The Andromeda Galaxy: The Rodney Dangerfield of the Night Sky
© Sonya Thomas-Haley
If you reside in the northern hemisphere, the Andromeda Galaxy is the only extra-galactic object visible to the naked eye, and that tells you it is either close (on a cosmic scale, anyway) or very bright, or both. The answer is both.
The Andromeda Galaxy (M31, NGC224) is an enormous spiral galaxy that is part of our Local Group. More than 20 little galaxies populate our neighborhood (the nearest being the Large and Small Magellanic Clouds, visible only in the southern hemisphere), but Andromeda and the Milky Way are the two big ones. In fact, Andromeda would qualify as a BIG galaxy just about anywhere on any scale.
From our perspective it is tilted about 13 degrees from edge-on. This is unfortunate, since it does not allow us a good view of its spiral arms. It is thus a little disappointing in a telescope, looking like a bright and elongated ball of fuzz. By coincedence, someone on a planet in Andromeda observing us would see us edge-on as well (so maybe we get ignored at their star parties, too). Since we travel around our galaxy's core much the same way planets circle a sun, in about 120 million years we will be on the opposite side of our galaxy and Andromeda will be invisible to us, obscured from view by our own dense galactic core.
Andromeda is about 110,000 light-years in diameter and is one of the most intrinsically luminous galaxies known. When viewing it, use the lowest magnification you can so you have the widest possible field of view. Don't expect to be able to resolve individual stars.
Andromeda is more than two million light-years away. That is an enormous distance, but on a cosmic scale still qualifies as being in our backyard. You are probably familiar with the concept of the expanding universe: everything is racing away from us. Well, Andromeda is an exception to that. We are on a collision course and will meet in about five billion years. No cause for alarm for our distant descendants, however (assuming they survive the death of our sun--but that's a whole other story). Enormous distances separate individual stars within galaxies so the stars themselves aren't likely to collide. There will be gravitational disruptions, with some stars switching allegiances between galaxies, and a burst of new star formation as clouds of interstellar dust get compressed, but that is about it.
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