The Coma Cluster
Image Credit: NASA, ESA, Hubble Heritage (STScI/AURA)
At a distance of about 99 Mpc from the Milky Way, the Coma Cluster is a major cluster containing over 1,000 identified galaxies. It is formally identified as Abell 1656. An even larger collection of galaxies referred to at the Coma Supercluster contains the Coma Cluster and the Leo Cluster along with others. Most of the galaxies in the Coma Cluster are elliptical galaxies, and its central region has two supergiant ellipticals with identifiers NGC 4874 and NGC 4889. There are also S0 galaxies but only a small number of spiral galaxies.
The above image is a mosaic of a small portion of Coma which was taken in 2006 by the Hubble Space Telescope to investigate how galaxies in rich clusters form and evolve. Most galaxies in Coma and other clusters are ellipticals, although some imaged here are clearly spirals. Almost every visible object in the picture is a galaxy.
The Coma cluster is significant historically as the first place where there was an indication of dark matter in addition to the calculated visible masses of the galaxies. In 1933 Fritz Zwicky used calculated masses and observed speeds to conclude that the galaxies were moving too fast to be accounted for by the visible masses of the galaxies.