Andromeda Galaxy, a spiral galaxy that is larger than the Milky Way (the galaxy to which Earth belongs) but similar to it in structure, and the closest to ours. (A galaxy is a large concentration of stars, dust, gas, and other material.) It is also known as M31. Because of the similarities between it and what astronomers have been able to map of the Milky Way, information obtained on the Andromeda galaxy helps us understand the structure of our own galaxy.

After the invention of the telescope in the 17th century, the Andromeda galaxy was called the Great Nebula of Andromeda (a name that is still sometimes used) and was thought to be a cloud of dust within the Milky Way. In the 19th century, some astronomers theorized that the Andromeda galaxy was not a dust cloud, but a huge group of stars so far away that telescopes of the time could not separate the individual stars. This theory was upheld in 1923 when Edwin Hubble, a United States astronomer, discovered some variable stars in the galaxys edges. In 1944, Walter Baade, a German astronomer who emigrated to the United States, was able to identify individual stars in its central portion.

The Andromeda galaxy is the only galaxy beyond the Milky Way that can be seen with the unaided eye, on fall and winter nights, from the middle latitudes of the Northern Hemisphere. It appears as a hazy spot in the constellation Andromeda, northwest of the Great Square of Pegasus.

The galaxy is about 2.5 million light-years from Earth. (A light-year is the distance that light travels in a vacuum in a yearabout 5.88 trillion miles [9.46 trillion km].) It is estimated to be about 160,000 light-years in diameter and to contain more than 2 trillion stars. The galaxy is wheel-shaped, but the view from Earth is nearly edge on, making it appear elongated.

The Andromeda and Milky Way galaxies are similar in some respects. They are members of the Local Group of Galaxies and comprise most of its mass. The Andromeda galaxys mass (amount of matter), comparable to the Milky Ways, is between hundreds of billions to over one trillion times the suns mass. The Andromeda is a spiral galaxy, like the Milky Way, with widespread projections of stars entwined around a distinct center. One difference is that the Andromeda galaxy is larger than the Milky Way and emits more light.

The galaxy is centered on a shining protuberance. A flat, winding disk, more than 200,000 light-years in diameter and about 1,000 light-years thick and a gigantic nimbus of material that reaches more than 1 million light-years surround it. The nimbus contains unidentified material, which is called dark matter. Dark matter makes up most of the galaxys mass. Within the nimbus, numerous satellite galaxies orbit the Andromeda galaxy. These galaxies are called dwarf galaxies. The nimbus also contains tidal streams or tidal tailsthe debris of dwarf galaxies torn apart by the Andromeda galaxys tidal forces (tidal force is the strain on a body caused by the difference in gravitational force on its sides). A supermassive black hole lies at the galaxys core (a black hole is a region in space that engulfs everything by virtue of its extremely powerful gravitational field). Its mass is about 140 million times that of the sun.

The Andromeda and Milky Way are being drawn together by their mutual gravitational pull. They might meet in a few billion years and form one colossal galaxy.