The Andromeda Galaxy /ænˈdrɒmɨdə/ is a spiral galaxy approximately 2.5 million light-years (2.4×10¹⁹ km) from Earth in theAndromeda constellation. Also known as Messier 31, M31, or NGC 224, it is often referred to as the Great Andromeda Nebula in older texts. The Andromeda Galaxy is the nearest spiral galaxy to our Milky Way galaxy, but not the nearest galaxy overall. It gets its name from the area of the sky in which it appears, the constellation of Andromeda, which was named after the mythological princess Andromeda. The Andromeda Galaxy is the largest galaxy of the Local Group, which also contains the Milky Way, the Triangulum Galaxy, and about 30 other smaller galaxies.

The Andromeda Galaxy is probably the most massive galaxy in the Local Group as well,^[7] despite earlier findings that suggested that the Milky Way contains more dark matter and could be the most massive in the grouping. The 2006 observations by the Spitzer Space Telescope revealed that M31 contains one trillion (10¹²) stars: at least twice the number of stars in the Milky Way galaxy, which is estimated to be 200–400 billion.