Group of galaxies which are mutually gravitationally bound. There are two important types of Concentration of 106 to 1012 stars, dust and gas, that are gravitationally bound. Our galaxy contains ~2 × 1011 stars. There are four main types of galaxies: • Elliptical
clusters: Regular and Irregular Galaxy Clusters. Regular clusters are huge spherical groupings of galaxies with large numbers of galaxies concentrated in their centers. They tend to contain thousands of galaxies and to have many bright elliptical and S0 type galaxies. Irregular clusters are not too centrally condensed with a somewhat non-spherical overall shape. They contain a few to hundreds of galaxies. Our Local Group (below) is an example of an irregular cluster of galaxies.
Some or all content above used with permission from J. H. Wittke.