COSMOS

Hubble inferred a uniformity in the spatial distribution of galaxies through number counts in deep photographic surveys of selected areas of the sky. This inference applies only to scales larger than several times 108 light-years. On smaller scales, galaxies tend to bunch together in clusters and superclusters, and Hubble deliberately avoided the more conspicuous examples in order not to bias his results. This clustering did excite debate among both observers and theorists in the earliest discussions of cosmology, particularly over the largest dimensions where there are still appreciable departures from homogeneity and over the ultimate cause of the departures. In the 1950s and early 1960s, however, attention tended to focus on homogeneous cosmological models because of the competing ideas of the big bang and steady state scenarios. Only after the discovery of the cosmic microwave backgroundwhich, together with the successes of primordial nucleosynthesis, signaled a clear victory for the hot big bang picturedid the issue of departures from homogeneity in the universe again attract widespread interest. From a more pragmatic point of view, clusters and groups of galaxies are important to cosmological studies because they are useful in establishing the extragalactic distance scale. A fundamental problem that recurs over and over again in astronomy is the determination of the distance to an object. Individual stars in star clusters and associations provide an indispensable tool in gauging distances within the Galaxy. The brightest starsin particular the brightest variable stars among the so-called Cepheid classallow the distance ladder to be extended to the nearest galaxies; but at distances much larger than 107 light-years individual stars become too difficult to resolve, at least from the ground, and astronomers have traditionally resorted to other methods. Clustering of galaxies Distant galactic cluster, as observed by the Hubble Space Telescope. This Clusters of galaxies fall into two morphological categories: regular and irregular. The regular clusters show marked spherical symmetry and have a rich membership. Typically, they contain thousands of galaxies, with a high concentration toward the centre of the cluster. Rich clusters, such as the Coma cluster, are deficient in spiral galaxies and are dominated by ellipticals and S0s. The irregular clusters have less well-defined shapes, and they usually have fewer members, ranging from fairly rich systems such as the Hercules cluster to poor groups that may have only a few members. Galaxies of all types can be found in irregular clusters: spirals and irregulars, as well as ellipticals and S0s. Most galaxies are to be found not in rich clusters but in loose groups. The Galaxy belongs to one such loose groupthe Local Group. genus of garden plants of the family Asteraceae, containing about 20 species native to tropical America. They have leaves opposite each other on the stem and heads of flowers that are borne along on long flower stalks or together in an open cluster. The disk flowers are red or yellow; the ray flowers, sometimes notched, may be white, pink, red, purple, or other colours. The common garden cosmos, from which most annual ornamental varieties have been developed, is C. bipinnatus. any of a series of unmanned Soviet satellites launched from the early 1960s. Cosmos satellites were used for a wide variety of purposes, including scientific research, navigation, and military reconnaissance. Cosmos 26 and 49 (both launched in 1964), for example, were equipped to measure the Earth's magnetic field. Others were employed to study certain technical aspects of spaceflight as well as physical phenomena in the Earth's upper atmosphere and in deep space. A number of them such as Cosmos 597, 600, and 602 were apparently used to collect intelligence information on the Yom Kippur War between the Arab states and Israel in October 1973. Some Cosmos spacecraft may have had the ability to intercept satellites launched by other nations. in astronomy, the entire physical universe consisting of all objects and phenomena observed or postulated. If one looks up on a clear night, one sees that the sky is full of stars. During the summer months in the Northern Hemisphere, a faint band of light stretches from horizon to horizon, a swath of pale white cutting across a background of deepest black. For the early Egyptians, this was the heavenly Nile, flowing through the land of the dead ruled by Osiris. The ancient Greeks likened it to a river of milk. Astronomers now know that the band is actually composed of countless stars in a flattened disk seen edge on. The stars are so close to one another along the line of sight that the unaided eye has difficulty discerning the individual members. Through a large telescope, astronomers find myriads of like systems sprinkled throughout the depths of space. They call such vast collections of stars galaxies, after the Greek word for milk, and call the local galaxy to which the Sun belongs the Milky Way Galaxy or simply the Galaxy. Every visible star is a sun in its own right. Ever since this realization first dawned in the collective mind of humanity, it has been speculated that many stars other than the Sun also have planetary systems encircling them. The related issue of the origin of the solar system, too, has always had special fascination for speculative thinkers, and the quest to understand it on a firm scientific basis has continued into the present day. Some stars are intrinsically brighter than the Sun; others, fainter. Much less light is received from the stars than from the Sun because the stars are all much farther away. Indeed, they appear densely packed in the Milky Way only because there are so many of them. The actual separations of the stars are enormous, so large that it is conventional to measure their distances in units of how far light can travel in a given amount of time. The speed of light (in a vacuum) equals 3 1010 cm/sec (centimetres per second); at such a speed, it is possible to circle the Earth seven times in a single second. Thus in terrestrial terms the Sun, which lies 500 light-seconds from the Earth, is very far away; however, even the next closest star, Proxima Centauri, at a distance of 4.3 light-years (4.1 1018 cm), is 270,000 times farther yet. The stars that lie on the opposite side of the Milky Way from the Sun have distances that are on the order of 100,000 light-years, which is the typical diameter of a large spiral galaxy. If the kingdom of the stars seems vast, the realm of the galaxies is larger still. The nearest galaxies to the Milky Way system are the Large and Small Magellanic Clouds, two irregular satellites of the Galaxy visible to the naked eye in the Southern Hemisphere. The Magellanic Clouds are relatively small (containing roughly 109 stars) compared to the Galaxy (with some 1011 stars), and they lie at a distance of about 200,000 light-years. The nearest large galaxy comparable to the Galaxy is the Andromeda galaxy (also called M31 because it was the 31st entry in a catalog of astronomical objects compiled by the French astronomer Charles Messier in 1781), and it lies at a distance of about 2,000,000 light-years. The Magellanic Clouds, the Andromeda galaxy, and the Milky Way system all are part of an aggregation of two dozen or so neighbouring galaxies known as the Local Group. The Galaxy and M31 are the largest members of this group. The Galaxy and M31 are both spiral galaxies, and they are among the brighter and more massive of all spiral galaxies. The most luminous and brightest galaxies, however, are not spirals but rather supergiant ellipticals (also called cD galaxies by astronomers for historical reasons that are not particularly illuminating). Elliptical galaxies have roundish shapes rather than the flattened distributions that characterize spiral galaxies, and they tend to occur in rich clusters (those containing thousands of members) rather than in the loose groups favoured by spirals. The brightest member galaxies of rich clusters have been detected at distances exceeding several thousand million light-years from the Earth. The branch of learning that deals with phenomena at the scale of many millions of light-years is called cosmologya term derived from combining two Greek words, kosmos, meaning order, harmony, and the world, and logos, signifying word or discourse. Cosmology is, in effect, the study of the universe at large. A dramatic new feature, not present on small scales, emerges when the universe is viewed in the largenamely, the cosmological expansion. On cosmological scales, galaxies (or, at least, clusters of galaxies) appear to be racing away from one another with the apparent velocity of recession being linearly proportional to the distance of the object. This relation is known as the Hubble law (after its discoverer, the American astronomer Edwin Powell Hubble). Interpreted in the simplest fashion, the Hubble law implies that roughly 1010 years ago, all of the matter in the universe was closely packed together in an incredibly dense state and that everything then exploded in a big bang, the signature of the explosion being written eventually in the galaxies of stars that formed out of the expanding debris of matter. Strong scientific support for this interpretation of a big bang origin of the universe comes from the detection by radio telescopes of a steady and uniform background of microwave radiation. The cosmic microwave background is believed to be a ghostly remnant of the fierce light of the primeval fireball reduced by cosmic expansion to a shadow of its former splendour but still pervading every corner of the known universe. The simple (and most common) interpretation of the Hubble law as a recession of the galaxies over time through space, however, contains a misleading notion. In a sense, as will be made more precise later in the article, the expansion of the universe represents not so much a fundamental motion of galaxies within a framework of absolute time and absolute space, but an expansion of time and space themselves. On cosmological scales, the use of light-travel times to measure distances assumes a special significance because the lengths become so vast that even light, traveling at the fastest speed attainable by any physical entity, takes a significant fraction of the age of the universe, roughly 1010 years, to travel from an object to an observer. Thus, when astronomers measure objects at cosmological distances from the Local Group, they are seeing the objects as they existed during a time when the universe was much younger than it is today. Under these circumstances, Albert Einstein taught in his theory of general relativity that the gravitational field of everything in the universe so warps space and time as to require a very careful reevaluation of quantities whose seemingly elementary natures are normally taken for granted. The observed expansion of the universe immediately raises the spectre that the universe is evolving, that it had a beginning and will have an end. The steady state alternative, postulated by a British school of cosmologists in 1948, is no longer considered viable by most astronomers. Yet, the notion that the Cosmos had a beginning, while common in many theologies, raises deep and puzzling questions for science, for it implies a creation eventa creation not only of all the mass-energy that now exists in the universe but also perhaps of space-time itself. The issue of how the universe will end seems, at first sight, more amenable to conventional analysis. Because the universe is currently expanding, one may ask whether this expansion will continue into the indefinite future or whether after the passage of some finite time, the expansion will be reversed by the gravitational attraction of all of the matter for itself. The procedure for answering this question seems straightforward: either measure directly the rate of deceleration in the expansion of the galaxies to extrapolate whether they will eventually come to a halt, or measure the total amount of matter in the universe to see if there is enough to supply the gravitation needed to make the universe bound. Unfortunately, astronomers' assaults on both fronts have been stymied by two unforeseen circumstances. First, it is now conceded that earlier attempts to measure the deceleration rate have been affected by evolutionary effects of unknown magnitude in the observed galaxies that invalidate the simple interpretations. Second, it is recognized that within the Cosmos there may be an unknown amount of hidden mass, which cannot be seen by conventional astronomical techniques but which contributes substantially to the gravitation of the universe. The hope is that, somehow, quantum physics will ultimately supply theoretical answers (which can then be tested observationally and experimentally) to each of these difficulties. The ongoing effort in particle physics to find a unified basis for all the elementary forces of nature has yielded promising new ways to think about the most fundamental of all questions regarding astronomical origins; it has offered a tentative prediction concerning the deceleration rate of the universe; and it has offered a plethora of candidates for the hidden mass of the universe. This article traces the development of modern conceptions of the Cosmos and summarizes the prevailing theories of its origin and evolution. Humanity has traveled a long road since self-centred societies imagined the creation of the Earth, the Sun, and the Moon as the main act, with the formation of the rest of the universe as almost an afterthought. Today it is known that the Earth is only a small ball of rock in a Cosmos of unimaginable vastness and that the birth of the solar system was probably only one event among many that occurred against the backdrop of an already mature universe. Yet, as humbling as the lesson has been, it has also unveiled a remarkable fact, one that endows the minutest particle in this universe with a rich and noble heritage. Events hypothesized to have occurred in the first few minutes of the creation of the universe turn out to have had profound influence on the birth, life, and death of galaxies, stars, and planets. Indeed, there is a direct, though tortuous, lineage from the forging of the matter of the universe in a primal furnace of incredible heat and light to the gathering on Earth of atoms versatile enough to serve as a chemical basis of life. The intrinsic harmony of the resultant worldview has great philosophical and aesthetic appeal and perhaps explains the resurgence of public interest in this subject. For detailed information on the structure and evolution of the major components of the Cosmos, see galaxy; star; star cluster; astronomical map; nebula; and solar system. The present article considers only aspects of these topics that satisfy one of three criteria: (1) they bear on the general issue of astronomical origins; (2) they are important to an integrated picture of how the universe evolved; or (3) they play a big role in forming humanity's growing vision of the miraculous unity that is the Cosmos. in astronomy, the entire physical universe consisting of all objects and phenomena observed or postulated. The principal constituents of the universe are the galaxies, stars and stellar groupings, and nebulae (clouds of interstellar gas and dust). Substantially smaller components include the solar system and any other assemblage of planets, satellites, comets, and meteoroids revolving around a central stellar body that may exist among the millions of galaxies. In addition to such objects and scattered matter, the universe contains gravitational fields and various forms of radiation. For a discussion of these and other significant components of the universe, see galaxy; star; star cluster; nebula; solar system; infrared source; radio source; and X-ray source. Throughout the centuries numerous theories have been proposed for the origin and structure of the universe. In ancient Greece, the Milesian school of pre-Socratic thinkers, including Thales, Anaximander, and Anaximenes of the 6th century BC, developed the view that the formation of the world occurred as a natural, rather than supernatural, sequence of events. This view was elaborated by the Pythagorean school of that era, which stressed the concept of an ordered cosmos governed by mathematical relations, and which culminated in the work of Leucippus and Democritus of the Atomist school. The Atomist view is expressed by the Roman poet Lucretius in De rerum natura (On the Nature of Things). He describes a boundless universe in which the interplay of atoms creates endless worlds in various stages of development and decay. These early pictures of the physical world were supplanted by the geocentric, finite cosmologies of Plato, Aristotle, and Ptolemy, which later were embodied in medieval theology. From the 16th century, the Copernican theory that the Sun is the centre of the universe, the meticulous celestial measurements of Tycho Brahe, the mathematical discoveries of Kepler, the observations and arguments of Galileo, and the theories of Newton, in a period of about 200 years, opened minds to the possibility of an apparently infinite universe whose centre has no specific location. The realization that stars may be arranged into a system of island universes, now known to be galaxies, emerged in the middle of the 18th century largely as a result of suggestions by such scientists and philosophers as Emanuel Swedenborg, Thomas Wright, Immanuel Kant, and Johann Heinrich Lambert. Such conjectures were supported by the observations of the English astronomer William Herschel. A qualitative picture of the Milky Way Galaxy as a flattened system of stars and nebulae, isolated in space, arose about 1785; however, it was not until the early 20th century that Harlow Shapley succeeded in assigning reliable dimensions to the system and estimating the location of the Sun. During the early 1920s another astronomer, Edwin Powell Hubble, determined conclusively that galaxies exist beyond the Milky Way system. Furthermore, Hubble's discovery (in 1929) that these external galaxies are apparently receding at speeds increasing with distance and Albert Einstein's general theory of relativity together established modern cosmology. According to the prevailing theoretical position, the universe originated from an explosionthe big bangabout 10,000,000,000 years ago. Immediately after the big bang, the universe consisted primarily of radiation, but as it expanded matter came to dominateroughly 1,000 years after the explosive beginning. Hubble's discovery and other more recent findings seem to suggest that the universe is still expanding and that it will continue to do so indefinitely (see also big-bang model; expanding universe). Early cosmological ideas Immediate issues that arise when anyone contemplates the universe at large are whether space and time are infinite or finite. And after many centuries of thought by some of the best minds, humanity has still not arrived at conclusive answers to these questions. Aristotle's answer was that the material universe must be spatially finite, for if stars extended to infinity, they could not perform a complete rotation around the Earth in 24 hours. Space must then itself also be finite because it is merely a receptacle for material bodies. On the other hand, the heavens must be temporally infinite, without beginning or end, since they are imperishable and cannot be created or destroyed. Except for the infinity of time, these views came to be accepted religious teachings in Europe before the period of modern science. The most notable person to publicly express doubts about restricted space was the Italian philosopher-mathematician Giordano Bruno, who asked the obvious question that, if there is a boundary or edge to space, what is on the other side? For his advocacy of an infinity of suns and earths, he was burned at the stake in 1600. In 1610 Kepler provided a profound reason for believing that the number of stars in the universe had to be finite. If there were an infinity of stars, he argued, then the sky would be completely filled with them and night would not be dark! This point was rediscussed by the astronomers Edmond Halley and Jean-Philippe-Loys de Chseaux of Switzerland in the 18th century, but it was not popularized as a paradox until Heinrich Wilhelm Olbers of Germany took up the problem in the 19th century. The difficulty became potentially very real with Hubble's measurement of the enormous extent of the universe of galaxies with its large-scale homogeneity and isotropy. His discovery of the systematic recession of the galaxies provided an escape, however. At first people thought that the redshift effect alone would suffice to explain why the sky is dark at nightnamely, that the light from the stars in distant galaxies would be redshifted to long wavelengths beyond the visible regime. The modern consensus is, however, that a finite age for the universe is a far more important effect. Even if the universe is spatially infinite, photons from very distant galaxies simply do not have the time to travel to the Earth because of the finite speed of light. There is a spherical surface, the cosmic event horizon (roughly 1010 light-years in radial distance from the Earth at the current epoch), beyond which nothing can be seen even in principle; and the number (roughly 1010) of galaxies within this cosmic horizon, the observable universe, are too few to make the night sky bright. When one looks to great distances, one is seeing things as they were a long time ago, again because light takes a finite time to travel to Earth. Over such great spans, do the classical notions of Euclid concerning the properties of space necessarily continue to hold? The answer given by Einstein was: No, the gravitation of the mass contained in cosmologically large regions may warp one's usual perceptions of space and time; in particular, the Euclidean postulate that parallel lines never cross need not be a correct description of the geometry of the actual universe. And in 1917 Einstein presented a mathematical model of the universe in which the total volume of space was finite yet had no boundary or edge. The model was based on his theory of general relativity that utilized a more generalized approach to geometry devised in the 19th century by the German mathematician Bernhard Riemann. Gravitation and the geometry of space-time The physical foundation of Einstein's view of gravitation, general relativity, lies on two empirical findings that he elevated to the status of basic postulates. The first postulate is the relativity principle: local physics is governed by the theory of special relativity. The second postulate is the equivalence principle: there is no way for an observer to distinguish locally between gravity and acceleration. The motivation for the second postulate comes from Galileo's observation that all objectsindependent of mass, shape, colour, or any other propertyaccelerate at the same rate in a (uniform) gravitational field. Einstein's theory of special relativity, which he developed in 1905, had as its basic premises (1) the notion (also dating back to Galileo) that the laws of physics are the same for all inertial observers and (2) the constancy of the speed of light in a vacuumnamely, that the speed of light has the same value (3 1010 cm/sec) for all inertial observers independent of their motion relative to the source of the light. Clearly, this second premise is incompatible with Euclidean and Newtonian precepts of absolute space and absolute time, resulting in a program that merged space and time into a single structure, with well-known consequences. The space-time structure of special relativity is often called flat because, among other things, the propagation of photons is easily represented on a flat sheet of graph paper with equal-sized squares. Let each tick on the vertical axis represent one light-year (9.46 1017 cm) of distance in the direction of the flight of the photon, and each tick on the horizontal axis represent the passage of one year (3.16 107 sec) of time. The propagation path of the photon is then a 45 line because it flies one light-year in one year (with respect to the space and time measurements of all inertial observers no matter how fast they move relative to the photon). The principle of equivalence in general relativity allows the locally flat space-time structure of special relativity to be warped by gravitation, so that (in the cosmological case) the propagation of the photon over thousands of millions of light-years can no longer be plotted on a globally flat sheet of paper. To be sure, the curvature of the paper may not be apparent when only a small piece is examined, thereby giving the local impression that space-time is flat (i.e., satisfies special relativity). It is only when the graph paper is examined globally that one realizes it is curved (i.e., satisfies general relativity). In Einstein's 1917 model of the universe, the curvature occurs only in space, with the graph paper being rolled up into a cylinder on its side, a loop around the cylinder at constant time having a circumference of 2pRthe total spatial extent of the universe. Notice that the radius of the universe is measured in a direction perpendicular to the space-time surface of the graph paper. Since the ringed space axis corresponds to one of three dimensions of the actual world (any will do since all directions are equivalent in an isotropic model), the radius of the universe exists in a fourth spatial dimension (not time) which is not part of the real world. This fourth spatial dimension is a mathematical artifice introduced to represent diagrammatically the solution (in this case) of equations for curved three-dimensional space that need not refer to any dimensions other than the three physical ones. Photons traveling in a straight line in any physical direction have trajectories that go diagonally (at 45 angles to the space and time axes) from corner to corner of each little square cell of the space-time grid; thus, they describe helical paths on the cylindrical surface of the graph paper, making one turn after traveling a spatial distance 2pR. In other words, always flying dead ahead, photons would return to where they started from after going a finite distance without ever coming to an edge or boundary. The distance to the other side of the universe is therefore pR, and it would lie in any and every direction; space would be closed on itself. Now, except by analogy with the closed two-dimensional surface of a sphere that is uniformly curved toward a centre in a third dimension lying nowhere on the two-dimensional surface, no three-dimensional creature can visualize a closed three-dimensional volume that is uniformly curved toward a centre in a fourth dimension lying nowhere in the three-dimensional volume. Nevertheless, three-dimensional creatures could discover the curvature of their three-dimensional world by performing surveying experiments of sufficient spatial scope. They could draw circles, for example, by tacking down one end of a string and tracing along a single plane the locus described by the other end when the string is always kept taut in between (a straight line) and walked around by a surveyor. In Einstein's universe, if the string were short compared to the quantity R, the circumference of the circle divided by the length of the string (the circle's radius) would nearly equal 2p = 6.2837853 . . . , thereby fooling the three-dimensional creatures into thinking that Euclidean geometry gives a correct description of their world. However, the ratio of circumference to length of string would become less than 2p when the length of string became comparable to R. Indeed, if a string of length pR could be pulled taut to the antipode of a positively curved universe, the ratio would go to zero. In short, at the tacked-down end the string could be seen to sweep out a great arc in the sky from horizon to horizon and back again; yet, to make the string do this, the surveyor at the other end need only walk around a circle of vanishingly small circumference. To understand why gravitation can curve space (or more generally, space-time) in such startling ways, consider the following thought experiment that was originally conceived by Einstein. Imagine an elevator in free space accelerating upward, from the viewpoint of a woman in inertial space, at a rate numerically equal to g, the gravitational field at the surface of the Earth. Let this elevator have parallel windows on two sides, and let the woman shine a brief pulse of light toward the windows. She will see the photons enter close to the top of the near window and exit near the bottom of the far window because the elevator has accelerated upward in the interval it takes light to travel across the elevator. For her, photons travel in a straight line, and it is merely the acceleration of the elevator that has caused the windows and floor of the elevator to curve up to the flight path of the photons. Let there now be a man standing inside the elevator. Because the floor of the elevator accelerates him upward at a rate g, he mayif he chooses to regard himself as stationarythink that he is standing still on the surface of the Earth and is being pulled to the ground by its gravitational field g. Indeed, in accordance with the equivalence principle, without looking out the windows (the outside is not part of his local environment), he cannot perform any local experiment that would inform him otherwise. Let the woman shine her pulse of light. The man sees, just like the woman, that the photons enter near the top edge of one window and exit near the bottom of the other. And just like the woman, he knows that photons propagate in straight lines in free space. (By the relativity principle, they must agree on the laws of physics if they are both inertial observers.) However, since he actually sees the photons follow a curved path relative to himself, he concludes that they must be bent by the force of gravity. The woman tries to tell him there is no such force at work; he is not an inertial observer. Nonetheless, he has the solidity of the Earth beneath him, so he insists on attributing his acceleration to the force of gravity. According to Einstein, they are both right. There is no need to distinguish locally between acceleration and gravitythe two are in some sense equivalent. But if that is the case, then it must be true that gravityreal gravitycan actually bend light. And indeed it can, as many experiments have shown since Einstein's first discussion of the phenomenon. It was the genius of Einstein to go even further. Rather than speak of the force of gravitation having bent the photons into a curved path, might it not be more fruitful to think of photons as always flying in straight linesin the sense that a straight line is the shortest distance between two pointsand that what really happens is that gravitation bends space-time? In other words, perhaps gravitation is curved space-time, and photons fly along the shortest paths possible in this curved space-time, thus giving the appearance of being bent by a force when one insists on thinking that space-time is flat. The utility of taking this approach is that it becomes automatic that all test bodies fall at the same rate under the force of gravitation, for they are merely producing their natural trajectories in a background space-time that is curved in a certain fashion independent of the test bodies. What was a minor miracle for Galileo and Newton becomes the most natural thing in the world for Einstein. To complete the program and to conform with Newton's theory of gravitation in the limit of weak curvature (weak field), the source of space-time curvature would have to be ascribed to mass (and energy). The mathematical expression of these ideas constitutes Einstein's theory of general relativity, one of the most beautiful artifacts of pure thought ever produced. The American physicist John Archibald Wheeler and his colleagues summarized Einstein's view of the universe in these terms: Curved spacetime tells mass-energy how to move; mass-energy tells spacetime how to curve. Contrast this with Newton's view of the mechanics of the heavens: Force tells mass how to accelerate; mass tells gravity how to exert force. Notice therefore that Einstein's worldview is not merely a quantitative modification of Newton's picture (which is also possible via an equivalent route using the methods of quantum field theory) but represents a qualitative change of perspective. And modern experiments have amply justified the fruitfulness of Einstein's alternative interpretation of gravitation as geometry rather than as force. His theory would have undoubtedly delighted the Greeks. Additional reading General works Review articles on a wide variety of modern astronomy and astrophysics topics written for the scientifically literate are found in Stephen P. Maran (ed.), The Astronomy and Astrophysics Encyclopedia (1992). Topical surveys of more limited scope are available in the Harvard Books on Astronomy series, especially such titles as Lawrence H. Aller, Atoms, Stars, and Nebulae, 3rd ed. (1991); Bart J. Bok and Priscilla F. Bok, The Milky Way, 5th ed. (1981); and Wallace Tucker and Riccardo Giacconi, The X-Ray Universe (1985). There are many introductory astronomy textbooks available that suppose little mathematical sophistication on the part of the reader; one of the most comprehensive is George O. Abell, David Morrison, and Sidney C. Wolff, Exploration of the Universe, 6th ed. (1991). An introduction that begins with the big bang and works forward in time is Donald Goldsmith, The Evolving Universe, 2nd ed. (1985). At a somewhat more advanced level is Frank H. Shu, The Physical Universe: An Introduction to Astronomy (1982). History of astronomy The standard reference is A. Pannekoek, A History of Astronomy (1961, reissued 1989; originally published in Dutch, 1951). Excellent accounts of early ideas can be found in J.L.E. Dreyer, A History of Astronomy from Thales to Kepler, 2nd ed. (1953); and Giorgio De Santillana, The Origins of Scientific Thought (1961, reissued 1970). A historical account of our understanding of galaxies and the extragalactic universe is Timothy Ferris, Coming of Age in the Milky Way (1988). William Sheehan, Worlds in the Sky (1992), summarizes our current understanding of the solar system. Planets Useful summaries are found in Bruce Murray (ed.), The Planets (1983), a collection of Scientific American articles. Also recommended is J. Kelly Beatty and Andrew Chaikin (eds.), The New Solar System, 3rd ed. (1990). The relationship of the origin of the solar system to theories of star formation is discussed at a technical level in David C. Black and Mildred Shapley Matthews (eds.), Protostars and Planets II (1985). Stars and other cosmic components A very readable work on stellar evolution is Robert Jastrow, Red Giants and White Dwarfs, new ed. (1990). Martin Cohen, In Darkness Born: The Story of Star Formation (1988), summarizes the processes of star formation. A classic text is Martin Schwarzschild, Structure and Evolution of the Stars (1958, reissued 1965). Stellar nucleosynthesis is the emphasis of Donald D. Clayton, Principles of Stellar Evolution and Nucleosynthesis (1968, reprinted 1983). Stan Woosley and Tom Weaver, The Great Supernova of 1987, Scientific American, 261(2):3240 (August 1989), is a popular review. The properties of gravitationally compact stellar remnants are discussed by Stuart L. Shapiro and Saul A. Teukolsky, Black Holes, White Dwarfs, and Neutron Stars (1983). Harry L. Shipman, Black Holes, Quasars, and the Universe, 2nd ed. (1980), is a more elementary treatment. Michael W. Friedlander, Cosmic Rays (1989), is an introduction. Galaxies Beautiful photographs of galaxies together with nontechnical commentary are contained in Timothy Ferris, Galaxies (1980). Equally enjoyable for the amateur and professional alike are Allan Sandage, The Hubble Atlas of Galaxies (1961); Halton Arp, Atlas of Peculiar Galaxies (1966, reprinted 1978); and Allan Sandage and G.A. Tammann, A Revised Shapley-Ames Catalog of Bright Galaxies, 2nd ed. (1987). An observational account of current ideas on the formation of our own galaxy is found in Sidney van den Bergh and James E. Hesser, How the Milky Way Formed, Scientific American, 268(1):7278 (January 1993). Extragalactic astronomy is discussed at a level appropriate for professionals in Allan Sandage, Mary Sandage, and Jerome Kristian (eds.), Galaxies and the Universe (1975, reprinted 1982); S.M. Fall and D. Lynden-Bell (eds.), The Structure and Evolution of Normal Galaxies (1981); and C. Hazard and Simon Mitton (eds.), Active Galactic Nuclei (1979). The problems of galaxy formation or galaxy clustering are described by Joseph Silk, The Big Bang, rev. and updated ed. (1989); and by P.J.E. Peebles, The Large-Scale Structure of the Universe (1980). Cosmology Several excellent semipopular accounts are available: Timothy Ferris, The Red Limit: The Search for the Edge of the Universe, 2nd rev. ed. (1983); Steven Weinberg, The First Three Minutes: A Modern View of the Origin of the Universe, updated ed. (1988); Nigel Calder, Einstein's Universe (1979, reissued 1982); Edward R. Harrison, Cosmology, the Science of the Universe (1981); Robert V. Wagoner and Donald W. Goldsmith, Cosmic Horizons (1982); and John Barrow and Joseph Silk, The Left Hand of Creation: The Origin and Evolution of the Expanding Universe (1983). Michael Rowan-Robinson, The Cosmological Distance Ladder (1985), provides a detailed discussion of how astronomers measure distances to galaxies and quasars. Stephen W. Hawking, A Brief History of Time (1988), is a discussion by a modern scientific icon on gravitation theory, black holes, and cosmology. Standard textbooks on general relativity and cosmology include P.J.E. Peebles, Physical Cosmology (1971); Steven Weinberg, Gravitation and Cosmology (1972); and Charles W. Misner, Kip S. Thorne, and John Archibald Wheeler, Gravitation (1973). The interface between particle physics and cosmology is the concern of G.W. Gibbons, Stephen W. Hawking, and S.T.C. Siklos (eds.), The Very Early Universe (1983). One of the best semipopular introductions to the modern attempts to unify the fundamental forces is P.C.W. Davies, The Forces of Nature, 2nd ed. (1986). Frank H. Shu Components of the universe Galaxies are where astronomers find stars, the major transformers of matter into energy in the universe. Paradoxically, it is also from the study of galaxies that astronomers first learned that there exist in the universe sources of energy individually much more powerful than stars. These sources are radio galaxies and quasars, and their discovery in the 1950s and '60s led to the establishment of a new branch of astronomy, high-energy astrophysics. Extragalactic radio sources Sources that emit a continuum of radio wavelengths and that lie beyond the confines of the Galaxy were divided in the 1950s into two classes depending on whether they present spatially extended or essentially starlike images. Radio galaxies belong to the former class, and quasars (short for quasi-stellar radio sources) to the latter. The distinction is somewhat arbitrary, because the ability to distinguish spatial features in cosmic radio sources has improved steadily and dramatically over the years, owing to Sir Martin Ryle's introduction of arrays of telescopes, which use aperture-synthesis techniques to enhance the angular resolution of a single telescope. Apart from the smaller angular extent that arises from being at a greater distance, many objects originally classified as quasars are now known to have radio structures that make them indistinguishable from radio galaxies. Not every quasar, however, is a radio galaxy. For every radio-loud quasar, there exist 20 objects having the same optical appearance but not the radio emission. These radio-quiet objects are called QSOs for quasi-stellar objects. Henceforth, the term quasars will be used to refer to both quasars and QSOs when the matter of radio emission is not under discussion. The most powerful extragalactic sources of radio waves are double-lobed sources (or dumbbells) in which two large regions of radio emission are situated in a line on diametrically opposite sides of an optical galaxy. The parent galaxy is usually a giant elliptical, sometimes with evide