D. The Clustering of Galaxies
Like stars, galaxies often appear together in groups and clusters. Groups may consist of a few galaxies and are often a part of larger galaxy clusters. Galaxies also often have small companion galaxies. Our Milky Way Galaxy is accompanied by the Large and Small Magellanic Clouds, which are both irregular galaxies visible from the southern hemisphere. The Andromeda Galaxy has two small companion elliptical galaxies, M32 and M110.
The Milky Way and our nearest neighbor galaxies form a collection of galaxies called the Local Group, which consists of about two dozen galaxies of various types - spiral, elliptical, and irregular. The nearest large cluster of galaxies is the Virgo Cluster. It covers a region in the sky about six degrees across in the constellation Virgo. It consists of over one hundred galaxies of many types, including spiral, elliptical, and irregular galaxies. The center of the Virgo Cluster is twenty million parsecs from Earth. Other clusters are farther, and some have asymmetric distribution of galaxies.
Some clusters are members of superclusters. The Local Group and Virgo Cluster are part of a supercluster that contains one hundred other clusters and is one hundred megaparsecs across. Superclusters are connected by lines (also referred to as filaments) of galaxies or clusters that run outside regions that do not have any galaxies (called voids). The study of these large structures in the Universe provides astronomers with observations that can be used to test their models in understanding how these structures form. Different models of how structure arises give different maps of the Universe.