GALAXY

any of the systems of stars and interstellar matter that make up the Cosmos. Many such assemblages are so enormous that they contain hundreds of billions of stars. Nature has provided an immensely varied array of galaxies, ranging from faint, diffuse dwarf objects to brilliant, spiral-shaped giants. Virtually all galaxies appear to have been formed soon after the universe began, and they pervade space, even into the depths of the farthest reaches penetrated by powerful modern telescopes. Galaxies usually exist in clusters, some of which in turn are grouped into larger clusters measuring hundreds of millions of light-years across. (A light-year is the distance traversed by light in one year, traveling at a velocity of 300,000 kilometres per second, or 650,000,000 miles per hour.) These so-called superclusters are separated by nearly empty voids, causing the gross structure of the universe to look somewhat like a network of sheets and chains of galaxies. Galaxies differ from one another in shape, with variations resulting from the way in which the systems were formed. Depending on the initial conditions in the pregalactic gas some 15,000,000,000 years ago, galaxies formed either as slowly turning, smoothly structured, round systems of stars and gas or as rapidly rotating pinwheels of such entities. Other differences between galaxies have been observed and are thought to reflect evolutionary changes. Some galaxies are rife with activity: they are the sites of star formation with its attendant glowing gas and clouds of dust and molecular complexes. Others, by contrast, are quiescent, having long ago ceased to form new stars. Perhaps the most conspicuous evolutionary changes in galaxies occur in their nuclei, where evidence suggests that in many cases supermassive objectsprobably black holesformed when the galaxies were young. Such phenomena occurred several billion years ago and are now observed as brilliant objects called quasars. The existence of galaxies was not recognized until the early 20th century. Since then, however, galaxies have become one of the focal points of astronomical investigation. The notable developments and achievements in the study of galaxies are surveyed here. Included in the discussion are the external galaxies (i.e., those lying outside the Milky Way Galaxy, the local galaxy to which the Sun and Earth belong), their distribution in clusters and superclusters, and the evolution of galaxies and quasars. For further details on the Milky Way Galaxy, see Milky Way Galaxy. For specifics about the components of galaxies, see star, nebula, and Cosmos. Principal schemes of classification Almost all current systems of galaxy classification are outgrowths of the initial scheme proposed by Hubble in 1926. In Hubble's scheme, which is based on the optical appearance of galaxy images on photographic plates, galaxies are divided into three general classes: ellipticals, spirals, and irregulars. His basic definitions are as follows: Elliptical galaxies Galaxies of this class have smoothly varying brightnesses, with the degree of brightness steadily decreasing outward from the centre. They appear elliptical in shape, with lines of equal brightness made up of concentric and similar ellipses. These galaxies are nearly all of the same colour: they are somewhat redder than the Sun. The extragalactic distance scale Before astronomers could establish the existence of galaxies, they had to develop a way to measure their distances. In an earlier section, it was explained how astronomers first accomplished this exceedingly difficult task for the nearby galaxies during the 1920s. Since that time, progress has been slow and the results far from satisfactory. Distance determinations for the nearest galaxies still remain uncertain by as much as 10 percent, and the scale of distances beyond the Local Group of galaxies is even more unsure, with an uncertainty of possibly a factor of two. The reason for this unfortunate situation is the great difficulty of observing distant galaxies in sufficient detail to recognize the necessary distance criteria accurately, to measure their characteristics, and to make certain that their characteristics are correctly interpreted. The process involved is one of many successive steps that are all closely tied to one another. Distances are first determined for a number of galaxies close to the Milky Way Galaxy, specifically those in the Local Group and a few in other nearby groups. For this step, criteria are used that have been calibrated within the Galaxy, where checks can be made between different methods and where the ultimate criterion is a geometrical one (basically involving trigonometric parallaxes and the moving-cluster method). Next, the nearby galaxies are used to set up new, brighter distance criteria that can be employed in more distant realms where only the brightest stars and other objects are discernible in galaxies. These in turn can be used to establish criteria based on the properties of whole galaxies so that distances can be found for systems so far away that no individual objects can be resolved within them. These last criteria then are applied to extend scientific knowledge to the most distant galaxies that can be detectedthose at the very edge of the visible universe. Considering the magnitude of the task, it is no surprise that the answers obtained are not yet highly certain. Astronomers have done remarkably well to measure such vast distances even with an uncertainty of a factor of two. The Local Group of galaxies is a concentration of approximately 35 galaxies, dominated by two large spirals, the Milky Way system and the Andromeda galaxy (see Table). For many of these galaxies, distances can be measured using the Cepheid periodluminosity law, which has been refined and made more accurate since Hubble first used it. For instance, the nearest external galaxy, the Large Magellanic Cloud, contains more than 2,000 Cepheid variables, which can be compared to Cepheids of known distance in the Galaxy to yield a distance determination of 150,000 light-years. This method has been employed for 10 galaxies of the Local Group. Most of the rest of the members are elliptical galaxies, which do not have Cepheid variables; their distances are measured by using Population II stars, such as RR Lyrae variables. Beyond the Local Group are two nearby groups for which the periodluminosity relation has been used: the Sculptor group and the M81 group. Both of these are small clusters of galaxies that are similar in size to the Local Group. They lie at a distance of from 10,000,000 to 15,000,000 light-years. While the periodluminosity relation is the primary distance criterion in this realm, others also are used. One of these is main-sequence fitting, a technique that compares the temperatures of individual stars (those in their normal, stable main-sequence phase) with similar stars in steller clusters in the Galaxy. Main-sequence fitting became effective in the 1980s, when powerful new detectors allowed accurate measurements of very faint stars in the few nearest galaxies. Also in the 1980s, astronomers developed a new method of effectively measuring distances to galaxies in the Local Group, in nearby groups, and even as far away as the Virgo cluster, lying at a distance of about 50,000,000 light-years. This method makes use of planetary nebulas, the ringlike shells that surround some stars in their late stages of evolution. Planetary nebulas have a variety of luminosities, depending on their age and other physical circumstances; however, it has been determined that the brightest planetary nebulas have an upper limit to their intrinsic brightnesses. This means that astronomers can measure the brightnesses of such nebulas in any given galaxy, find the upper limit to the apparent brightnesses, and then immediately calculate the distance of the galaxy. Another recent method involves the use of the nova phenomenon, which can be detected in galaxies in the Local Group and beyondthat is to say, as remote as the Virgo cluster. Once distances have been established for these nearby galaxies and groups, new criteria are calibrated for extension to fainter galaxies. Examples of the many different criteria that have been tried are the luminosities of the brightest stars in the Galaxy, the diameters of the largest H II regions, supernova luminosities, the spread in the rotational velocities of stars and interstellar gas, and the luminosities of globular clusters. All of these criteria have difficulties in their application because of dependencies on galaxy type, composition, luminosity, and other characteristics, so that the results of several methods must be intercompared and cross-checked. Such distance criteria allow astronomers to measure the distances to galaxies out to a few hundred million light-years. Beyond 100,000,000 light-years, another method becomes possible. The expansion of the universe, at least for the immediate neighbourhood of the Local Group (within 1,000,000,000 light-years or so), is linear enough that the radial velocity of a galaxy is a reliable distance indicator. The velocity is directly proportional to the distance in this interval, so that once a galaxy's radial velocity has been measured all that must be known is the constant of proportionality, which is called the Hubble constant. Although there still remains some uncertainty in the correct value of the Hubble constant, research in the 1990s has indicated that the constant has a value very near 25 kilometres per second per 1,000,000 light-years. Radial velocity for nearby galaxies and groups is affected by the Local Group's motion with respect to the general background of galaxies toward a concentration of galaxies and groups of galaxies centred on the Virgo cluster, all of which make up a local supercluster. Radial velocities cannot give reliable distances beyond a few billion light-years because, in the case of such galaxies, astronomers are looking so far back in time that they do not know whether the expansion rate of the universe was then the same as it is now. The light that is observed today was emitted several billion years ago when the universe was much younger and smaller than it is at present. To find the distances of very distant galaxies, astronomers have to avail themselves of methods that make use of the total properties of the very brightest galaxies. Most commonly it is assumed that the brightest galaxies in clusters all have the same true luminosity (this appears to be the case for relatively nearby clusters for which distances can be measured in other ways) and that therefore measuring the apparent brightness of the brightest galaxy in a distant cluster will give its distance. This method, as well as others that make use of the cluster environment, have yielded distance estimates as large as 10,000,000,000 to 20,000,000,000 light-years for the most distant galaxies detectable. Physical properties of external galaxies Size and mass The range in intrinsic size for the external galaxies extends from the smallest systems, such as the nearby dwarf galaxy GR8 with a diameter of about 5,000 light-years, to giant radio galaxies, the extent of which (including their radio-bright lobes) is more than 3,000,000 light-years. Normal large spiral galaxies, such as the Andromeda galaxy, have diameters of 100,000 to 500,000 light-years. The total masses of galaxies are not well known, largely because of the uncertain nature of the hypothesized invisible dark halos that surround many, or possibly all, galaxies. The total mass of material within the radius out to which the light of a galaxy can be detected is known for many hundreds of systems. The range is from about 100,000 to roughly 1,000,000,000,000 times the Sun's mass. The mass of a typical large spiral is about 500,000,000,000 suns. The study of the origin and evolution of galaxies and the quasar phenomenon has only just begun. Many models of galaxy formation and evolution have been constructed on the basis of assumptions about conditions in the early universe, which are in turn based on models of the expansion of the Cosmos after the big bangthe primordial explosion from which the Cosmos is thought to have originated. Prevailing theory has it that at crucial points in time there condensed from the expanding matter smaller clouds (protogalaxies) that could collapse under their own gravitational field and eventually form galaxies. At the time when the mass of such a stable perturbation in the cloud was approximately 1012 solar masses, the galaxies formed. It is still not known whether the clusters of galaxies emerged first or whether they resulted as accumulations of already formed galaxies. Following the separation of mass into individual galaxies, the next step probably depended on the characteristics of the particular clump of matter involved, especially on its mass and angular momentum. The latter quantity was the most likely determinant of the form of the galaxy that eventually evolved. It is thought that a protogalaxy with a large amount of angular momentum tended to form a flat, rapidly rotating system (a spiral galaxy), whereas one with very little angular momentum developed into a more nearly spherical system (an elliptical galaxy). Calculations show that a galaxy very gradually becomes dimmer and redder as time progresses and its constituent stars evolve. There is some evidence from very distant galaxiesthose whose light was emitted billions of years ago when they were youngerthat the effects of this kind of slow evolution can actually be seen. A more spectacular example of galaxy evolution is provided by quasars. From the statistics of the frequency of different redshifts, which represent different distances and different epochs in the past, it has been determined that the quasar phenomenon occurred most frequently a few billion years after the big bang (the exact amount of time is uncertain because astronomers do not as yet know enough about the geometry or the age of the universe). It appears that conditions did not become suitable for quasar formation until after the galaxies had formed and separated. Today quasars are quite rare, but many galaxies have miniature versions of them in their nuclei in the form of less massive yet remarkably energetic objects. Paul W. Hodge any of the billions of systems of stars and interstellar matter that make up the Cosmos. Galaxies vary considerably in size, composition, and structure, but nearly all of them are arranged in groups, or clusters, of from a few to as many as 10,000 members each. The diameters of galaxies are generally measured in tens of thousands of light-years. The distance between galaxies within a cluster averages approximately 1,000,0002,000,000 light-years, and the spaces between clusters of galaxies may be a hundred times as great. Each galaxy is composed of innumerable starsmost likely from hundreds of million to more than a trillion stars. In many galaxies, as in the Milky Way Galaxy, clouds of interstellar gas and dust particles known as nebulas can be detected. The majority of known galaxies fall into one of two major classes: spirals and ellipticals. Roughly 70 percent of the bright galaxies in the sky are of the spiral variety, including the Milky Way Galaxy. A spiral galaxy has a main disk of stars 50,000 to 150,000 light-years in diameter and a thickness up to a tenth as great. Embedded in the disk are the spiral arms, winding out from the centre like those of a pinwheel. The arms contain the greatest concentration of a spiral galaxy's interstellar gas and dust, and it is in these regions that star formation can occur. Among newly formed stars are the occasional short-lived, highly luminous ones; their presence in the arms makes them conspicuous on telescopic photographs. Surrounding the central nucleus of a spiral galaxy is a large nuclear bulge, which is nearly spherical in most cases and may have a diameter of up to half that of the disk. Outside the nuclear bulge and disk is a sparse, more or less spherical halo of star clusters, individual stars, and perhaps other matter. The halo may extend far beyond the disk and contain most of the galaxy's mass. Spiral galaxies are generally subdivided into normal and barred types. In the latter, the arms begin either from the ends of a straight bar of stars and interstellar matter passing through the nucleus or from a circular ring surrounding the bar, rather than from the nucleus. Spirals are still further subdivided according to three well-correlated parameters: the size of the nucleus, the tightness of the winding of the arms, and the smoothness of the arms. An elliptical galaxy has a telescopic image that reveals a symmetrical distribution of stars in a spherical or spheroidal shape. Such galaxies range from the rare giant ellipticals, which extend several hundred thousand light-years across, to dwarf ellipticals of only a few million stars. The dwarf ellipticals are by far the most common kind of galaxy, although none is conspicuous in the sky. The projected images of ellipticals range from nearly circular to extremely elongated. Their flattening, however, is not due to rotation, and it is not known whether their true shapes are oblate or prolate spheres. A small number of galaxies do not fit neatly into the usual scheme, and they are classified as irregular. There are also some special classes, a few of which are considered here. The first of these are the S0 galaxies, which are usually found in rich clusters of galaxies. They resemble spirals but have no spiral arms, possibly because their interstellar matter is stripped away as they move through intracluster gas. A second type of unusual star system is the cD galaxy. Such galaxies are supergiant ellipticals that often occur at or near the centre of rich galactic clusters. They are thought to result from the merging, or fusion, of several galaxies that have collided. Active nuclei galaxies constitute one other notable class of unusual galaxies. Their central nuclei show evidence of spectacular or violent activity. These range from Seyfert galaxies to quasars, the latter generally believed to be exceedingly bright nuclei of very remote galaxies. Additional reading Edwin Hubble, The Realm of the Nebulae (1936, reprinted 1982), is a classic account of the early history of extragalactic research written by one of the principal investigators. Allan Sandage, The Hubble Atlas of Galaxies (1961), discusses galaxy classification and includes marvelous full-page illustrations of different types of galaxies. Allan Sandage, Mary Sandage, and Jerome Kristian, Galaxies and the Universe (1976, reprinted 1982), contains a comprehensive collection of review articles. Paul W. Hodge, Galaxies (1986), is a nonmathematical introduction. Paul W. Hodge (comp.), The Universe of Galaxies (1984), a collection of Scientific American articles, covers most topics of modern galactic research. Michael Rowan-Robinson, The Cosmological Distance Ladder (1985), elaborately and comprehensively reviews the distance problem. Sidney Van den Bergh and Christopher J. Pritchet (eds.), The Extragalactic Distance Scale (1988), collects technical symposium research papers of varying lengths. Paul W. Hodge