Clusters of galaxies should not be confused with either star clusters, which are inside galaxies, or with globular clusters, which usually orbit galaxies.
Still larger than galaxy clusters are superclusters. A very large aggregation of galaxies known as the Great Attractor, dominated by the Norma Cluster, is massive enough to affect the local expansion of the universe (Hubble's law).
Notable galaxy clusters in the distant, high-redshift universe include SPT-CL J0546-5345, the most massive galaxy cluster ever found in the early universe.
Basic properties
Galaxy clusters typically have the following properties.
They contain 50 to 1,000 galaxies, hot X-ray emitting gas and large amounts of dark matter. Details are described in the "Composition" section.
The distribution of these three components is approximately the same in the cluster.
They have total masses of 1014 to 1015 solar masses.
They typically have a diameter from 2 to 10 megaparsecs.
The spread of velocities for the individual galaxies is about 800–1000 km/s.
The intracluster medium or ICM has gas between the galaxies with a temperature of 7-9 keV.
Composition
There are three main components of a galaxy cluster. They are:
Name of the components
Mass fraction
Description
Galaxies
1%
In optical observations only galaxies are visible
Gas between galaxies, inside the cluster
9%
Plasma between the galaxies at high temperature – emit x-ray radiation
Dark matter
90%
Most massive component, cannot be seen, inferred by gravitational interactions
