''M31: Andromeda Galaxy
The Andromeda Galaxy, also known as M31 (object 31 on the catalog of Messier) or NGC 224, is the other giant spiral galaxy in the Local Group, together with our galaxy, the Milky Way. It is at a distance of approximately 2.36 million light years or 725 kpc, in the direction of the constellation Andromeda.
With a mass of about 1.5 times more than the Milky Way, it is the dominant galaxy of the Local Group, which is formed by a couple dozen of small galaxies plus three large spirals: Andromeda, Milky Way and M33.
Projections indicate that the Andromeda Galaxy is on a collision course with the Milky Way, closing at a speed of about 140 kilometers per second. Impact is predicted in about 3 billion years; the two galaxies will probably merge to form a giant elliptical.
This galaxy plays an important role in galaxy studies, since it is the nearest giant spiral. Edwin Hubble identified extragalactic cepheid variable stars for the first time on astronomical photos of this galaxy, confirming the extragalactic nature of this "nebula," as it was previously called.
The Andromeda Galaxy was observed in 905 by the Arab astronomer 'Abd Al-Rahman Al Sufi, who described it as a "small cloud". The first description of the object based on telescopic observation was given by Simon Marius (1612), who is often wrongly credited as the discoverer of the Andromeda Galaxy.