The name given to a large swarm of galaxies connected to one another through mutual gravitational attraction. Clusters of galaxies can range from dozens to thousands of members, covering regions of space tens of millions of light years in diameter. The lower limit for membership of a cluster is about fifty galaxies; below this level, a collection of connected galaxies is usually known simply as a 'group'. Clusters themselves can combine to form even more immense structures known as superclusters.
Structure and Composition
In visible light, galaxy clusters appear as collections of individual galaxies, but in fact these visible galaxies form only a fraction of the mass of a typical cluster. Between the galaxies of the cluster is a region of gas known as the intracluster medium or ICM, accounting for about ten times the mass of the galaxies themselves. Though transparent to visible light, the energy of this medium is detectable in the X-ray spectrum.
Far more significant than either of these elements is dark matter, accounting for ten times again the combined mass of the galaxies and the ICM. This dark matter is currently impossible to detect by any direct means, but its gravitational effects on the cluster as a whole reveal its presence.
The Milky Way Galaxy is not considered to belong to a true galaxy cluster; it forms part of an aggregation of just over fifty galaxies termed the Local Group. The nearest galaxy cluster to the Milky Way is the Virgo Cluster, containing about two thousand individual galaxies and centred on a point about fifty million light years from the Milky Way. The Local Group is not entirely unconnected to the Virgo Cluster; it forms an outlying part a vast accumulation of clusters and groups known as the Virgo Supercluster.
On approximately the opposite side the Virgo Cluster from the Local Group is another cluster termed the Fornax Cluster, some sixty million light years from the Milky Way, and beyond that lies the Eridanus Cluster. These are minor groupings in comparison with the Virgo Cluster, containing only a few hundred galaxies each. The Ursa Major Cluster is also about sixty million light years from the Milky Way, and is divided into two distinct subgroups.
These are the major clusters within a hundred million light years of the Milky Way. In the same direction as the Virgo Cluster, but about 330 million light years from the Milky Way, is another large cluster termed the Coma Cluster, while at 500 million light years' distance is the Hercules Cluster. Beyond even these, the structures of clusters and superclusters of galaxies extend outward into the universe.
The term 'galaxy cluster' is not to be confused with 'galactic cluster', an entirely different concept despite the similarity of its name. A galactic cluster is a cluster of stars within a galaxy, of the type more usually referred to as an 'open cluster'.