|Dwarf spheroidal galaxies
A subtype of dwarf ellipticals (dwarf elliptical galaxy), which are companion to the Milky Way and other similar galaxies. The first example of such objects was discovered by Harlow Shapley (1938) in the constellation Sculptor. 22 such galaxies are known currently to orbit the Milky Way and at least 36 exist in the Local Group of galaxies. Nearby galaxy clusters such as the Virgo, Fornax, Centaurus, and Coma clusters contain hundreds to thousands of individual dSph galaxies. These galaxies have very low surface brightnesses, as low as only 1% that of the sky background. They are also among the smallest, least luminous galaxies known. Most of the radiation from dSph galaxies is emitted by stars in the optical portion of the electromagnetic spectrum. The lack of strong emission lines, infrared, or radio emission suggests that these galaxies are generally devoid of interstellar medium. The velocities of stars within dSph galaxies are so high that them must be disrupting. However, the bulk of mass in these galaxies might be undetected. Dynamical models that include dark matter do adequately explain the velocity dispersion of the stars in all dSph systems. In the most extreme cases, only 1% of the mass of the galaxy is visible. Many of the Local Group dSph galaxies show evidence for star formation more recent than 10 Gyr.