M 31

The Andromeda Galaxy (/ænˈdrɒmədə/) is a spiral galaxy approximately 2.5 million light-years (2.4 × 1019 km) from Earth[4] in the Andromeda 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 galaxy (Milky Way), but not the closest 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 our galaxy (Milky Way), the Triangulum Galaxy, and about 30 other smaller galaxies. Although the largest, the Andromeda Galaxy may not be the most massive, as recent findings suggest that the Milky Way contains more dark matter and could be the most massive in the grouping.[11] The 2006 observations by the Spitzer Space Telescope revealed that M31 contains one trillion (1012) stars:[8] at least twice the number of stars in our own galaxy, which is estimated to be 200–400 billion.[12]