Galaxies are not evenly distributed throughout the universe. Gravity tends to make galaxies clump together. Small clumps of galaxies are called “groups.” Our galaxy, the Milky Way, belongs to a group that we call the Local Group; it contains about 30 known galaxies (although there may be several undiscovered dwarf galaxies in the group, so the actual number is not known).
Galaxy clusters typically contain hundreds or even thousands of galaxies. The picture at the right shows a famous cluster called Abell 957. The cluster is named after George Abell, an American astronomer who published a catalog of galaxy clusters in 1958.
Think about how you know which galaxies are part of Abell 957, and which are just other galaxies at different distances in the same part of the sky. Click on ten or so galaxies that you think are part of the cluster, both spirals and ellipticals, and save them in your notebook. How are these galaxies similar? How are they different?