The Hercules cluster, Abell 2151 is the dominant galaxy of the Hercules supercluster which contains 12 Abell galaxy clusters and spans some 6 degrees of sky. The Hercules supercluster was first recognized by Harlow Shapely in the 1930's. The supercluster is split into a northern and southern (larger) superclusters which are physically connected by a bridge of galaxies and reside at distances of 400 and 500 million light years respectively. Abell 2151 is the richest cluster within the supercluster in terms of galactic members, especially large spirals which make up 50% of the cluster. Abell 2151 is itself made of subclusters designated N, C, and E. The bumpy distribution of intracluster gas and presence of distinct subclusters suggest that A2151 is relatively unevolved and is still undergoing collapse and coalescence. Current thinking is that galaxy clusters form from large scale mergers of smaller subclusters and groups. The mergers are responsible for changes in the physical dynamics and structure of the clusters and intercluster medium.
The Hercules cluster (A 2151) and Hercules supercluster are part of a huge sheetlike megastructure of galaxies known as the "Great Wall". The Great Wall also includes the Coma and Leo clusters and extends along a filament some 500 million light years in length terminating with the Hercules cluster at one end. The Great Wall was the first megastructure of its kind discovered by the American astronomers Margaret Geller and John Huchra in the 1980's. Many more wall structures are now known to exist. Superclusters are the largest gravitationally bound structures known in the cosmos. They in turn are comprised of smaller galaxy clusters and groups. Superclusters can reach enormous sizes up to several hundred million light years and are often juxtaposed against large voids in space where few galaxies exist. Their existence indicates that galaxies in the universe are not evenly distributed but are arranged in large coherent structures of clusters, superclusters, walls and filaments. Astronomers estimate that approximately 10 million superclusters may exist in the observable universe.
An extensive catalogue of galaxy clusters was compiled by the American astronomer George O. Abell (1927-1983). He published his original catalogue in 1958 which was later completed posthumously in 1987 and includes approximately 4000 clusters out to a redshift of Z=0.2.