Messier 31: The Andromeda Galaxy


The Andromeda Galaxy is a spiral galaxy containing a trillion stars (about twice as many as our Milky Way). At 2.2 million light years away it's the most distant object visible to the naked eye, should you have favourable sky conditions.

The two galaxies, Andromeda and the Milky Way, are slowly approaching, at 110 kilometres/second (68 miles/sec) which means that in around four billion years the two will collide. In fact M31 was itself formed just this way; around 10 billion years ago smaller galaxies collided and merged, eventually producing the Andromeda Galaxy.

The galaxy is found just west of nu Andromedae and you should have easily found it by following the instructions found at Mirach.

The Andromeda Galaxy is best seen either with binoculars or with a low magnification telescope, when its full dimension of over four degrees can be seen. Even if you won't see the magnificent details that can only be discerned by means of astrophotography, it's a marvellous site in large binoculars (at least 80mm) which bring out the dust lanes.

M31 contains many smaller galaxies, termed 'dwarf galaxies'. A medium-sized telescope will bring out M32 and M110, both close by. M32 is thought to have already collided with M31 in the past, and much of its mass may have been absorbed by the larger galaxy. There is a massive black hole at the centre of M32.

M110 may presently be in contact with M31 with some of its mass being absorbed by M31. Both M32 and M110 are - or have been - active in forming new stars.


Finally this constellation is resident to one of the closest stars to our solar system, at 11.64 light years away. To find the star could be a challenge; place M31 lower southeast in your view, then move one binocular field northwest; Groombridge 34 is just above the bright star 26 Andromedae.

Groombridge 34 is even more interesting in that it's a double star of two red dwarfs orbiting a common centre of mass with a period of 2600 years. There is also a second component (C) which forms a visual binary with the other two but is not part of the system. The two components, compared with the primary, offer the following data: AB: 8.3, 11.4; 65, 34.8" and AC: 8.3, 11.8; 171, 62.2".

26 Andromedae is a wide binocular double: 6.1, 7.0; 275, 38".

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1999-2014 by Richard Dibon-Smith.