Dr. N.L. Kachhara
Andromeda Galaxy as shown in fig 6.10 is the nearest spiral galaxy to our own Milky Way. It is approximately 2.5 million light years away in the constellation Andromeda.
Andromeda is the largest galaxy of the Local Group, which consists of the Andromeda Galaxy, The Milky Way Galaxy, the Triangulum Galaxy, and about 30 other smaller galaxies. Although the largest, it may not be the most massive, as recent findings suggest that the Milky Way contains more dark matter and may be the most massive in the grouping. Andromeda Galaxy contains about one trillion (1012) stars, greatly exceeding the number of stars in our own galaxy. It is easily visible to the naked eye; the full angular diameter of the galaxy is seven times that of the full moon. The diameter and mass of Andromeda Galaxy is estimated to be 220000 lights years and 1.23x1012 solar masses. Spectroscopic studies have provided detailed measurements of the rotational velocity of this galaxy at various radii from the core. The velocity is high at a reaches of 1300 light years. As we move out it decreases then increases and decreases again. These velocity measurements imply a concentrated mass of about 6x109 solar masses in the nucleus. M31 is known to harbour a dense and compact star cluster at its center. The nucleus is in fact double, consisting of two concentrations separated by 1.5 parsecs, a few light years. The brighter concentration, P1, is offset from the center of galaxy. The dimmer concentration, P2, falls at the true center of the galaxy and contains a 108 solar mass black hole. Like the Milky Way, Andromeda galaxy has satellite galaxies, consisting of 14 known dwarf galaxies. The best known and most readily observed satellite galaxies are M32 and M110.
Current measurements suggest that Andromeda Galaxy is approaching us at 100 to 140 kilometer per second. The Milky Way may collide with it in 3 to 4 billion years, depending on the importance of unknown lateral components to the galaxies' relative motion. It they collide, it is thought that the Sun and the other stars in the Milky Way will probably not collide with the stars of the Andromeda Galaxy, but that the two galaxies will merge to form a single elliptical galaxy over the course of about a billion years.
