Infrared image of the Andromeda Galaxy (M31) recorded by the Infrared Astronomical Satellite (IRAS). Located at a distance of 2.2 million light years, M31 is the closest major galaxy to our own Milky Way. It is a member of our Local Group of galaxies, which occupies a region of space nearly five million light years across. The existence of M31 was realised in 964 A.D. by the Persian Astronomer Al-Sufi, who recorded it as a little cloud"; its nature as a galaxy was not defined until this century. The Andromeda Galaxy's diameter is almost 150,000 light years, and it contains approximately 300,000 million stars."