Virgo Cluster

(astronomy) A cluster of galaxies which is the nearest to the galaxy that includes the sun; the cluster is centered in the constellation Virgo and is about 1.6 × 10⁷ light-years (1.51 × 10²³ m) from earth.

The nearest large cluster of galaxies, forming a conspicuous clump in the distribution of easily visible elliptical and spiral galaxies. The clump lies mostly in the constellation of Virgo but also extends into neighboring constellations, especially Coma Berenices. See also Constellation; Virgo.

Besides the 100 or so bright galaxies in Virgo, more than 2000 fainter galaxies show up in deeper searches of the cluster. Furthermore, the cluster includes the Southern Extension, where several dozen additional galaxies lie. G. de Vaucouleurs promoted the idea, now universally recognized, that the Virgo Cluster is so extensive in size that it includes a giant halo of many outlying groups, including the Local Group, to which the Milky Way Galaxy belongs. He called this structure the Local Supercluster. See also Local Group; Universe.

The Virgo Cluster is an example of a mixed-population cluster, with both early-type (elliptical and S0) and late-type (spiral and irregular) galaxies present. As the nearest large cluster, it provides a good testing ground for theories of how the dense cluster environment affects the properties of galaxies. The most obvious peculiarity of the Virgo galaxies is the limited extent of the disks of the spirals. The spiral galaxies have possibly been stripped of their outer gas by encounters with other galaxies in the cluster. A related phenomenon is the presence of anemic galaxies, spirals that have unusually low surface brightness and inconspicuous spiral arms.

Determining the distance to the Virgo Cluster has been an important task of twentieth-century astronomy. The Virgo Cluster forms an essential stepping stone for the cosmic distance scale, as it provides a connection between local distance criteria and the use of the Hubble law of gauging distances. The best measurements give a mean distance of 16 megaparsecs (5 × 10⁷ light-years). When the infall velocity of the Milky Way Galaxy toward the center of the Virgo Cluster is taken into account, this distance indicates that the local cosmic value of the Hubble constant, which relates expansion velocity to distance, is about 70 kilometers per second per megaparsec. See also Cosmology; Hubble constant.