Andromeda << an DROM uh duh, >> Galaxy , also known as M31, is the closest large galaxy to our home galaxy, the Milky Way. It lies about 2.5 million light-years from Earth. A light-year is the distance that light travels in a vacuum in a year—about 5.88 trillion miles (9.46 trillion kilometers). The Andromeda Galaxy is one of the farthest objects visible to the unaided eye. It appears on fall and winter nights in the Northern Hemisphere, northwest of the Great Square of Pegasus in the constellation Andromeda.
The Andromeda Galaxy appears as a thin disk tilted to our line of sight. Like the Milky Way, it is a spiral galaxy, with sweeping arms of stars wrapped about a distinct center. The Andromeda Galaxy’s mass (amount of matter) measures from hundreds of billions to more than one trillion times that of the sun, comparable to the mass of the Milky Way. But the Andromeda Galaxy is larger and gives off more light than the Milky Way. These two galaxies account for the vast majority of the mass and light of the Local Group of galaxies. See Local Group .
The Andromeda Galaxy consists of a bright central bulge surrounded by a flat spiral disk and a vast halo of material. Its core contains a supermassive black hole. A black hole is a region of space whose gravitational pull is so strong that nothing can escape from it. This particular black hole has a mass about 140 million times that of the sun. The bulge extends thousands of light-years from the core. The disk measures more than 200,000 light-years in diameter but only about 1,000 light-years thick. The halo reaches more than 1 million light-years across. It contains most of the galaxy’s mass in the form of unidentified material called dark matter (see Dark matter ). Many small satellite galaxies called dwarf galaxies orbit the Andromeda Galaxy within the halo. The halo also contains tidal streams, or tidal tails, the remains of satellite galaxies shredded by the Andromeda Galaxy’s tidal forces. Tidal forces are strains caused by the uneven pull of gravity near a massive body.
Loading the player...Andromeda Galaxy
The mutual gravitational pull of the Andromeda Galaxy and the Milky Way is causing them to move toward each other. In a few billion years, they will probably collide and form a single large galaxy.
