A group of galaxies containing many hundreds of members extending over a radius of up to a few megaparsecs (there also exist small groups of galaxies, such as the Local Group, with a few tens of members). The richest and most regular clusters, such as the Coma cluster, with thousands of members, are gravitationally bound systems; it is not certain whether other less regular and less concentrated clusters are also bound. As well as galaxies, the clusters contain hot intracluster gas, at temperatures between 10⁷ and 10⁸ K; this can be detected by its X-ray emission. On a scale larger than clusters there are also superclusters, with extents of the order of a hundred megaparsecs, containing about a hundred galaxies. It is not known whether superclusters are gravitationally bound. See also missing mass.