In 1958 the great extragalactic astronomer George Abell of UCLA compiled a catalog of galaxy clusters. It was extended by Abell and collaborators in 1987 and eventually contained 4,076 clusters.
Abell 2256 is a rich cluster lying in Ursa Minor. It contains more than 500 members and lies about 800 million light-years away. The cluster spans some 10 million light-years, about the same diameter as our Local Group of galaxies, which contains fewer than 100 galaxies.
The brightest galaxy in Abell 2256, NGC 6331, glows meekly at magnitude 15.4. The second brightest member is the slightly fainter UGC 10726. Overall, the cluster spans some 140’, but most of the galaxies lie within a central core.
The cluster has been studied by Chandra and other instruments and is notable for three x-ray emitting subclusters. Aside from the main cluster, there is the remnant of an older merger, and also a bright, bullet-like system of in falling gas.
