The mass-luminosity ratio is the ratio of the mass of a system, expressed in solar masses, to its visual luminosity, expressed in solar luminosities. The Milky Way Galaxy has a mass-luminosity ratio in its inner regions of 10, indicating that the typical star is a dwarf of mass about half that of the Sun. A rich cluster of galaxies (one containing many galaxies within a relatively compact region of space) such as the Coma Cluster has a mass-luminosity ratio of about 200, indicating the presence of a considerable amount of dark matter.