M31 (Hubble class SA(s)b) is our neighbourhood big galaxy in the Local Group and lies a tad over 2.54 million l.y. away from us. Though the galaxy is slightly larger than us in lineal terms (220Kly vs 190Kly), both galaxies are around about the same mass (about 1.2 trillion solar masses). Although, it does have considerably more stars, at present (1 trillion vs 400-500 billion, or so). This overabundance of stars is due to a collision that occurred around 3-4Ga ago, when M32 (ngc221), the small oval on M31's upper flank, collided with M31 and was stripped of most of its stars and mass. Originally the third largest member of the Local Group, M32 collided head on with M31 and triggered a massive starburst, which added around 40-50% of M31's current stellar population and puffed the galaxy up to its present lineal size. In the last 2Ga, star formation has tailed off in M31 to the point of becoming nearly inactive. The galaxy is also brighter than ours, with an absolute mag of -21.52 vs -20.9 for our galaxy. However, the Milky Way is producing considerably more stars than M31, 3-5 solar masses per year c.f. 1 solar mass per year. At this rate, the MW will eventually overtake M31 in luminosity in around 1.5Ga.

Both galaxies have supermassive black holes in their centres, though M31's is much larger than ours (110-230 million solar masses vs 4.1 million solar masses). They also have vast intergalactic halos containing many satellite galaxies, M32 and M110 being the most prominent satellites of M31. M110 (ngc225), the bright oval to M31's lower right, is a dwarf elliptical galaxy of Hubble class dE5pec. The "pec" designation (peculiar) being for the presence of young, bright blue stars and interstellar dust and gas near the galaxy's nucleus, which is unusual in elliptical galaxies. At its distance from us, being about the same as M31, its apparent size of 21.9 arcminutes gives a true diameter of 16181l.y. (4.963Kpc). Unlike M32, M110 doesn't appear to have a supermassive black hole in its nucleus.

M31 is approaching our galaxy at a velocity of around 110km/sec, so that in about 4Ga, both galaxy will begin to collide with one another. Over a period of several hundred million to a billion years, both galaxies will pull one another apart and merge into either a large elliptical or lenticular galaxy. At that time, a massive starburst will occur in the colliding system with much of the rest of the gas within the system being used up in that starburst. What will happen to our solar system is unknown, but it's possible that the interaction may eject the solar system from the Milky Way and/or we may end up joining the remnant of M31 as it merges with the remnant of the Milky Way. Although, it's more likely we will stay within the Milky Way. Astronomers have given the merged system the rather tacky name of the "Milkomeda" Galaxy

Pic taken with t20 (iTelescope, NMSO) 1 hour (10x3min L, 5x2min each RGB).