If you’ve ever seen a picture of another galaxy, odds are that it was of this one, the Andromeda galaxy, also known as M31. At a distance of 2.54 million light years from us, it is closer and hence appears larger than any other galaxy (apart from the two Magellanic Clouds in the southern hemisphere that are small satellites of our Milky Way). And it is on a collision course with us and will collide with our Milky Way in about four billion years.
The Andromeda Galaxy is the most distant object and the only spiral galaxy outside our Milky Way able to be seen with the naked eye as a fuzzy patch in a dark sky on a moonless night.
It was long assumed to be one of the nearest gaseous nebulae in own Milky Way. Later some astronomers came to believe it was another “island universe” similar to and far beyond the edge of the Milky Way. The debate was settled in 1925 by Edwin Hubble (for whom the space telescope was named) when he found and measured a number of Cepheid variable stars contained therein.
Andromeda is a fairly tightly-wound spiral galaxy that we see about 23 degrees away from being edge-on. Because of this, it is difficult to study its spiral structure. You can see very noticable dark dust clouds interspersed among the spiral arms, especially to the lower right (the portion of the disk nearest to us) and near the top. Like the Milky Way, Andromeda also has two prominent satellite galaxies - M110 (the fuzzy patch in the lower right corner) and M32 (the smaller and more circular fuzzy patch at about the 8 o’clock position and a little closer to the nucleus of M31).
(I took this image in late February when Andromeda was fairly low in the west. I hope to obtain better results next summer or autumn when it will be placed high in the sky.)