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galaxy clusters Most galaxies lie at great distances from the Earth and the Milky Way Galaxy, and tend to form groups or clusters held together by their mutual gravitational attraction. These groups or clusters are separated by voids characterized by relatively empty space apparently devoid of luminous matter. The Milky Way Galaxy is part of the Local Group, which also includes the large Andromeda Spiral galaxy, the Large and Small Magellanic Clouds, and about 20 smaller galaxies. Most of the galaxies in the Local Group are elliptical and dwarf irregular systems, and the diameter of the local group is about 1 megaparsec.
Several types of galaxy clusters exist. Regular clusters are spherical with a dense central core, and are classified based on how many galaxies reside within 1.5 megaparsecs of the cluster center. Most regular clusters have a radius of 1-10 megaparsecs and masses of 1015 solar masses. In some cases such as the Coma cluster, there may be thousands of galaxies within 1.5 megaparsecs of the center of the cluster, making this one of the densest large-scale regions in the universe.
Irregular clusters generally have slightly lower mass (~1012-1014 solar masses) than regular clusters
Cluster of galaxies called Abell 2218, taken by Hubble Space Telescope (NASA, A. Fruchter and the ERO Team, STScI, ST-ECF)
and have no well-defined center. An example of an irregular cluster is the Virgo cluster.
Galaxies and galaxy clusters are further grouped into even larger structures. Superclusters typically consist of groups or chains of clusters with masses of about 1016 solar masses. The Milky Way Galaxy is part of one supercluster, centered on the Virgo cluster, and has a size of about 15 megaparsecs. In contrast, the largest known superclusters, like that associated with the Coma cluster, are about 100 megaparsecs across. Recent advances in astronomers' ability to map the distribution of matter in space reveal that about 90 percent of all galaxies are located within a network of superclusters that permeates the known universe.
The largest-scale structures known in the universe include an understanding that these galaxy clusters and superclusters form a bubbly type of distribution of galaxies and clusters, with voids that are about 25 megaparsecs across separating sheets and filaments of galaxy clusters. The Great Wall is one such structure—a sheet of galaxies about 100 megaparsecs long, located about 100 megaparsecs from Earth.
Studies of the redshifts of galaxies from Earth reveal that many groups of galaxies on scales of 60 megaparsecs across are moving in a relatively coherent way, as if they are linked by some large-scale structure. Consistent with this idea is the determination that the Milky Way Galaxy, along with our local group, is moving at about 600 km/sec toward an area in space about 45 megaparsecs away in the Centau-rus Supercluster, known as the Great Attractor. The mass in this area is calculated to be in excess of 5 X 1016 solar masses, and is likely a diffuse concentration of matter about 400 million light years across, perhaps a supercluster itself.
See also astronomy; astrophysics; cosmology; galaxies.
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