galaxy cluster

A galaxy cluster (CL) is a group of on the order of hundreds to thousands of galaxies bound to each other by gravity. A smaller such group of galaxies is known as a galaxy group. The vast majority of galaxies are in such clusters and groups. The cluster often has an intergalactic medium (IGM) (aka intracluster medium, ICM) including plasma. The typical mass of a cluster is on the order of 10¹⁵ solar masses. A number of clusters grouped together is termed a supercluster. Shock waves from matter striking a galaxy cluster's ICM produce X-rays, which allow X-ray astronomical surveys to discover clusters, including very distant clusters, the furthest currently known (CL J1001+0220) being at redshift 2.5. Example galaxy clusters:

• Abell 1689
• Abell 85
• Coma Cluster
• Fornax Cluster
• Hercules Cluster (note there is also a Hercules stellar cluster)
• Perseus Cluster (Abell 426)
• Virgo Cluster
• Zwicky 3146

(Prominent galaxy clusters known before 1980 are generally listed in the Abell Catalog of galaxy clusters.) Shock can also result in a stronger magnetic field, resulting in synchrotron radiation in the radio range, producing various types of radio sources, i.e., radio relics, radio halos and/or radio phoenixes. The abbreviation CL is sometimes used in designators for galaxy clusters, e.g., for CL J1001+0220.

The term cluster can also refer to stellar clusters such as open clusters, sometimes confusingly called galactic clusters (meaning clusters within a galaxy, i.e., the Milky Way).