Galaxy groups and clusters are the largest
gravitationally-bound objects. They form the densest part of the
large scale structure of the Universe. In models for the gravitational formation of structure with
cold dark matter, the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain from ten to thousands of galaxies. The clusters themselves are often associated with larger groups called
superclusters.
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. Collections of galaxies which are held together by the force of their mutual gravity. Small clusters, with perhaps a few dozen members, are known as groups. Our own galaxy, the Milky Way, is a member of a galaxy group known as the
Local Group . The Andromeda galaxy (Messier 31) is also a member of this Local Group. Clusters contain hundreds or thousands of galaxies and, typically, have diameters of a few tens of millions of light years. Different clusters have different shapes, although two broad classifications exist. Spherical clusters contain predominantly elliptical galaxies and often have a
cD galaxy at their centre. It is thought that many galactic mergers have taken place with these clusters. Irregular clusters, in which the galaxies do not come into close contact with one another, contain predominantly spiral galaxies. In between the galaxies, the intergalactic medium is composed of very tenuous but very hot gas. This gas is detectable because it emits X-rays.
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