Groups of galaxies (termed clusters—not to be confused with the globular and open clusters of stars that occur within galaxies) are major features of the universe. Their sizes vary enormously, with some galactic clusters containing only a few galaxies and others with several thousands. Moreover, the galactic clusters themselves are part of still larger clusters of clusters—superclusters. Most astronomers believe superclusters to be the universe’s largest units. But some astronomers believe that even clusters of superclusters are possible, forming a hierarchical organization of increasingly larger units.
Much of our knowledge about galactic clusters comes from a pioneering 12-year study by C. Donald Shane and Carl Wirtanen at the Lick Observatory near San Francisco, California. They divided the sky into a number of areas (cells) and counted the galaxies above a certain magnitude in each. At the end of the survey (which involved counting more than 1 million galaxies) they found that some cells contained 10 or more whereas others had none; the average was one galaxy per cell. They also compiled a galaxy distribution map, which showed that, at least to an average distance of about 400 million parsecs, galaxies are grouped into clusters of various shapes.
The Local Group
Our own galaxy—the Milky Way—is part of a galactic cluster generally known as the Local Group. It contains about 32 galaxies and extends across a spherical volume of space 1 million parsecs in diameter. The Local Group is a fairly typical galactic cluster in terms of the types of galaxies it contains: there are three large spiral galaxies—the Milky Way, the Andromeda Spiral (M31), and the Triangulum Spiral (M33), which are between 15,000 and 50,000 parsecs across; four medium-sized, irregular galaxies with diameters between 3,000 and 12,000 parsecs, including the Large and Small Magellanic clouds (also called Nebecula Major and Minor); and about 25 elliptical galaxies, most of which are relatively small (four are regular ellipticals with diameters between 2,000 and 5,000 parsecs, whereas the remainder are dwarf ellipticals, mostly less than 2,000 parsecs across). Two of the regular elliptical galaxies are companions of Andromeda.
The largest member of the Local Group is the Andromeda Spiral, followed by the Milky Way, the Triangulum Spiral, and the Clouds of Magellan. Of these, only Andromeda and the Clouds of Magellan can be completely seen from the earth with the naked eye—and the Clouds are visible only from the Southern Hemisphere.
Two other galaxies are sometimes also included in the Local Group—Maffei I, an elliptical galaxy, and Maffei II, a spiral. Extremely difficult to observe by optical means because they are obscured by dust, they were not discovered until 1968 (by the Italian astronomer Paolo Maffei). More recent studies indicate, however, that they are not actually in the Local Group, but are part of a neighboring galactic cluster.
Other galactic clusters
Millions of galaxies have been detected in our part of the universe, and many are parts of clusters. Some clusters are extremely rich in galaxies—the larger ones may have as many as 10,000 members. In the immediate vicinity of the Local Group are several smaller clusters, each containing about 12 galaxies. The nearest one that is rich in galaxies is the Virgo cluster (in the constellation Virgo), about 15,000,000 to 20,000,000 parsecs distant from the earth. It is large (about 2 million parsecs across) and contains thousands of galaxies of all types. Other notable rich clusters include the Coma Cluster (in the constellation Coma Berenices) and the Perseus Cluster (in Perseus). All three emit X rays, a phenomenon that some astronomers believe to be characteristic of rich clusters. In addition, the galaxies in the Coma Cluster are concentrated in its center—another feature common to many clusters.
It is now thought that the Local Group, the small neighboring clusters, and the Virgo Cluster form part of a much larger cluster of clusters—called the Local Supercluster. It is believed to contain about a hundred individual galactic clusters and to be about f 00 million parsecs in diameter.
Explaining galactic groups
Since the birth of the universe in the big bang and the subsequent evolution of galactic systems, the galaxies have been—and still are—receding from each other. Despite this general movement, galaxies remain in clusters, each of which seems to be reasonably stable. This relative stability results from the mutual gravitational attraction that binds the galaxies within each cluster, although each galaxy is still sufficiently free to move within its cluster. In the Local Group, the Andromeda Spiral is moving toward the Milky Way.
Starting from the assumption that clusters are stable, it is possible to calculate the gravitational force necessary to maintain that stability, which can, in turn, be used to calculate the total mass of the cluster. But the results of such calculations reveal an interesting anomaly: the galactic clusters appear to contain less mass in the galaxies seen within them than is necessary to maintain their stability. The Virgo Cluster, for example, is observed to have 50 times less mass than it should have. This phenomenon—known as the missing mass problem—has not yet been satisfactorily explained, although one possible reason is that the galaxies within each cluster are still settling down after their formation, and so the clusters are not really stable. Alternatively, there may not be any missing mass because there may exist between the galaxies large amounts of dark, invisible matter (such as dust or gas) that has not yet been detected.