The Andromeda galaxy is the nearest spiral galaxy to the Milky Way, our galaxy. Andromeda is sometimes called Messier 31, M31 or NGC 224 by astronomers. It is about 2.6 million light years away from us. In other words, we see it as it was 2.6 million years ago.

The galaxy looks like a dim, fuzzy star to the naked eye: We can only see the central part through a telescope.

Andromeda is named after the princess in Greek mythology called Andromeda (Ανδρομέδα).

Andromeda is the largest galaxy of the Local Group, which consists of the Andromeda galaxy, the Milky Way galaxy, the Triangulum galaxy, and about 30 other smaller galaxies. Although the largest, Andromeda may not be the most massive. Recent findings suggest that the Milky Way contains more dark matter and may be the most massive in the group.

The 2006 observations by the Spitzer Space Telescope revealed that M31 contains a trillion stars (1012). This is more than the number of stars in our own galaxy, which is estimated to be c. 200-400 billion.

Andromeda is estimated to be 7.1×1011 solar masses. In comparison, a 2009 study estimated that the Milky Way and Andromeda are about equal in mass,

At an apparent magnitude of 3.4, the Andromeda galaxy is notable for being one of the brightest Messier objects. It visible to the naked eye on moonless nights even when viewed from areas with moderate light pollution. Although it appears more than six times as wide as the full Moon when photographed by a larger telescope, only the brighter central region is visible to the naked eye. Being both large and bright, it is one of the farthest objects that can be seen without a telescope or binoculars.

The Andromeda galaxy is approaching the Milky Way at about 100 to 140 kilometres per second (62 to 87 mi/s), so it is one of the few blue shifted galaxies. The Andromeda galaxy and the Milky Way are thus expected to collide in perhaps 4.5 billion years. The Milky Way–Andromeda collision is likely to make the galaxies merge, to form a giant elliptical galaxy. Such events do happen in galaxy groups. However, 4.5 billion years is a vast period of time, roughly equal to the time the Earth has existed as a planet.