The Andromeda galaxy is a spiral galaxy classified as type SA (s) b in the Hubble sequence, with a diameter of 220,000 light years it is the largest galaxy of the local group, and is the most distant object visible to the naked eye from the earth.
The first written observation of the Andromeda Galaxy dates back to 964 and was conducted by the Persian astronomer Abd al-Rahmān al-Sūfi, who described it as a "small cloud" in his Book of fixed stars. The first description of the object based on telescopic observations was made by Simon Marius on December 15, 1612 who defined it as "the light of a candle observed through a translucent horn". Charles Messier later inserted it in his famous catalog with the number 31 in the year 1764. Estimates of the mass of the Andromeda Galaxy, including dark matter, give a mass value including dark matter less than that of the Milky Way despite its dimensions are higher, but in fact M31 contains many more stars than the Milky Way and has a considerably larger diameter. The Andromeda Galaxy is approaching the Milky Way at a speed of about 400,000 km / h, therefore it is one of the few galaxies to show a blue shift; given the movement of the Sun within our Galaxy, it is found that the two galaxies are approaching at a speed of 100–140 km / s. The two galaxies could thus collide in an estimated time of 2.5 billion years and in that case they will probably merge, giving rise to an elliptical galaxy of large proportions. However, the tangential velocity of M31 with respect to the Milky Way is not well known, thus creating uncertainty as to when the collision will occur and how it will proceed. The Andromeda Galaxy hosts a very dense and compact cluster of stars in its real center. In large telescopes it is possible to observe the stars immersed in the surrounding diffuse bulge. The brightness of the nucleus exceeds that of the brightest globular clusters. In 1991, studying the images obtained with the Hubble Space Telescope of the innermost regions of the nucleus, it was discovered that the galaxy hosts a double nucleus, formed by two concentrations separated by about 5 light years. The brightest concentration, cataloged as P1, is off-center from the true galactic center, while the smaller concentration, P2, falls exactly in the center and contains a black hole of 108 solar masses. The spiral arms of the Andromeda Galaxy are marked by a series of H II regions that Baade described as a row of beads. They appear very frequent, although they are more separate from each other and less frequent than in our Galaxy. The rectified images of the galaxy show a common spiral galaxy with arms wound clockwise; there are continuous major arms separated from each other by a minimum of 13 000 light years and can be followed from the outside up to a distance of about 1 600 light years from the core; this can be seen by the displacement of neutral hydrogen clouds from the stars. Associated with M31 there are about 460 globular clusters. The most massive of these, cataloged as Mayall II, is nicknamed "Globular One" and has a higher luminosity than any other known globular cluster in the Local Group of galaxies. Mayall II contains a few million stars and is twice as bright as Omega Centauri, the brightest globular cluster known in the Milky Way. Like the Milky Way, the Andromeda Galaxy also has a system of satellite galaxies consisting of 14 known dwarf galaxies and the best known and easiest to observe are M32 and M110.