The Coma Cluster (Abell 1656), along with the Leo Cluster, is one of two major clusters compromising the Coma Supercluster. It contains over 1000 identified galaxies. Most of the galaxies in the center of the Coma Cluster are elliptical galaxies including both dwarf and giant. However the center is dominated by NGC 4874 and NGC 4889, two giant elliptical galaxies. The brightest galaxies are visible, a few degrees north of the galactic pole, with an amateur telescope larger than 20 cm. The visible galaxies have apparent magnitudes of 12-14. The mean distance of the cluster from Earth is 99 Mpc.
The galaxies within the Coma Cluster are overwhelmingly elliptical, only a few are spirals of a younger age; most of them are near the outskirts of the cluster. Although many of the galaxies were previously identified, the full extent of the cluster was really only revealed by the astronomers at Mount Palomar Observatory in the 1950’s.
Although poorly constrained, about 90% of the mass of the cluster is in the form of dark matter.
An X-ray source centered at 1300+28 in the direction of the Coma cluster of galaxies was reported before August 1966. The X-ray was observed by a balloon; however, it was unable to detect the source. That was in 1964. Later, the Uhuru satellite observed a strong source of X-rays close to the center of the coma cluster, garnering the name Coma X-1.