The Andromeda Galaxy has been known about from ancient times, with the Persian astronomer Al-Sufi describing it as "little cloud" in his 964 AD work entitled "Book of Fixed Stars". Visible to the naked eye from a moderately dark sky site on a moonless night, its apparent size stretches across a full 3° of sky, or the span of 6 full moon diameters. Lying at a distance of about 2.5 million light years from Earth and traveling on an approach path towards our galaxy, Andromeda may collide with the Milky Way in about 3 billion years. If all goes according to prediction, about a billion years later the enormous forces twisting and tearing at the two galaxies will form them into a giant elliptical galaxy along with a lot of tidal debris.

In 1923 Edwin Hubble, using the Mount Wilson 100" reflector, observed a Cepheid variable star in M31 and was thus able to determine the intergalactic distance and true nature of M31 as a distinct galaxy apart from the Milky Way. Hubble's distance estimate was off by a factor greater than 2, an error not discovered until the 200" Palomar giant trained its large eye on M31 in the 1950's.

The above image shows M31 along with its companion dwarf elliptical galaxies, M 32 (NGC 221) left of the nucleus and M 110 (NGC 205) at a 5 o'clock position from the nucleus.