhaps most, meteors (meteor and meteoroid) occur in showers (see meteor shower) and follow orbits that seem to be identical with those of certain comets, thus pointing to a cometary origin. For example, each May, when Earth crosses the orbit of Halley's Comet, the Eta Aquarid meteor shower occurs. Micrometeorites ( interplanetary dust particles), the smallest meteoroidal particles, can be detected from Earth-orbiting satellites or collected by specially equipped aircraft flying in the stratosphere and returned for laboratory inspection. Since the late 1960s numerous meteorites have been found in the Antarctic on the surface of stranded ice flows (see Antarctic meteorites). Detailed analyses have shown that some of these meteorites have come from the Moon and others from Mars. Yet others contain microscopic crystals whose isotopic proportions are unique and appear to be dust grains that formed in the atmospheres of different stars.

Determinations of age and chemical composition
      The age of the solar system, taken to be close to 4.6 billion years, has been derived from measurements of radioactivity in meteorites, lunar samples, and Earth's crust. Abundances of isotopes of uranium, thorium, and rubidium and their decay products, lead and strontium, are the measured quantities.

      Assessment of the chemical composition of the solar system is based on data from Earth, the Moon, and meteorites as well as on the spectral analysis of light from the Sun and planets. In broad outline, the solar system abundances of the chemical elements decrease with increasing atomic weight. Hydrogen atoms are by far the most abundant, constituting 91 percent; helium is next, with 8.9 percent; and all other types of atoms together amount to only 0.1 percent.

Theories of origin
      The origin of Earth, the Moon, and the solar system as a whole is a problem that has not yet been settled in detail. The Sun probably formed by condensation of the central region of a large cloud of gas and dust, with the planets and other bodies of the solar system forming soon after, their composition strongly influenced by the temperature and pressure gradients in the evolving solar nebula. Less-volatile materials could condense into solids relatively close to the Sun to form the terrestrial planets. The abundant, volatile lighter elements could condense only at much greater distances to form the giant gas planets. After the early 1990s astronomers confirmed that stars other than the Sun have one or more planetlike objects revolving around them. Studies of the properties of these solar systems have both supported and challenged astronomers' theoretical models of how Earth's solar system formed. (See also solar system: Origin of the solar system (solar system).)

      The origin of the planetary satellites is not entirely settled. As to the origin of the Moon, the opinion of astronomers had long oscillated between theories that saw its origin and condensation simultaneous with formation of Earth and those that posited a separate origin for the Moon and its later capture by Earth's gravitational field. Similarities and differences in abundances of the chemical elements and their isotopes on Earth and Moon had challenged each group of theories. Finally, in the 1980s a model emerged that has gained the support of most lunar scientists—that of a large impact on Earth with the expulsion of material that subsequently formed the Moon. (See Moon: Origin and evolution (Moon).) For the outer planets with their multiple satellites, many very small and quite unlike one another, the picture is less clear. Some of these moons have relatively smooth icy surfaces, whereas others are heavily cratered; at least one, Jupiter's Io, is volcanic. Some of the moons may have formed along with their parent planets, and others may have formed elsewhere and been captured. (For additional discussion of the solar system and its components, see cosmos: Planetary systems (Cosmos).)

Study of the stars (star)
Measuring observable stellar properties
 The measurable quantities in stellar astrophysics include the externally observable features of the stars: distance, temperature, radiation spectrum and luminosity, composition (of the outer layers), diameter, mass, and variability in any of these. Theoretical astrophysicists use these observations to model the structure of stars and to devise theories for their formation and evolution. Positional information can be used for dynamical analysis, which yields estimates of stellar masses.

      In a system dating back at least to the Greek astronomer-mathematician Hipparchus in the 2nd century BC, apparent stellar brightness (m) is measured in magnitudes. Magnitudes are now defined such that a first-magnitude star is 100 times brighter than a star of sixth magnitude. The human eye cannot see stars fainter than about sixth magnitude, but modern instruments used with large telescopes can record stars as faint as about 30th magnitude. By convention, the absolute magnitude (M) is defined as the magnitude that a star would appear to have if it were located at a standard distance of 10 parsecs. These quantities are related through the expression m − M = 5 log10 r − 5, in which r is the star's distance in parsecs.

      The magnitude scale is anchored on a group of standard stars. An absolute measure of radiant power is luminosity, usually expressed in ergs per second (ergs/sec). (Sometimes the luminosity is stated in terms of the solar luminosity, 3.86 × 1033 ergs/sec.) Luminosity can be calculated when m and r are known. Correction might be necessary for the interstellar absorption of starlight.

      There are several methods for measuring a star's diameter. From the brightness and distance the luminosity (L) can be calculated, and from observations of the brightness at different wavelengths the temperature (T) can be calculated. Because the radiation from many stars can be well approximated by a Planck blackbody spectrum (see Planck's radiation law), these measured quantities can be related through the expression L = 4πR2σT4, thus providing a means of calculating R, the star's radius. In this expression, σ is the Stefan-Boltzmann constant, 5.67 × 10−5 ergs/cm2K4sec, in which K is the temperature in kelvins. (The radius R refers to the star's photosphere, the region where the star becomes effectively opaque to outside observation.) Stellar angular diameters can be measured through interference effects. Alternatively, the intensity of the starlight can be monitored during occultation by the Moon, which produces diffraction fringes whose pattern depends on the angular diameter of the star. Stellar angular diameters of several milliarcseconds can be measured, but so far only for relatively bright and close stars.

      Many stars occur in binary systems (binary star) (see binary star), with the two partners in orbits around their mutual centre of mass. Such a system provides the best measurement of stellar masses. The period (P) of a binary system is related to the masses of the two stars (m1 and m2) and the orbital semimajor axis (mean radius; a) via Kepler's third law: P2 = 4π2a3/G(m1 + m2). (G is the universal gravitational constant.) From diameters and masses, average values of the stellar density can be calculated and thence the central pressure. With the assumption of an equation of state (state, equation of), the central temperature can then be calculated. For example, in the Sun the central density is 158 grams per cubic cm; the pressure is calculated to be more than one billion times the pressure of Earth's atmosphere at sea level and the temperature around 15 million K (27 million °F). At this temperature, all atoms are ionized, and so the solar interior consists of a plasma, an ionized gas with hydrogen nuclei (i.e., protons), helium nuclei, and electrons as major constituents. A small fraction of the hydrogen nuclei possess sufficiently high speeds that, on colliding, their electrostatic repulsion is overcome, resulting in the formation, by means of a set of fusion reactions, of helium nuclei and a release of energy (see proton-proton cycle). Some of this energy is carried away by neutrinos, but most of it is carried by photons to the surface of the Sun to maintain its luminosity.

      Other stars, both more and less massive than the Sun, have broadly similar structures, but the size, central pressure and temperature, and fusion rate are functions of the star's mass and composition. The stars and their internal fusion (and resulting luminosity) are held stable against collapse through a delicate balance between the inward pressure produced by gravitational attraction and the outward pressure supplied by the photons produced in the fusion reactions.

      Stars that are in this condition of hydrostatic equilibrium are termed main-sequence stars, and they occupy a well-defined band on the Hertzsprung-Russell (H-R) diagram (Hertzsprung–Russell diagram), in which luminosity is plotted against colour index or temperature. Spectral classification, based initially on the colour index, includes the major spectral types O, B, A, F, G, K and M, each subdivided into 10 parts (see star: Stellar spectra (star)). Temperature is deduced from broadband spectral measurements in several standard wavelength intervals. Measurement of apparent magnitudes in two spectral regions, the B and V bands (centred on 4350 and 5550 angstroms, respectively), permits calculation of the colour index, CI = mB − mV, from which the temperature can be calculated.

      For a given temperature, there are stars that are much more luminous than main-sequence stars. Given the dependence of luminosity on the square of the radius and the fourth power of the temperature (R2T4 of the luminosity expression above), greater luminosity implies larger radius, and such stars are termed giant stars or supergiant stars. Conversely, stars with luminosities much less than those of main-sequence stars of the same temperature must be smaller and are termed white dwarf stars. Surface temperatures of white dwarfs typically range from 10,000 to 12,000 K (18,000 to 21,000 °F), and they appear visually as white or blue-white.

      The strength of spectral lines of the more abundant elements in a star's atmosphere allows additional subdivisions within a class. Thus, the Sun, a main-sequence star, is classified as G2 V, in which the V denotes main sequence. Betelgeuse, a red giant with a surface temperature about half that of the Sun but with a luminosity of about 10,000 solar units, is classified as M2 Iab. In this classification, the spectral type is M2, and the Iab indicates a giant, well above the main sequence on the H-R diagram.

Star formation and evolution
 The range of physically allowable masses for stars is very narrow. If the star's mass is too small, the central temperature will be too low to sustain fusion reactions. The theoretical minimum stellar mass is about 0.08 solar mass. An upper theoretical limit of approximately 100 solar masses has been suggested, but this value is not firmly defined. Stars as massive as this will have luminosities about one million times greater than that of the Sun.

      A general model of star formation and evolution has been developed, and the major features seem to be established. A large cloud of gas and dust can contract under its own gravitational attraction if its temperature is sufficiently low. As gravitational energy is released, the contracting central material heats up until a point is reached at which the outward radiation pressure balances the inward gravitational pressure, and contraction ceases. Fusion reactions take over as the star's primary source of energy, and the star is then on the main sequence. The time to pass through these formative stages and onto the main sequence is less than 100 million years for a star with as much mass as the Sun. It takes longer for less massive stars and a much shorter time for those much more massive.

      Once a star has reached its main-sequence stage, it evolves relatively slowly, fusing hydrogen nuclei in its core to form helium nuclei. Continued fusion not only releases the energy that is radiated but also results in nucleosynthesis, the production of heavier nuclei.

      Stellar evolution has of necessity been followed through computer modeling because the timescales for most stages are generally too extended for measurable changes to be observed, even over a period of many years. One exception is the supernova, the violently explosive finale of certain stars. Different types of supernovas can be distinguished by their spectral lines and by changes in luminosity during and after the outburst. In Type Ia, a white dwarf star attracts matter from its nearby companion; when the white dwarf's mass exceeds about 1.4 solar masses, the star implodes and is completely destroyed. Type II supernovas are not as luminous as Type Ia and are the final evolutionary stage of stars more massive than about eight solar masses.

      The nature of the final products of stellar evolution depend on stellar mass. Some stars pass through an unstable stage in which their dimensions, temperature, and luminosity change cyclically over periods of hours or days. These so-called Cepheid variables (Cepheid variable) serve as standard candles for distance measurements (see above Determining astronomical distances (astronomy)). Some stars blow off their outer layers to produce planetary nebulas (planetary nebula). The expanding material can be seen glowing in a thin shell as it disperses into the interstellar medium, while the remnant core, initially with a surface temperature as high as 100,000 K (180,000 °F), cools to become a white dwarf. The maximum stellar mass that can exist as a white dwarf is about 1.4 solar masses and is known as the Chandrasekhar limit. More-massive stars may end up as either neutron stars or black holes.

      The average density of a white dwarf is calculated to exceed one million grams per cubic cm. Further compression is limited by a quantum condition called degeneracy (see degenerate gas), in which only certain energies are allowed for the electrons in the star's interior. Under sufficiently great pressure, the electrons are forced to combine with protons to form neutrons. The resulting neutron star will have a density in the range of 1014–1015 grams per cubic cm, comparable to the density within atomic nuclei. The behaviour of large masses having nuclear densities is not yet sufficiently understood to be able to set a limit on the maximum size of a neutron star, but it is thought to be in the region of three solar masses.

      Still more-massive remnants of stellar evolution would have smaller dimensions and would be even denser that neutron stars. Such remnants are conceived to be black holes (black hole), objects so compact that no radiation can escape from within a characteristic distance called the Schwarzschild radius (see gravitational radius (Schwarzschild radius)). This critical dimension is defined by Rs = 2GM/c2. (Rs is the Schwarzschild radius, G is the gravitational constant, M is the object's mass, and c is the speed of light.) For an object of three solar masses, the Schwarzschild radius would be about three kilometres. Radiation emitted from beyond the Schwarzschild radius can still escape and be detected.

      Although no light can be detected coming from within a black hole, the presence of a black hole may be manifested through the effects of its gravitational field, as, for example, in a binary star system. If a black hole is paired with a normal visible star, it may pull matter from its companion toward itself. This matter is accelerated as it approaches the black hole and becomes so intensely heated that it radiates large amounts of X-rays from the periphery of the black hole before reaching the Schwarzschild radius. A few candidates for stellar black holes have been found—e.g., the X-ray source Cygnus X-1. Each of them has an estimated mass clearly exceeding that allowable for a neutron star, a factor crucial in the identification of possible black holes. (Supermassive black holes that do not originate as individual stars are thought to exist at the centres of active galaxies; see below Study of other galaxies and related phenomena (astronomy).)

      Whereas the existence of stellar black holes has been strongly indicated, the existence of neutron stars was confirmed in 1968 when they were identified with the then newly discovered pulsars, objects characterized by the emission of radiation at short and extremely regular intervals, generally between 1 and 1,000 pulses per second and stable to better than a part per billion. Pulsars are considered to be rotating neutron stars, remnants of some supernovas. (For additional discussion of stars and stellar evolution, see cosmos: Stars and the chemical elements (Cosmos).)

Study of the Milky Way Galaxy
      Stars are not distributed randomly throughout space. Many stars are in systems consisting of two or three members separated by less than 1,000 AU. On a larger scale, star clusters may contain many thousands of stars. Galaxies (galaxy) are much larger systems of stars and usually include clouds of gas and dust.

      The solar system is located within the Milky Way Galaxy, close to its equatorial plane and about 8.7 kiloparsecs from the galactic centre. The galactic diameter is about 30 kiloparsecs, as indicated by luminous matter. There is evidence, however, for nonluminous matter—so-called dark matter (see cosmos: Dark matter (Cosmos))—extending out nearly twice this distance. The entire system is rotating such that, at the position of the Sun, the orbital speed is about 220 km per second (almost 500,000 miles per hour) and a complete circuit takes roughly 240 million years. Application of Kepler's third law leads to an estimate for the galactic mass of about 100 billion solar masses. The rotational velocity can be measured from the Doppler shifts (see Doppler effect) observed in the 21-cm emission line of neutral hydrogen and the lines of millimetre wavelengths from various molecules, especially carbon monoxide. At great distances from the galactic centre, the rotational velocity does not drop off as expected but rather increases slightly. This behaviour appears to require a much larger galactic mass than can be accounted for by the known (luminous) matter. Additional evidence for the presence of dark matter comes from a variety of other observations. The nature and extent of the dark matter (or missing mass) constitutes one of today's major astronomical puzzles.

      There are about 100 billion stars in the Milky Way Galaxy. Star concentrations within the galaxy fall into three types: open clusters (star cluster), globular clusters, and associations (see star cluster). open clusters lie primarily in the disk of the galaxy; most contain between 50 and 1,000 stars within a region no more than 10 parsecs in diameter. stellar associations tend to have somewhat fewer stars; moreover, the constituent stars are not as closely grouped as those in the clusters and are for the most part hotter. globular clusters, which are widely scattered around the galaxy, may extend up to about 100 parsecs in diameter and may have as many as a million stars. The importance to astronomers of globular clusters lies in their use as indicators of the age of the galaxy. Because massive stars evolve more rapidly than do smaller stars, the age of a cluster can be estimated from its H-R diagram. In a young cluster the main sequence will be well-populated, but in an old cluster the heavier stars will have evolved away from the main sequence. The extent of the depopulation of the main sequence provides an index of age. In this way, the oldest globular clusters have been found to be about 14 billion ± 1 billion years old, which should therefore be the minimum age for the galaxy.

Investigations of interstellar matter
      The interstellar medium, composed primarily of gas and dust, occupies the regions between the stars. On average, it contains less than one atom in each cubic centimetre, with about 1 percent of its mass in the form of minute dust grains. The gas, mostly hydrogen, has been mapped by means of its 21-cm emission line. The gas also contains numerous molecules. Some of these have been detected by the visible-wavelength absorption lines that they impose on the spectra of more-distant stars, while others have been identified by their own emission lines at millimetre wavelengths. Many of the interstellar molecules are found in giant molecular clouds, wherein complex organic (organic compound) molecules have been discovered.

      In the vicinity of a very hot O- or B-type star, the intensity of ultraviolet radiation is sufficiently high to ionize the surrounding hydrogen out to a distance as great as 100 parsecs to produce an H II region, known as a Strömgren sphere. Such regions are strong and characteristic emitters of radiation at radio wavelengths, and their dimensions are well calibrated in terms of the luminosity of the central star. Using radio interferometers, astronomers are able to measure the angular diameters of H II regions even in some external galaxies and can thereby deduce the great distances to those remote systems. This method can be used for distances up to about 30 megaparsecs. (For additional information on H II regions, see nebula: Diffuse nebulae (H II regions) (nebula).)

      Interstellar dust grains (interplanetary dust particle) (see nebula: Interstellar dust (nebula)) scatter and absorb starlight, with the effect being roughly inversely proportional to wavelength from the infrared to the near ultraviolet. As a result, stellar spectra tend to be reddened. Absorption amounts typically to about one magnitude per kiloparsec but varies considerably in different directions. Some dusty regions contain silicate materials, identified by a broad absorption feature around a wavelength of 10 μm. Other prominent spectral features in the infrared range have been sometimes, but not conclusively, attributed to graphite grains and polycyclic aromatic hydrocarbons.

      Starlight often shows a small degree of polarization (a few percent), with the effect increasing with stellar distance. This is attributed to the scattering of the starlight from dust grains that have been partially aligned in a weak interstellar magnetic field. The strength of this field is estimated to be a few microgauss, very close to the strength inferred from observations of nonthermal cosmic radio noise. This radio background has been identified as synchrotron radiation, emitted by cosmic-ray electrons traveling at nearly the speed of light and moving along curved paths in the interstellar magnetic field. The spectrum of the cosmic radio noise is close to what is calculated on the basis of measurements of the cosmic rays (cosmic ray) near Earth.

       cosmic rays constitute another component of the interstellar medium. Cosmic rays that are detected in the vicinity of Earth comprise high-speed nuclei and electrons. Individual particle energies, expressed in electron volts (eV; 1 eV = 1.6 × 10−12 erg), range with decreasing numbers from about 106 eV to more than 1020 eV. Among the nuclei, hydrogen nuclei are the most plentiful at 86 percent, helium nuclei next at 13 percent, and all other nuclei together at about 1 percent. Electrons are about 2 percent as abundant as the nuclear component. (The relative numbers of different nuclei vary somewhat with kinetic energy, while the electron proportion is strongly energy-dependent.)

      A minority of cosmic rays detected in Earth's vicinity are produced in the Sun, especially at times of increased solar activity (as indicated by sunspots and solar flares). The origin of galactic cosmic rays has not yet been conclusively identified, but they are thought to be produced in stellar processes such as supernova explosions, perhaps with additional acceleration occurring in the interstellar regions. (For additional information on interstellar matter, see Milky Way Galaxy: The general interstellar medium (Milky Way Galaxy) and cosmos: Interstellar clouds (Cosmos); for additional information on cosmic rays and galactic nonthermal radio emission, see cosmos: Cosmic rays and magnetic fields (Cosmos).)

Observations of the galactic centre
      The central region of the Milky Way Galaxy is so heavily obscured by dust that direct observation has become possible only with the development of astronomy at nonvisual wavelengths—namely, radio, infrared, and, more recently, X-ray and gamma-ray wavelengths. Together, these observations have revealed a nuclear region of intense activity, with a large number of separate sources of emission and a great deal of dust. Detection of gamma-ray emission at a line energy of 511,000 eV, which corresponds to the annihilation of electrons and positrons (the antimatter counterpart of electrons), along with radio mapping of a region no more than 20 AU across, points to a very compact and energetic source, designated Sagittarius A*, at the centre of the galaxy (see Sagittarius A). Whether this source is powered by a supermassive black hole or some very close and hot stars remains to be determined. (For additional information on the Milky Way Galaxy, see cosmos: The Milky Way Galaxy (Cosmos).)

Study of other galaxies and related phenomena
 Galaxies (galaxy) are normally classified into three principal types according to their appearance: spiral, elliptical, and irregular. Galactic diameters are typically in the tens of kiloparsecs and the distances between galaxies typically in megaparsecs.

      Spiral galaxies—of which the Milky Way system is a characteristic example—tend to be flattened, roughly circular systems with their constituent stars strongly concentrated along spiral arms. These arms are thought to be produced by traveling density waves, which compress and expand the galactic material. (For an explanation of the density wave theory, see cosmos: Dynamics of ellipticals and spirals (Cosmos).) Between the spiral arms exists a diffuse interstellar medium of gas and dust, mostly at very low temperatures (below 100 K [−280 °F, −170 °C]). Spiral galaxies are typically a few kiloparsecs in thickness; they have a central bulge and taper gradually toward the outer edges.

      Ellipticals show none of the spiral features but are more densely packed stellar systems. They range in shape from nearly spherical to very flattened and contain little interstellar matter. Irregular galaxies number only a few percent of all stellar systems and exhibit none of the regular features associated with spirals or ellipticals.

      Properties vary considerably among the different types of galaxies. Spirals typically have masses in the range of a billion to a trillion solar masses, with ellipticals having values from 10 times smaller to 10 times larger and the irregulars generally 10–100 times smaller. Visual galactic luminosities show similar spreads among the three types, but the irregulars tend to be less luminous. In contrast, at radio wavelengths the maximum luminosity for spirals is usually 100,000 times less than for ellipticals or irregulars.

       quasars are objects whose spectra display very large redshifts, thus implying (in accordance with the Hubble law) that they lie at the greatest distances (see above Determining astronomical distances (astronomy)). They were discovered in 1963 but remained enigmatic for many years. They appear as starlike (i.e., very compact) sources of radio waves—hence their initial designation as quasi-stellar radio sources, a term later shortened to quasars. They are now considered to be the exceedingly luminous cores of distant galaxies. These energetic cores, which emit copious quantities of X-rays and gamma rays, are termed active galactic nuclei and include the object Cygnus A and the nuclei of a class of galaxies called Seyfert galaxies (Seyfert galaxy). They may be powered by the infall of matter into supermassive black holes (see cosmos: Black-hole model for active galactic nuclei (Cosmos)).

      The Milky Way Galaxy is one of the Local Group of galaxies, which contains more than three dozen members and extends over a volume about one megaparsec in diameter. Two of the closest members are the Magellanic Clouds, irregular galaxies about 50 kiloparsecs away. At about 740 kiloparsecs the Andromeda Galaxy is one of the most distant in the Local Group. Some members of the group are moving toward the Milky Way system, while others are traveling away from it. At greater distances all galaxies are moving away from the Milky Way Galaxy. Their speeds (as determined from the redshifted wavelengths in their spectra) are generally proportional to their distances. The Hubble law relates these two quantities (see above Determining astronomical distances (astronomy)). In the absence of any other method, the Hubble law continues to be used for distance determinations to the farthest objects—that is, galaxies and quasars for which redshifts can be measured. (For additional information on external galaxies, see cosmos: Galaxies (Cosmos).)

       cosmology is the scientific study of the universe (Cosmos) as a unified whole, from its earliest moments through its evolution to its ultimate fate. The currently accepted cosmological model is the big bang (big-bang model). In this picture, the expansion of the universe started in an intense explosion about 10–20 billion years ago. In this primordial fireball, the temperature exceeded one trillion K, and most of the energy was in the form of radiation. As the expansion proceeded (accompanied by cooling), the role of the radiation diminished, and other physical processes dominated in turn. Thus, after about three minutes, the temperature had dropped to the one-billion-K range, making it possible for nuclear reactions of protons to take place and produce nuclei of deuterium and helium. (At the higher temperatures that prevailed earlier, these nuclei would have been promptly disrupted by high-energy photons.) With further expansion, the time between nuclear collisions had increased and the proportion of deuterium and helium nuclei had stabilized. After a few hundred thousand years, the temperature must have dropped sufficiently for electrons to remain attached to nuclei to constitute atoms. Galaxies are thought to have begun forming after a few million years, but this stage is very poorly understood. Star formation probably started much later, after at least a billion years, and the process continues today.

      Observational support for this general model comes from several independent directions. The expansion has been documented by the redshifts observed in the spectra of galaxies. Furthermore, the radiation left over from the original fireball would have cooled with the expansion. Confirmation of this relic energy came in 1965 with one of the most striking cosmic discoveries of the 20th century—the observation, at short radio wavelengths, of a widespread cosmic radiation corresponding to a temperature of almost 3 K (about −454 °F or −270 °C). The shape of the observed spectrum is an excellent fit to the theoretical Planck blackbody spectrum (see Planck's radiation law). (The present best value for this temperature is 2.73 K, but it is still called three-degree radiation or the cosmic microwave background; see cosmos: Microwave background radiation (Cosmos).) The spectrum of this cosmic radio noise peaks at approximately one-millimetre wavelength, which is in the far infrared, a difficult region to observe; however, the spectrum has been well mapped at many wavelengths from that point through the radio region. Additional support for the big bang theory comes from the observed cosmic abundances of deuterium and helium. Normal stellar nucleosynthesis cannot produce their measured quantities, which fit well with calculations of production during the early stages of the big bang.

      Surveys of the cosmic background radiation have indicated that it is extremely uniform in all directions (isotropic). Calculations have shown that it is difficult to achieve this degree of isotropy unless there was a very early and rapid inflationary period before the expansion settled into its present mode. Nevertheless, the isotropy poses problems for models of galaxy formation. It has been conjectured that galaxies originate from turbulent conditions that produce local fluctuations of density, toward which more matter would then be gravitationally attracted. Such density variations are difficult to reconcile with the isotropy required by observations of the 3 K radiation. This problem of galaxy formation has produced theories that seem successful in some dimensions, less so in others. There is as yet no model that is accepted in totality.

      The very earliest stages of the big bang are less well understood. The conditions of temperature and pressure that prevailed prior to the first microsecond require the introduction of theoretical ideas of subatomic particle physics. Subatomic particles are usually studied in laboratories with giant accelerators, but the region of particle energies of potential significance to the question at hand lies beyond the range of accelerators currently available. Fortunately, some important conclusions can be drawn from the observed cosmic helium abundance, which is dependent on conditions in the early big bang. The observed helium abundance sets a limit on the number of families of certain types of subatomic particles that can exist.

      The age of the universe can be calculated in several ways. Assuming the validity of the big bang model, one attempts to answer the question: How long has the universe been expanding in order to have reached its present size? The numbers relevant to calculating an answer are Hubble's constant (i.e., the current expansion rate), the density of matter in the universe, and the cosmological constant, which allows for change in the expansion rate. In 2003 a calculation based on a fresh determination of Hubble's constant yielded an age of 13.7 billion ± 200 million years, although the precise value depends on certain assumed details of the model used. Independent estimates of stellar ages have yielded values less than this, as would be expected, but other estimates, based on supernova distance measurements, have arrived at values of about 15 billion years, still consistent, within the errors. In the big bang model the age is proportional to the reciprocal of Hubble's constant, hence the importance of determining H as reliably as possible. For example, a value for H of 100 km/sec/Mpc would lead to an age less than that of many stars, a physically unacceptable result.

      A small minority of astronomers have developed alternative cosmological theories that are seriously pursued. The overwhelming professional opinion, however, continues to support the big bang model.

      Finally, there is the question of the future behaviour of the universe: Is it open? That is to say, will the expansion continue indefinitely? Or is it closed, such that the expansion will slow down and eventually reverse, resulting in contraction? (The final collapse of such a contracting universe is sometimes termed the “big crunch.”) So-called dark energy, a kind of repulsive force that is now believed to be a major component of the universe, appears to be the decisive factor in predictions of the long-term fate of the cosmos. If this energy is a cosmological constant (as proposed in 1917 by Albert Einstein (Einstein, Albert) to correct certain problems in his model of the universe), then the result would be a “big chill.” In this scenario, the universe would continue to expand, but its density would decrease. While old stars would burn out, new stars would no longer form. The universe would become cold and dark. The scenario changes, however, if the dark energy is not a cosmological constant. Accelerated expansion could end, and a big crunch would remain a possibility. A very speculative possibility is that the dark energy would cause a runaway acceleration of the expansion of the universe, ending in a “big rip.” The dark (nonluminous) matter component of the universe, whose composition remains unknown, is not considered sufficient to close the universe and cause it to collapse; it now appears to contribute only a fourth of the density needed for closure.

      An additional factor in deciding the fate of the universe might be the mass of neutrinos. For decades the neutrino had been postulated to have zero mass, although there was no compelling theoretical reason for this to be so. From the observation of neutrinos generated in the Sun and other celestial sources such as supernovas, in cosmic-ray interactions with Earth's atmosphere, and in particle accelerators, investigators have concluded that neutrinos have some mass, though only an extremely small fraction of the mass of an electron. Although there are vast numbers of neutrinos in the universe, the sum of such small neutrino masses appears insufficient to close the universe. (For additional discussion of the big bang theory, alternative cosmologies, and the age of the universe and its ultimate fate, see cosmos: Cosmological models (Cosmos).)

The techniques of astronomy
      Astronomical observations involve a sequence of stages, each of which may impose constraints on the type of information attainable. Radiant energy is collected with telescopes and brought to a focus on a detector, which is calibrated so that its sensitivity and spectral response are known. Accurate pointing and timing are required to permit the correlation of observations made with different instrument systems working in different wavelength intervals and located at places far apart. The radiation must be spectrally analyzed so that the processes responsible for radiation emission can be identified.

Telescopic observations
      Before Galileo Galilei (Galileo)'s use of telescopes (telescope) for astronomy in 1609, all observations were made by naked eye, with corresponding limits on the faintness and degree of detail that could be seen. Since that time, telescopes have become central to astronomy. Having apertures much larger than the pupil of the human eye, telescopes permit the study of faint and distant objects. In addition, sufficient radiant energy can be collected in short time intervals to permit rapid fluctuations in intensity to be detected. Further, with more energy collected, a spectrum can be greatly dispersed and examined in much greater detail.

      Optical telescopes are either refractors or reflectors that use lenses or mirrors, respectively, for their main light-collecting elements (objectives). Refractors are effectively limited to apertures of about 100 cm (approximately 40 inches) or less because of problems inherent in the use of large glass lenses. These distort under their own weight and can be supported only around the perimeter; an appreciable amount of light is lost due to absorption in the glass. Large-aperture refractors are very long and require large and expensive domes. The largest modern telescopes are all reflectors, the very largest composed of many segmented components and having overall diameters of about 10 metres (33 feet). Reflectors are not subject to the chromatic problems of refractors, can be better supported mechanically, and can be housed in smaller domes because they are more compact than the long-tube refractors.

      The angular resolving power (or resolution) of a telescope is the smallest angle between close objects that can be seen clearly to be separate. Resolution is limited by the wave nature of light. For a telescope having an objective lens or mirror with diameter D and operating at wavelength λ, the angular resolution (in radians) can be approximately described by the ratio λ/D. Optical telescopes can have very high intrinsic resolving powers; in practice, however, these are not attained for telescopes located on Earth's surface, because atmospheric effects limit the practical resolution to about one arc second. Sophisticated computing programs can allow much-improved resolution, and the performance of telescopes on Earth can be improved through the use of adaptive optics, in which the surface of the mirror is adjusted rapidly to compensate for atmospheric turbulence that would otherwise distort the image. In addition, image data from several telescopes focused on the same object can be merged optically and through computer processing to produce images having angular resolutions much greater than that from any single component.

      The atmosphere does not transmit radiation of all wavelengths equally well. This restricts astronomy on Earth's surface to the near ultraviolet, visible, and radio regions of the electromagnetic spectrum, with some relatively narrow “windows” in the nearer infrared. Longer infrared (infrared astronomy) wavelengths are strongly absorbed by atmospheric water vapour and carbon dioxide. Atmospheric effects can be reduced by careful site selection and by carrying out observations at high altitudes. Most major optical observatories (astronomical observatory) are located on high mountains, well away from cities and their reflected lights. Infrared telescopes have been located atop Mauna Kea (see Mauna Kea Observatory) in Hawaii and in the Canary Islands where atmospheric humidity is very low. Airborne telescopes designed mainly for infrared observations—such as (until 1995) on the Kuiper Airborne Observatory, a jet aircraft fitted with astronomical instruments—operate at an altitude of about 12 km (40,000 feet) with flight durations limited to a few hours. Telescopes for infrared, X-ray, and gamma-ray observations have been carried to altitudes of more than 30 km (100,000 feet) by balloons. Higher altitudes can attained during short-duration rocket flights for ultraviolet observations. Telescopes for all wavelengths from infrared to gamma rays have been carried by robotic spacecraft observatories such as the Hubble Space Telescope and the Wilkinson Microwave Anisotropy Probe, while cosmic rays have been studied from space by the Advanced Composition Explorer.

      Angular resolution better than one milliarcsecond has been achieved at radio wavelengths by the use of several radio telescopes in an array. In such an arrangement, the effective aperture then becomes the greatest distance between component telescopes. For example, in the Very Large Array (VLA), operated near Socorro, N.M., by the National Radio Astronomy Observatory, 27 movable radio dishes are set out along tracks that extend for nearly 21 km. In another technique, called very long baseline interferometry (VLBI), simultaneous observations are made with radio telescopes thousands of kilometres apart; this technique requires very precise timing.

      Earth is a moving platform for astronomical observations. It is important that the specification of precise celestial coordinates be made in ways that correct for telescope location, the position of Earth in its orbit around the Sun, and the epoch of observation, since Earth's axis of rotation moves slowly over the years. Time measurements are now based on atomic clocks rather than on Earth's rotation, and telescopes can be driven continuously to compensate for the planet's rotation, so as to permit tracking of a given astronomical object.

Use of radiation detectors
      Although the human eye remains an important astronomical tool, detectors capable of greater sensitivity and more rapid response are needed to observe at visible wavelengths and, especially, to extend observations beyond that region of the electromagnetic spectrum. Photography has been used in astronomy since the late 19th century and continues to be an essential tool (see technology of photography: Astronomical photography (photography, technology of)). Long-duration exposures may be needed to reveal faint objects. This integrative property of photography, however, smooths out rapid variations in radiation intensity; to study these variations, electronic methods must be used. Photography also provides an archival record. A photograph of a particular celestial object may include the images of many other objects that were not of interest when the picture was taken but that become the focus of study years later. When quasars were discovered in 1963, for example, photographic plates exposed before 1900 and held in the Harvard College Observatory were examined to trace possible changes in position or intensity of the radio object newly identified as quasar 3C 273. Also, major photographic surveys, such as those of the National Geographic Society and Palomar Observatory, can provide a historic base for long-term studies.

      Photographic film converts only a few percent of the incident photons into images, whereas efficiencies of better than 80 percent can be achieved by any of several electronic methods of detection. The greater sensitivity and intrinsically rapid response of such methods are exploited for tracking exceedingly rapid variations in intensity. For example, pulsars that emit their radiation at millisecond intervals can be followed and their pulse shapes monitored. The arrival of individual photons can be recorded with photomultiplier tubes or with more advanced and sensitive detectors, such as charge-coupled devices (CCDs). Special photographic materials can be employed for the shortest infrared wavelengths, but semiconductor detectors that operate at very low (cryogenic) temperatures are used for wavelengths longer than a few micrometres. In detectors of this kind, the absorbed photons produce a minute temperature increase or a change in electrical resistance that is recorded as a signal; individual photons are not recorded. Reception of radio waves is based on the production of a small voltage in an antenna rather than on photon counting. Individual X-ray and gamma-ray photons possess sufficient energy to be detectable through the ionization that they produce.

      Spectral analysis (spectroscopy) (see spectroscopy) involves measuring the intensity of the radiation as a function of wavelength or frequency. In some detectors, such as those for X-rays and gamma rays, the energy of each photon can be measured directly. Photographic film is sensitive to photons over a wide range of wavelengths. For low-resolution spectroscopy, broadband filters suffice to select wavelength intervals. Greater resolution can be obtained with prisms, gratings, and interferometers. (For additional information on astronomical radiation detectors, see telescope: Advances in auxiliary instrumentation (telescope).)

Solid cosmic samples
      As a departure from the traditional astronomical approach of remote observing, certain more recent lines of research involve the analysis of actual samples under laboratory conditions. These include studies of meteorites, rock samples returned from the Moon, cometary dust samples returned by space probes, and interplanetary dust particles collected by aircraft in the stratosphere or by spacecraft. In all such cases, a wide range of highly sensitive laboratory techniques can be adapted for the often microscopic samples. Chemical analysis can be supplemented with mass spectroscopy (see mass spectrometry), allowing isotopic composition to be determined. Radioactivity and the impacts of cosmic-ray particles can produce minute quantities of gas, which then remain trapped in crystals within the samples. Carefully controlled heating of the crystals (or of dust grains containing the crystals) under laboratory conditions releases this gas, which then is analyzed in a mass spectrometer. X-ray spectrometers, electron microscopes, and microprobes are employed to determine crystal structure and composition, from which temperature and pressure conditions at the time of formation can be inferred.

Theoretical approaches
      Theory (scientific theory) is just as important as observation in astronomy. It is required for the interpretation of observational data; for the construction of models of celestial objects and physical processes, their properties, and their changes over time; and for guiding further observations. Theoretical astrophysics is based on laws of physics that have been validated with great precision through controlled experiments. Application of these laws to specific astrophysical problems, however, may yield equations too complex for direct solution. Two general approaches are then available. In the traditional method, a simplified description of the problem is formulated, incorporating only the major physical components, to provide equations that can be either solved directly or used to create a numerical model that can be evaluated (see numerical analysis). Successively more-complex models can then be investigated. Alternatively, a computer program can be devised that will explore the problem numerically in all its complexity. Computational science is taking its place as a major division alongside theory and experiment. The test of any theory is its ability to incorporate the known facts and to make predictions that can be compared with additional observations.

Impact of astronomy
      No area of science is totally self-contained. Discoveries in one area find applications in others, often unpredictably. Various notable examples of this involve astronomical studies. Newton's laws of motion and gravity (see also celestial mechanics: Newton's laws of motion (celestial mechanics)) emerged from the analysis of planetary and lunar orbits. Observations during the 1919 solar eclipse provided dramatic confirmation of Albert Einstein (Einstein, Albert)'s general theory of relativity, which gained further support with the discovery and tracking of the binary pulsar designated PSR 1913+16. (See relativity: Experimental evidence for general relativity (relativity) and Gravitational waves (relativity).) The behaviour of nuclear matter and of some elementary particles is now better understood as a result of measurements of neutron stars and the cosmological helium abundance, respectively. Study of the theory of synchrotron radiation was greatly stimulated by the detection of polarized visible radiation emitted by high-energy electrons in the supernova remnant known as the Crab Nebula. Dedicated particle accelerators are now being used to produce synchrotron radiation to probe the structure of solid materials and make detailed X-ray images of tiny samples, including biological structures (see spectroscopy: Synchrotron sources (spectroscopy)).

      Astronomical knowledge also has had a broad impact beyond science. The earliest calendars were based on astronomical observations of the cycles of repeated solar and lunar positions. Also, for centuries, familiarity with the positions and apparent motions of the stars through the seasons enabled sea voyagers to navigate with moderate accuracy. Perhaps the single greatest effect that astronomical studies have had on our modern society has been in molding its perceptions and opinions. Our conceptions of the cosmos and our place in it, our perceptions of space and time, and the development of the systematic pursuit of knowledge known as the scientific method have been profoundly influenced by astronomical observations. In addition, the power of science to provide the basis for accurate predictions of such phenomena as eclipses and the positions of the planets and later, so dramatically, of comets has shaped an attitude toward science that remains an important social force today.

Michael Wulf Friedlander

Additional Reading

Roger A. Freedman and William J. Kaufman III, Universe, 7th ed. (2005); Michael Zeilik, Astronomy: The Evolving Universe, 9th ed. (2002); and Michael A. Seeds, Foundations of Astronomy, 9th ed. (2007), are introductory texts.

Traditional histories of astronomy include John Lankford (ed.), History of Astronomy: An Encyclopedia (1997); and John North, The Norton History of Astronomy and Cosmology (also published as The Fontana History of Astronomy and Cosmology, 1994). David Leverington, A History of Astronomy from 1890 to the Present (1995), offers a good review of the period cited. Anthony F. Aveni, Empires of Time: Calendars, Clocks, and Cultures, rev. ed. (2002), provides an introduction to archaeoastronomy.

Guides and handbooks
Ian Ridpath (ed.), Norton's Star Atlas and Reference Handbook, Epoch 2000.0, 20th ed. (2004), is a popular atlas with explanatory material. Stephen James O'Meara, The Messier Objects (also published as The Messier Objects Field Guide, 1998), provides a guide to the objects in this famous catalog. Martin Mobberley, The New Amateur Astronomer (2004), is a general guide to telescopes and observing. Listings of observational information are found in U.S. Naval Observatory Nautical Almanac Office and Great Britain Nautical Almanac Office, The Astronomical Almanac (annual); and Royal Astronomical Society of Canada, The Observer's Handbook (annual). Kenneth R. Lang, A Companion to Astronomy and Astrophysics: Chronology and Glossary with Date Tables (2006), is a dictionary of technical terms, with names of scientists, values of astronomical quantities, and brief historical notes, and Astrophysical Formulae, 3rd ed. rev. and enlarged, 2 vol. (1999), is a comprehensive reference source with extensive formulas and background data. Arthur N. Cox (ed.), Allen's Astrophysical Quantities, 4th ed. (2000), is a standard reference updated every few years.

Current knowledge
Works dealing with forefront areas of research and directed to the nonspecialist include F.A. Aharonian, Very High Energy Cosmic Gamma Radiation (2004); Paul Halpern and Paul Wesson, Brave New Universe: Illuminating the Darkest Secrets of the Cosmos (2006); D.A. Lorimer and M. Kramer, Handbook of Pulsar Astronomy (2005); James B. Kaler, Extreme Stars: At the Edge of Creation (2001); A.C. Fabian, K.S. Pounds, and R.D. Blandford, Frontiers of X-ray Astronomy (2004); Andreas Eckart, Rainer Schödel, and Christian Straubmeier, The Black Hole at the Center of the Milky Way (2005); and Simon Singh, Big Bang: The Origin of the Universe (2004).

Up-to-date reviews of specialized topics are found in the Annual Review of Astronomy and Astrophysics and Annual Review of Earth and Planetary Sciences. Their first chapters are often devoted to reminiscences by major scientists. Major professional journals include The Astronomical Journal (10/year) and The Astrophysical Journal (3/month), both published for the American Astronomical Society; the Monthly Notices of the Royal Astronomical Society (3/month); and Astronomy and Astrophysics (4/month), managed by the European Southern Observatory for a consortium of European astronomical societies. An excellent publication is Sky and Telescope (monthly), directed to the serious amateur astronomer and still of interest to the professional.Michael Wulf Friedlander

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Universalium. 2010.

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