Although a number of minor galaxies lodge closer to our Milky Way galaxy, the Andromeda galaxy is the closest major galaxy. Excluding the Large and Small Magellanic Clouds (which we can’t see from our northerly latitudes), the Andromeda galaxy is the brightest galaxy in all the heavens. It’s the most distant thing you can see with your unaided eye, a smudge of light larger than a full moon.
The Andromeda galaxy and our Milky Way galaxy reign as the two most massive and dominant galaxies within the Local Group of Galaxies. The Andromeda Galaxy is the largest galaxy of the Local Group, which, in addition to the Milky Way, also contains the Triangulum Galaxy, and about 30 other smaller galaxies. Both the Milky Way and the Andromeda galaxies lay claim to about a dozen satellite galaxies. Both are some 100,000 light-years across, containing enough mass to make billions of stars.
At one time, the Great Andromeda galaxy was called the Great Andromeda nebula. Astronomers thought this patch of light was composed of glowing gases, or was perhaps a solar system in the process of formation. It wasn’t until the 20th century that astronomers were able to resolve the Andromeda spiral nebula into individual stars.
This discovery lead to a controversy about whether the Andromeda spiral nebula and other spiral nebulae lie within or outside the Milky Way. In the 1920′s Edwin Hubble finally put the matter to rest, when he used Cepheid variable stars within the Andromeda galaxy to determine that it is indeed an island universe residing beyond the bounds of the Milky Way galaxy.
How to find the Andromeda Galaxy. From mid-northern latitudes, you can see M31 – also called the Andromeda galaxy – for at least part of every night, all year long. But most people see the galaxy first in autumn, when it’s high enough in the sky to be seen from nightfall till daybreak. In late September and early October, the Andromeda galaxy shines in your eastern sky at nightfall, swings high overhead around midnight (1 a.m. daylight saving time) and stands rather high in the west at the onset of morning dawn. Winter evenings are also good for viewing the Andromeda galaxy.
If you are far from city lights, and it’s a moonless night – and you’re looking on an autumn or winter evening – it’s possible you’ll simply notice the galaxy in your night sky. It’s looks like a hazy patch in the sky, as wide across as a full moon.
But if you look, and don’t see the galaxy – yet you know you’re looking at a time when it’s above the horizon – you can star-hop to find the galaxy. You’ll be hopping to the Andromeda galaxy from the Great Square of Pegasus. In autumn, the Great Square of Pegasus looks like a great big baseball diamond in the eastern sky. Envision the bottom star of the Square’s four stars as home plate, then draw an imaginary line from the “first base” star though the “third base” star to locate two streamers of stars flying away from the Great Square. These stars belong to the constellation Andromeda the Princess.
On each streamer, go two stars north (left) of the third base star, locating the stars Mirach and Mu Andromedae. Draw a line from Mirach through Mu Andromedae, going twice the Mirach/Mu Andromedae distance. You’ve just landed on the Andromeda galaxy, which looks like a smudge of light to the unaided eye. If you can’t see the Andromeda galaxy with the eye alone, by all means use binoculars.
At 2.3 million light-years, the Great Andromeda galaxy (Messier 31) rates as one of the most distant objects you can see with the unaided eye. It is also the closest and brightest spiral galaxy to our Milky Way galaxy.
The Andromeda galaxy (M31) is at RA: 0h 42.7m; Dec: 41o 16′ north