The Andromeda galaxy is the closest galaxy to our own Milky Way. It is also known as Messier 31 or M31. Some other synonyms for the Andromeda galaxy include NGC 224, GC 108, and PGC 2557. It is estimated to be around 2.5 million light-years away from Earth and is classified as a spiral galaxy. The Andromeda galaxy is also notable for being the most massive galaxy in the Local Group of galaxies, which includes the Milky Way and about 54 other smaller satellite galaxies. Its luminous center is powered by a supermassive black hole, and it is home to over a trillion stars.