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The Difference Between Clusters & Superclusters

Galaxies are not scattered randomly throughout the universe. Rather than being isolated islands in space, they are often found clumped together in clusters. In turn, these clusters may themselves be clumped into ̶0;clusters of clusters̶1; called superclusters. The most obvious difference between clusters and superclusters is simply one of scale, but there are also more subtle differences in terms of shape and dynamics.

Size
- The Local Group of galaxies, of which our own Milky Way is part, is too small to qualify as a galaxy cluster: It is about 3 million light years in diameter and contains only about 50 galaxies, most of which are dwarfs. More typical of a true cluster is the Virgo cluster, some 60 million light years from Earth. This contains over 2,000 galaxies within a diameter of 9 million light years. Both the Virgo cluster and the Local Group, together with about a hundred other clusters and groups, lie within the much larger Virgo supercluster, which is about 100 million light years across.
Shape
- Most galaxy clusters are shaped roughly like a three-dimensional ellipsoid. Some, like the Virgo cluster, have an irregular structure with no clearly defined center. Others are more regular, with a near-spherical shape and increasing density toward the center. A well-known example of the latter type is the Coma cluster, consisting of more than 3,000 galaxies in an approximately spherical volume 20 million light years across. In contrast to the rounded shape of clusters, superclusters are usually flatter and thinner, sometimes approximating a disk shape.
Dynamics
- One of the reasons for the difference in shape between clusters and superclusters is that clusters, being smaller, have had time to reach a stable state of equilibrium. They are bound together by gravity, just as individual galaxies are. This is not true of superclusters -- they are so enormous that there has not been enough time since the beginning of the universe for them to achieve equilibrium. As such, superclusters are effectively still in the process of formation.
Even Larger Scales
- Despite the enormous size of superclusters, they are not the largest structures in the universe. Aggregations of superclusters form vast sheets and filaments in space, interspersed by huge cosmic voids. The latter account for more than 90 percent of the volume of the universe, yet they are virtually empty of galaxies. The ̶0;walls̶1; between these voids are arguably the largest structures in existence. The Sloan Great Wall, for example, discovered in 2003 using data from the Sloan Digital Sky Survey, is estimated to be over a billion light years in size.

Astronomy