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galaxies(gal-ăks-eez) Giant assemblies of stars, gas, and dust into which most of the visible matter in the Universe is concentrated. Each galaxy exists as a separate, though not always entirely independent, system held together and organized largely by the gravitational interactions between its various components. When capitalized, the term denotes specifically our own system, the Milky Way Galaxy. Owing to their cloudlike appearance when viewed by eye through simple telescopes, galaxies other than our own were once known as ‘nebulae’. This term is now strictly applied only to clouds of interstellar gas and dust.
The majority of galaxies are composed of two structural components, the central spheroidal bulge and the flattened disk. The bulge is usually supported against gravitational collapse by the anisotropic velocity dispersions of its stars, whereas a disk is maintained by rotation. Galaxies are divided into broad categories based on the relative importance of these two components.
Elliptical galaxies are those systems that have no disk component. They appear in photographs as fuzzy elliptical patches of light, diminishing smoothly in brightness from the center outward with no obvious internal structure. The shape of the outline varies from almost circular through to narrow ellipses about three times as long as they are wide. Traditionally the latter were regarded as oblate (flattened) systems but recent analyses show that prolate (elongated) systems are also possible. Most elliptical galaxies are probably triaxial ellipsoids with their three axes of different lengths. The shape of the outline is the basis of the Hubble classification. Elliptical galaxies were long thought to be devoid of gas but X-ray and radio observations have shown them to have a significant and complex interstellar medium, with very hot (107 K) gas coexisting with clouds of neutral hydrogen. Some galaxies even show low-level emission-line activity. The stars within them are predominantly old (spectral types K and M).
Galaxies with a second, disk-shaped component in addition to the bulge are known as spiral galaxies. They form flattened systems containing prominent spiral arms of interstellar matter and bright young stars that wind outward from a dense central bulge or nucleus. Although two-armed spirals are the most common, systems with one arm or even, very occasionally, three arms have also been observed. In normal spirals the arms emerge directly from the nucleus, usually from opposite sides; in barred spirals the arms emanate from each end of a bright central bar that extends across the nucleus. Both types exist in a wide range of forms. At one extreme there are galaxies with large dominant bulges and thin tightly wound spiral arms; at the other extreme there are galaxies with inconspicuous nuclei and prominent loosely wound spiral arms. The shape and structure of spirals and barred spirals is the basis of Hubble's classification.
Spiral galaxies are rich in gas and dust, most of which is distributed in clouds along the spiral arms. Stars in the nuclei of spirals appear to be predominantly well advanced in their evolution and the brightest individual stars observed there are red giants of population II. A fairly smooth axially symmetric distribution of old stars also extends beyond the nucleus. However, owing to the intrinsic faintness of most of its constituent stars, this system is much less conspicuous than the spiral arms embedded within it.
Although star formation is undoubtedly still taking place in the spiral arms, what triggers this process is still uncertain, as is the origin of the arms themselves. It has been proposed, in the density-wave theory, that the spiral structure is maintained over a long period by gravitational effects. Alternatively, it has been suggested that the spiral structure arises and persists as a result of self-propagating star formation involving supernovae. All spiral galaxies are in differential rotation, stars in the outer parts completing their orbits on average more slowly than those nearer the center. As far as can be ascertained, the spiral arms always trail away from the direction of rotation.
Galaxies possessing a large bulge and small disk, intermediate between the spirals and ellipticals, are known as S0 galaxies or lenticular galaxies. The disk shows no evidence for spiral arms, although dark dust clouds are sometimes seen when the galaxy presents an edge-on aspect. The stars within S0 galaxies are predominantly old and little gas is apparent.
Irregular galaxies are those without discernable symmetry in shape or structure. They vary enormously in appearance but all are below average in size and contain large amounts of interstellar matter. In the early Hubble classification this term was applied to any galaxy that could not be fitted into the elliptical or spiral classes. Hubble's Irr II galaxies are now reclassified as starburst, active, and interacting galaxies. The dwarf galaxies presently labelled as irregular correspond to Hubble's Irr I galaxies.
The colors of galaxies vary according to the age of the stars responsible for most of the light. In general, ellipticals and S0s are the most red and irregulars, because of their high content of very young stars, are the most blue. Differences in the dominant spectral type are also reflected in the mass to luminosity ratio, spirals and irregulars being, on average, brighter than ellipticals or S0s of similar mass.
The brightest and most massive galaxies are the cD galaxies , which resemble giant ellipticals; they have absolute magnitudes of about –22.5 and masses in excess of 1012 solar masses. cD galaxies are located exclusively at the gravitational center of clusters of galaxies. They possess an extra component in the form of a very faint but extensive (up to radii of 100 kiloparsecs) envelope of stars. They are the rarest galaxies, and usually contain the most powerful radio sources in the nearby Universe.
Ellipticals also display the greatest variation in mass, ranging down to extreme dwarfs (about 106 solar masses) that are no brighter than the most luminous globular clusters. Spirals appear to exist only as large or giant systems, with masses typically of the order of 1010 or 1011 times that of the Sun. No irregulars are as bright as the giant spirals and some are extreme dwarfs.
Of the 1000 brightest galaxies, about 75% are spiral, 20% elliptical, and 5% irregular. However, when allowance is made for the many dwarf galaxies, the true proportions turn out to be nearer to 30:60:10. Few galaxies exist in total isolation. Double and multiple systems are common and many galaxies are also members of larger groups known as clusters of galaxies. Clusters can in turn form loosely bound aggregates called superclusters.
How and why galaxies have evolved to their present shapes is still uncertain although it appears that spirals contain much more angular momentum than ellipticals. Turbulence and vorticity in the early Universe may play a role. However, it is generally conceded now that the principal types of galaxy represent separate species rather than one species seen at different stages in its evolution. There is, in fact, no direct evidence that any other galaxy is significantly older than our own Galaxy (which is thought to be about 12 thousand million years old), and none younger apart from the minuscule extragalactic H II regions. All galaxies appear to contain a mixture of stellar populations.
giant systems of stars resembling our star system, the Milky Way System, which contains the solar system. The outdated terms for galaxies “extragalactic nebulas” and “anagalactic nebulas” reflect the fact that they are visible in the sky as bright, nebulous patches outside the boundaries of the Milky Way (our galaxy), which is thus for them a “zone of avoidance.” Galaxies are not visible in this zone because of the concentration of dark, light-absorbing dust matter near the equatorial plane of our galaxy.
The nature of galaxies became known in the 1920’s after the American astronomer E. Hubble discovered that the closest galaxies consist of a multitude of very faint stars that, when observed through small telescopes, merge into a solid bright patch—a nebula. Among the brightest individual stars, he succeeded in detecting variable stars of the cepheid type, whose apparent magnitude, when measured, makes it possible to determine the distance to the systems they belong to. In this way it was conclusively established that galaxies are located far beyond our galaxy and have comparable dimensions. It turned out that the closest galaxies to us were the Magellanic Clouds, which resemble scraps of the Milky Way and are 46 kiloparsecs away (about 150,000 light-years). In diameter they are several times smaller than our galaxy and are evidently its companions. Distances to remote galaxies are estimated according to the red shift—the shift in the spectral lines of galaxies caused by the Doppler effect. This shift increases statistically as the distance to a galaxy increases. The distance to the remotest galaxies distinguishable on photographs made with the aid of the largest telescopes is more than 1 billion parsecs (more than 3 billion light-years).
In the 1920’s and 1930’s, Hubble worked out the principles for a structural classification of galaxies according to which three classes are identified: spiral galaxies, elliptical galaxies, and irregular galaxies. Spiral galaxies are characterized by two relatively bright arms situated in a spiral around the center. The arms emerge either from a bright center (such galaxies are designated by S) or from the ends of a luminous bar intersecting the nucleus (designated SB). Elliptical galaxies (E) have the shape of ellipsoids. Irregular galaxies (I) have irregular shapes. According to the degree of clustering in the arms, spiral galaxies are divided into subtypes a, b, and c. In the first subtype, the arms are amorphous; in the second, show some clustering; and in the third, show well-marked clustering, and the nucleus is always dim and small. In the second half of the 1940’s, W. Baade (USA) established that the extent of clustering of spiral arms and their blueness grow with an increase in their content of hot blue stars and with an increase in their aggregates and diffuse nebulas. The central parts of spiral galaxies are yellower than the arms and contain old stars (Population II stars according to Baade), while the flat spiral arms consist of young stars (Population I stars). The density of the distribution of stars in space grows with proximity to the equatorial plane of spiral galaxies. This plane is the system’s plane of symmetry, and most of the stars in rotating around the center of a galaxy remain close to it. The periods of rotation vary from 107 to 109 years. In the process, the inner parts rotate like a solid, but on the periphery the angular and linear velocities of rotation decrease upon receding from the center. However, in some cases an even smaller nucleus —the “kernel”—located within the center rotates the most rapidly. Irregular galaxies, which are also flat star systems, rotate in a similar fashion. Elliptical galaxies consist of Population II stars. Only the most compressed of such systems have been observed to rotate. As a rule, they have no cosmic dust, setting them apart from irregular and especially spiral galaxies, which contain large quantities of light-absorbing dust matter. In spiral galaxies dust matter constitutes between several thousandths and one-hundredth of their total mass. As a result of the concentration of dust matter toward the equatorial plane, it forms a dark zone in galaxies that are turned to us sideways and have the form of a spindle.
Radio astronomical observations have made it possible to detect clusters of neutral hydrogen in galaxies. Its mass is relatively small in Sa spiral galaxies, reaches several percent in Sb galaxies, and totals as high as 10 percent of the mass of stars in Sc galaxies as well as in irregular galaxies. For the most part, the neutral hydrogen—the principal part of the gaseous component of galaxies—is located in a narrow equatorial layer, but individual clouds are also observed far from it, where there are no very hot stars capable of ionizing it and bringing it to a luminous state.
Subsequent observations showed that the above classification is inadequate to systematize the diversity of shapes and properties of galaxies. For instance, galaxies have been discovered that, in a certain sense, occupy an intermediate position between spiral and elliptical galaxies (identified as SO). These galaxies have an enormous dense region in the center and a flat disk surrounding it but no spiral arms. In the 1960’s numerous ring- and disk-shaped galaxies were discovered with all gradations of abundance of hot stars and dust. As far back as the 1930’s elliptical dwarf galaxies were discovered in the constellations Fornax and Sculptor with very low surface brightness; the brightness was so small that these galaxies, which are among the closest to us, and even their central parts can be seen only with difficulty against the sky. On the other hand, in the early 1960’s numerous distant compact galaxies were discovered, among which the most distant of their type were indistinguishable from stars, even through the most powerful telescopes. They are distinguished from stars by the spectrum, in which bright lines of emission are visible with enormous red shifts corresponding to great distances at which even the brightest solitary stars cannot be seen. In contrast to ordinary distant galaxies, which because of a combination of the true distribution of energy in their spectrum and the red shift appear reddish, the most compact galaxies (also called quasistellar galaxies) are blue in color. As a rule, these objects are hundreds of times brighter than ordinary supergiant galaxies, but there are also fainter ones. Many galaxies have been found to emit radio-frequency radiation of a nonthermal nature, which arises, according to the theory of Soviet astronomer I. S. Shklovskii, when electrons and heavier charged particles traveling at velocities close to the velocity of light are slowed down in a magnetic field (so-called synchrotron radiation). The particles acquire such velocities as a result of tremendous explosions inside the galaxies.
Compact distant galaxies possessing powerful nonthermal radio emission are called N galaxies. Starlike sources with such radiation are called quasars (quasistellar radio sources), and galaxies with powerful radio emission and noticeable angular dimensions are called radio galaxies. All these objects are extremely far from us, which hampers their study. Radio galaxies with especially powerful nonthermal radio emission mostly have an elliptical shape, but spiral ones are also found. The so-called Seyfert galaxies are of great interest. The spectra of their small centers contain many very wide, bright stripes, attesting to powerful expulsions of gas from their centers at speeds reaching several thousand kilometers per second. Some Seyfert galaxies have been found to have very weak nonthermal radio emission. It is not ruled out that the optical radiation of such nuclei, as in quasars, is not produced by stars and is also of a nonthermal nature. Strong nonthermal radio emission is possibly a temporary stage in the development of quasistellar galaxies. Radio galaxies near us have been studied more fully, specifically by methods of optical astronomy. Some of them have been found to have features that have not yet been completely explained. For example, in the giant elliptical galaxy Centaurus A, an extraordinarily powerful dark band has been detected along its diameter. Another radio galaxy consists of two close elliptical galaxies that are linked by a bar of stars.
In studying the irregular galaxy M82 in the constellation Ursa Major, the American astronomers A. Sandage and C. Lynds concluded in 1963 that about 1.5 million years ago a tremendous explosion occurred in its center as a result of which jets of hot hydrogen had been expelled in all directions at a velocity of about 1,000 km/sec. The resistance of the interstellar medium hampered the spread of the jets of gas in the equatorial plane, and they were directed primarily in two opposite directions along the galaxy’s axis of rotation. This explosion evidently also produced a multitude of electrons at velocities close to the velocity of light, which were the cause of the nonthermal radio emission.
Long before the explosion in M82 was detected, the Soviet astronomer V. A. Ambartsumian explained numerous other facts by advancing a hypothesis of possible explosions in the nuclei of galaxies. In his opinion, such a substance exists even now in the center of some galaxies, and it can split into parts during explosions that are accompanied by strong radio emission. Thus, radio galaxies are galaxies whose nuclei are in the process of disintegrating. The expelled dense parts, while continuing to break up, will possibly form new sister galaxies or smaller-mass companions of galaxies. In the process, the dispersion of the fragments can reach enormous speeds. Research has shown that many groups and even clusters of galaxies are disintegrating; their members are receding without limit from one another, as if they had all been generated by an explosion.
Also not yet explained are the causes of the formation of so-called interacting galaxies, detected in 1957-58 by the Soviet astronomer B. A. Vorontsov-Vel’iaminov. These are pairs or tight groups of galaxies in which one or several members have obvious distortions of shape and appendages; sometimes they are immersed in a common luminous envelope. Thin filaments connecting a pair of galaxies can also be observed as can “tails” directed away from a neighboring galaxy, seemingly repelled by it. The filaments are sometimes dual, which indicates that the distortions of the shapes of interacting galaxies cannot be attributed to tidal phenomena. Often a large galaxy links up, by means of one of its arms, sometimes a deformed one, with its companion. All of these parts, like the galaxies themselves, consist of stars and sometimes diffuse matter.
Galaxies are often found in space in pairs and larger groups, sometimes in clusters containing hundreds of galaxies. Our galaxy with the Magellanic Clouds and other close galaxies probably also makes up a separate local cluster of galaxies. The Magellanic Clouds and our galaxy are evidently immersed in a common hydrogen cloud. Groups and clusters vary in terms of the types of galaxies they contain. Sometimes they contain only spiral and irregular galaxies, sometimes only elliptical galaxies, and sometimes all three. The nearest ones to us are a rarefied cloud of galaxies in Ursa Major and an irregular cluster in the constellation Virgo. Both contain galaxies of all types. The very abundant and compact cluster of E and S0 galaxies located in the constellation Coma Berenices numbers thousands of members.
The luminosities and dimensions of the galaxies vary widely. Supergiant galaxies have luminosities 1011 times greater than the luminosity of the sun, and quasars, on the average, are another 100 times brighter; the faintest of the known dwarf galaxies are comparable to ordinary globular star clusters in our galaxy. Their luminosity is about 105 times the sun’s. The dimensions of the galaxies differ widely, varying from tens of parsecs to tens of thousands of parsecs.
The space between galaxies, especially that within clusters of galaxies, at times seems to contain cosmic dust. Radio telescopes do not reveal in them a detectable amount of neutral hydrogen, but cosmic rays penetrate it just like electromagnetic radiation.
About 1,500 bright galaxies are known (up to the 13th stellar magnitude). The Morphological Catalog of Galaxies (four volumes), compiled in the USSR (publication was completed in 1968), contains data on 30,000 galaxies brighter than the 15th stellar magnitude. It covers three-quarters of the entire sky. Several billion galaxies up to the 21st stellar magnitude are accessible to a 5-m telescope. Such galaxies are distinguishable from the faintest stars only by the slightly washed-out quality of their images.
REFERENCESEigenson, M. S. Vnegalakticheskaia astronomiia. Moscow, 1960.
Stroenie zvezdnykh sistem. Edited by P. N. Kholopov. Moscow, 1962. (Translated from English.)
Agekian, T. A. Zvezdy, galaktiki, metagalaktika. Moscow, 1966.
Baade, W. Evoliutsiia zvezd i galaktik. Moscow, 1966. (Translated from English.)
Struve, O. L., and V. Zebergs. Astronomiia 20 veka. Moscow, 1968. (Translated from English.)
B. A. VORONTSOV-VEL’IAMINOV