The Messier catalog number of the Andromeda Galaxy is M31.
M31 is also known as NGC 224 and is the nearest major galaxy to the Milky Way. It is a barred spiral galaxy in the constellation Andromeda and one of the largest members of the Local Group, the collection of galaxies that includes the Milky Way, the Triangulum Galaxy and many dwarf galaxies.
French astronomer Charles Messier cataloged the object as number 31 in 1764. The catalogue was originally created to identify fuzzy objects that could be mistaken for comets, but many of its entries turned out to be galaxies, nebulae and star clusters. A common mix-up is M32: that is a smaller elliptical galaxy orbiting Andromeda, not Andromeda itself. M33 is the separate Triangulum Galaxy, while M101 is the Pinwheel Galaxy. Under dark skies, M31 can be seen without a telescope as a faint, hazy patch.