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The Coma Cluster of Galaxies
The Coma Cluster of Galaxies (also know as Abell 1656) is one of the densest clusters known— it contains thousands of galaxies. Each of these galaxies houses billions of stars— just as our own Milky Way Galaxy does. Although nearby when compared to most other clusters, light from the Coma Cluster still takes about 300 million years to reach us. The Coma Cluster is so big it takes light about 20 million years just to go from one side to the other! Most galaxies in Coma and other clusters are ellipticals, while most galaxies outside of clusters are spirals.
The Hubble Space Telescope took a close-up of the boxed area in the picture above.