GROUPS AND CLUSTERS OF GALAXIESAlthough isolated galaxies exist, many galaxies occur in groups or clusters. Groups are defined
as assemblages of a few (3 to 50 or so), while clusters are more populous, some having many thousands of members. The Milky Way galaxy is part of a small group called the Local Group of galaxies, which has some 30 other members, including the Magellanic Clouds and the Andromeda galaxy. Three of the members are spirals, 13 are dwarf ellipticals, and the rest are irregulars. Some of the latter, such as the Phoenix dwarf, are hard to classify, being intermediate in character between the irregular and the elliptical classes.The Local Group is particularly important for several reasons. Because its members are all close by, it is the source of much of the information known about the details of galaxy structure and content. It is also useful for the analysis of group dynamics. For other groups and clusters, astronomers can measure only one component of member velocities (the component in the line of sight, called the radial velocity) and they have a map of positions in only two dimensions (the plane of the sky). For Local Group members a three-dimensional map is available, and astronomers can measure transverse as well as radial velocities of the nearest members. Thus it is possible to work out the dynamics of the group in detail and to study its stability, its past history, and its probable future.The nearest large cluster of galaxies is the Virgo cluster, whose extent in the sky is more than 12 degrees. It has a sufficiently large mass to affect the motion of our Local Group, which is falling into it with a velocity of more than 200 km/sec. The cluster has at least 2,500 member galaxies and probably more. Its radial velocity with respect to the Earth is 1,150 km/sec. Some members, falling in toward its center from the far side, are moving so fast that their velocity of approach is larger than the cluster's bulk cosmic velocity of recession.Fairly early in the study of galaxy clusters, Shapley and others recognized that these are not the largest structures in the universe. Later work demonstrated the existence of what came to be called superclusters, which consist of several loosely connected clusters often occupying volumes as large as 200 to 300 million light-years in diameter. Even larger complexes seem to exist in certain areas. For example, the motions of nearby galaxies appear to be influenced by a gravitational attraction in a particular direction toward an accumulation of galaxies termed "the Great Attractor."When it became possible to plot out the three-dimensional distribution of galaxies, other features of their distribution in space became apparent. Large-scale surveys of galaxy red shifts carried out by Margaret Geller, John Huchra, and colleagues showed that some areas of space are relatively empty. Called voids, they can be as large as superclusters. Other areas have high densities of galaxies arranged in what are called walls. The general distribution of galaxies shows a high degree of organization, which is an important clue to the large-scale evolution of the universe.