George Abell authored one of the most extensively used catalogs of galaxy clusters, which assigns clusters with a number and classifies them based on their visible characteristics. A cluster's richness indicates how many galaxies it contains. For example, for a cluster to be classified as "rich" it must have at least a few hundred galaxies. Clusters are considered "regular" if they are rich, spherical, and have a higher concentration of galaxies at the center. If a cluster does not match these characteristics it is called an "irregular" cluster.

The Coma Galaxy Cluster, listed as Abell 1656, is a regular cluster more than 350 million light years away from earth in the constellation of Coma Berenices, near the north galactic pole. It is estimated that over 1000 galaxies are part of the Coma Cluster, making it very rich. Coma is nearly spherical and has a one-megaparsec diameter consisting mainly of elliptical and highly-flattened eliptical galaxies. These galaxies have been estimated to be approximately 15 billion years old.

Coma was discovered to be an x-ray source in March 1969 by John Meekins et al. Two giant elliptical galaxies, NGC 4889 and NGC 4874, dominate the foreground of Coma. Like other galaxy clusters, Coma's center consists of a diffuse cloud of hot gas which emits x-rays and is held by the cluster's gravity. The gas, which is invisible to the naked eye, was discovered in 1971 through x-ray observation.