University of California, San Diego
Center for Astrophysics & Space Sciences

Gene Smith's Astronomy Tutorial

Clusters of Galaxies

Clusters of Galaxies
Most galaxies are found in gravitationally-bound groups called "clusters". Clusters can be rich, with several thousand galaxies, or poor, with only 20 or 30 members. The Local Group, the cluster to which our own Milky Way galaxy belongs, is made up of about 30 galaxies.

Clusters of galaxies are classified by their properties;richness (number of members),shape (spherical,flattened, or irregular),and galactic content (spiral-rich,spiral-poor, or elliptical-rich),for example. Some are strong radio sources, while others emit x-rays. The richest nearby cluster is Virgo, 60 million light years from the Milky Way. It contains about 2500 galaxies, mostly ellipticals.

The Local Group of Galaxies

We reside in a small group called the Local Group which is dominated by two giant spiral galaxies, Andromeda and our own Milky Way. In addition to Messier 33, an intermediate mass Sc galaxy, there are 15 ellipticals and 13 irregular galaxies in the cluster, including the Magellanic Clouds, our Galaxy's satellites, Messier 32 and NGC 205, satellites of Andromeda. The group has a size of about 3 million l.y., and has a total mass of 5 x 1012M

The Virgo Cluster

The Virgo Cluster, about 50 million l.y. away, is the nearest regular cluster of galaxies with several hundred members. Our Local Group is an outlying member of a "supercluster" of galaxies of which the Virgo Cluster is the dominant member.

The Hercules Cluster of Galaxies

The Hubble Space Telescope has provided the first opportunity to look back into the early universe at clusters. Billions of years ago, clusters contained many more spiral galaxies than they do today. They were probably disrupted over time by collisions and mergers within the clusters.

HST Image of CL0024+1654

CL 0024+1654 is a large cluster of galaxies located 5 billion light-years from Earth. It is distinctive because of its richness (large number of member galaxies), and its magnificent gravitational lens. The blue loops in the foreground are lensed images of a spiral galaxy located behind the cluster.

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Prof. H. E. (Gene) Smith
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Last updated: 26 April 1999