The giant spiral assemblage of several billion stars that is home to the Sun and its family of planets, including Earth, is only one of the billions of star systems known to exist in the universe (see EXTRAGALACTIC SYSTEMS). Because it is our star system, however, it is usually called the Galaxy. The Milky Way, another name for it, is the portion visible to the naked eye. In the 20th century, astronomers determined that the Galaxy is a disk-shaped object, far larger than most of the galaxies in its neighborhood (see LOCAL GROUP OF GALAXIES). Its visible disk is about 100,000 LIGHT YEARS wide but only about 2,000 light-years thick. A halo of other materials, including star clusters, surrounds the disk. The total mass of the Galaxy can be measured by studying the motions of individual stars and clouds of hydrogen gas in different parts of the galaxy and by applying CELESTIAL MECHANICS to calculate a total mass that will account for the observed motions. The mass can also be determined from the motions of the Galaxy's small satellite galaxies, especially the nearby dwarf elliptical galaxies, and globular clusters. Recent computations by both methods agree that the Galaxy's mass is possibly 1,000 to 2,000 billion times the mass of the Sun. As the Sun's mass is about average for a star in the Galaxy, the total number of stars must also be of this order. Most of these stars are invisible from the Earth, however, because the solar system lies in the dense plane of the Galaxy, where interstellar dust obscures all but its nearer parts. The area around the Galaxy is populated by about twenty galaxies that make up a small cluster called the local group. Most of these neighbors, such as the MAGELLANIC CLOUDS, are far smaller and less luminous than the Galaxy. The only other large galaxy is the ANDROMEDA GALAXY, which is more than 2 million light-years away, is somewhat larger and more luminous than our own galaxy and is visible to the naked eye. The Andromeda spiral differs slightly in shape from our galaxy, having a larger smooth, amorphous central bulge and spiral arms that are less patchy. The Sun lies a little more than 30,000 light-years from the center of the Galaxy. From our vantage point, the Galaxy appears thicker toward its center, in the direction of the constellation Sagittarius, and somewhat thinner in other directions. However, because of the obscuration by dust, which limits our view in all directions, it is difficult to realize from observation that we are not near the center of the system. DETERMINATION OF GALACTIC STRUCTURE Until the 1920s it was thought that the system of stars outlined by the Milky Way was the entire universe; early attempts to understand the structure of the Galaxy were thought of as studies of the universe itself. In 1784, Sir William HERSCHEL attempted to determine the structure of the Milky Way (he referred to his work as exploring the "construction of the heavens") by making extensive star counts through telescopes, recording the number of stars in various directions, and plotting the results in a series of maps. Assuming that all stars had approximately the same brightness and that the Galaxy was uniformly dense, Herschel calculated the extent of the system of stars in various directions and concluded that we live in the central region of a flat, round arrangement of stars that extends far along the Milky Way. A much more accurate view of the Galaxy resulted from Harlow Shapley's studies of globular clusters, begun in 1914. Shapley realized that dust obscured large numbers of stars along the Milky Way and discovered that making star counts was not as good a way of gauging the size of the Galaxy as determining the size of the system of globular clusters that lie above and below the obscuration of the Milky Way plane. Using this method, he determined that the Galaxy is about ten times larger than previously thought, and that the Sun is located at a considerable distance from the center. He found that the clusters make up a thin, spherical halo that surrounds a bright flat disk. The detailed structure of the disk was difficult to discover because of the dust, but several astronomers, especially Jacobus C. Kapteyn in Holland and Bart J. Bok in the United States, pursued the task of plotting the distribution of stars to try to find a pattern, particularly a pattern of spiral arms like those seen in many other galaxies. Only fragments of structure emerged, however, and they were found to resemble scraps of spiral arms only when stellar associations were discovered during the late 1940s. An important breakthrough occurred in 1951, when Harvard scientists Harold Ewen and Edward Purcell made the first radio detection of the 21-cm emission line of neutral hydrogen gas in the Milky Way. By 1954, Dutch and Australian radio astronomers were ready to assemble a radio map of the Galaxy. Since radio waves pass through the dust unimpeded, this map was far more accurate than those based on visual observations. The result clearly showed a complex and beautiful spiral structure, very much like that of the giant galaxy Messier 101 or the Whirlpool Galaxy, Messier 51. Our present view of the Galaxy is based on highly detailed radio maps of neutral atomic hydrogen gas and other sources, including hot gas clouds and gas-dust complexes that emit radiation from various molecules and parts of molecules, such as water, carbon monoxide, and hydroxyl. The Galaxy consists of a slightly warped, scalloped disk of heavy-element-rich stars, gas clouds, and dust, surrounded by a tenuous spherical halo of old, heavy-element-poor stars and star clusters. The halo extends to about 85,000 light-years from the center, and is enveloped by the corona, which reaches to at least 200,000 light-years. In recent years astronomers have begun to examine the core of the Milky Way in other wavelengths (see INFRARED ASTRONOMY; RADIO ASTRONOMY; X-RAY ASTRONOMY). Infrared studies have revealed a small number of fast-moving red supergiant stars within 5 light-years of the center. Strong radio and X-ray emissions from the same area suggest that a BLACK HOLE may exist in that region, and that it may be generating the extremely hot gases spiralling around the galactic center at speeds of up to 700,000 kilometers per hour. Heavy-metal synthesis, which accompanies the formation of new stars, is also thought to occur in that region. Farther out are dramatic radiowave-emitting filaments perpendicular to the galactic plane; these arcs of matter, approximately 150 light-years long, suggest that a huge magnetic field exists around the galactic core. Unrelated to these arcs are three bizarre, threadlike structures, uniformly bright and about one light-year wide and more than 100 light-years long, that cut across the central galactic regions. These threads remain unexplained but may be magnetic in structure. COMPOSITION OF THE GALAXY Our galaxy, like most well-studied spiral galaxies, chiefly consists of stars, gas, and dust. A census of the visible part of the Galaxy indicates that most of the mass is in the stars, with only about 2% in the form of gas (mostly hydrogen) and about 0.01% in the form of dust. The stars of the Galaxy have been divided into two kinds, called Population I and Population II. Population I stars are prevalent in the spiral arms and include stars of all ages, from over ten billion years to only a few hundred thousand years old. They all contain elements heavier than helium, in amounts comparable to those found in the Sun. Population II stars, on the other hand, are found in the bulge around the galactic nucleus and in the spherical halo, which includes both the thin envelope of stars surrounding the disk and the globular clusters. All Population II stars are approximately 12 to 15 billion years old, and all are deficient in their amounts of heavy elements, some by factors of more than a hundred. These are the Galaxy's oldest inhabitants, and they are frequently offered as evidence that the Galaxy itself is 15 billion years old. The total amount of stars, gas, and dust in the Galaxy does not quite equal the total measured mass. Although there has been considerable recent controversy about the matter, it may be necessary to account for the difference by suggesting that the Galaxy contains matter in some undetected, invisible form, such as molecular hydrogen, black holes (collapsed and invisible stars), or meteoroids. Recent investigations suggest that the "missing mass" of the Galaxy might be found in the corona. The composition of the corona is unknown, but it is estimated that it contains between 100 and 200 billion solar masses. DYNAMICS OF THE GALAXY In the early 20th century, as the mystery of the structure of the Galaxy was being unraveled, the motions of stars were also being determined. Astronomers recorded the slow perceived position changes of stars (proper motion) and their motions toward or away from Earth (radial velocities); the latter are easily measured by the Doppler shift of the stars' spectral lines. In 1904, Kapteyn found that stars did not move at random but in two streams flowing in opposite directions along the Milky Way, one converging in the constellation of Orion and the other converging 180 deg away, in Scutum. The Swedish astronomer Bertil Lindblad showed that this streaming motion is simply the result of the rotation of the Galaxy. Stars traveling in nearly circular orbits around the galactic center with the Sun will have larger motions relative to the Sun either toward or away from the center--depending on whether they are approaching the nearest or farthest points in their elliptical orbits--than in the direction of motion. Therefore, we preferentially see motions towards us or away from us in these directions. In 1927, Jan H. Oort of the Netherlands showed that the motions of stars in different parts of the Milky Way could be used to derive the properties of the rotation of the Galaxy, including the speed of the Sun through space. When modern values are used in Oort's equations, it is found that the Sun's velocity is approximately 250 km/sec in its orbit around the galactic center. The velocity for stars at larger distances from the center is smaller; in the inner part of the Galaxy the velocities are similar to those of solid bodies. These velocity differences cause differential rotation in the disk, and they may be the primary cause of the spiral shape of the arms (and also, incidentally, of the rotation of the bodies in the solar system, including the Earth). The dynamics of the spiral arms are still only imperfectly understood. Differential rotation will make spiral arms out of almost any structural feature in a galaxy, but the arms should only last a fraction of the age of the galaxy. It would lead to a rapid winding-up of the arms in the 50 or so rotations that have occurred since the Galaxy was formed. One possible explanation is that the arms are not constant physical entities, but are waves of high star density moving more slowly than the stars. Stars slow down and pile up temporarily in an arm because of its higher gravitational field, then pass out of the arm and proceed until they encounter the next arm. Determinations have been made of the Galaxy's movement as a whole, relative to the rest of the universe. For example, high-altitude measurements were made of the universe's BACKGROUND RADIATION, the residual glow of the so-called "big bang" that is assumed to have occurred in the first moments of the universe (see COSMOLOGY). The measurements indicated that the Galaxy is moving, relative to the universe, in the same direction as the constellation Leo lies relative to the Earth, and with a velocity of more than 600 km/sec (373 mi/sec). The galaxy is also moving at about 100 km/sec (62 mi/sec), relative to the center of mass of the local group of galaxies. The local group, in turn, is moving at a comparable velocity relative to the supercluster of galaxies to which it belongs. Some astronomers have proposed that the flow is moving toward a huge, distant region of space that has been called the Great Attractor, but others have since disputed this theoretical structure. RADIATION FROM THE GALAXY From a distance, the Galaxy could be detected by a wide variety of means, since it emits radiation at almost all possible wavelengths: it is optically bright, emitting the equivalent of approximately 200 billion Suns in optical (visible) radiation; it is a strong source of radio-line emission, especially from its large mass of neutral hydrogen; it is a source of radio continuum noise, both from the hot gas clouds in its arms and from its dense, hot nucleus; its huge, dark, cool complexes of dust and gas emit infrared emission; it shines in the ultraviolet region of the spectrum because of its large number of very hot, recently formed stars; and it gives off X radiation from many sources. Paul W. Hodge Bibliography: Bok, Bart J. and Priscilla F., The Milky Way, 5th ed. (1981); Catchpole, R. M., "A Window on Our Galaxy's Core," Sky & Telescope, February 1988; Corwin, H. G., and Bottinelli, L., eds., The World of Galaxies (1990); Hodge, Paul W., Galaxies (1986); Shuter, W. L., Kinematics, Dynamics and Structure of the Milky Way (1983).