Milky Way

Milky Way, spiral galaxy with a radius of about 50,000 light-years, containing some 100 billion stars. The Milky Way is shaped like a flat disk about 10,000 light-years thick in most places, about 30,000 light-years at the center. It is a modest-sized galaxy. Our solar system is in one of its spiral arms, just over 30,000 light-years from the galactic center. The galaxy rotates about a roughly spherical nucleus, the sun circling the galactic center once every 230 million years or so. The Milky Way is surrounded by a spheroidal halo some 165,000 light-years in diameter composed of gas, dust, occasional stars, and globular clusters. The name of the galaxy is derived from its appearance as a hazy, milk like band of stars in the night sky. Irregular dark patches are caused by intervening clouds of gas dust.

See also: Galaxy; Solar System.