An aggregation of galaxies, which may or may not be bound together by gravity. For example, our Galaxy, the Milky Way, is a member of the Local Group, a rather small cluster of which the only other large member is the Andromeda Galaxy. At the other extreme are the Abell clusters which contain many hundreds or even thousands of galaxies in a region just a few million light years across; prominent nearby examples are the Virgo and Coma Clusters. Between these two extremes, galaxies appear to be clustered in systems of varying density. The densest Abell clusters are held together by their own self-gravity. Such rich clusters are filled with X-ray emitting gas, at temperatures of up to 10⁸ K, and tend to have giant elliptical galaxies at their centre. The less rich and more extended systems may not be bound by gravity. See also large-scale structure.