General Facts:
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It can go by the name Messier 31, M31, or NGC 224. In older texts, its name appears as the Great Andromeda Nebula. Andromeda is the nearest spiral galaxy to the Milky Way, but may not be the closest galaxy, that galaxy is the Canis Major Dwarf.
Named after the mythological princess Andromeda and located in the Andromeda Constellation, it is the largest galaxy in the Local Group compendium of galaxies. The Local Group consists of the Andromeda Galaxy, the Milky Way, the Triangulum Galaxy, and about 30 other smaller galaxies.
Mass
Although it is the largest galaxy in the Local Group, Andromeda may not be the most massive because of dark matter speculations associated with the Milky Way. Recent findings put the Milky Way containing more dark matter than Andromeda and thus may be the most massive galaxy in the Local Group.
Studies estimate that Andromeda contains 7.1×1011 solar masses. In comparison, a 2009 study estimated that the Milky Way and Andromeda are about equal in mass. But on the other hand a 2006 study put the mass of the Milky Way at ~80% of the mass of Andromeda. Clearly, there is still work that needs to be done in this area to make a final determination about mass density between the two galaxies.
Magnitude
With an apparent magnitude (brightness) of 3.4, the Andromeda Galaxy is notable for being one of the brightest Messier objects.
The magnitude scales is a bit confusing. The brightest objects will have a negative magnitude number, while the faintest objects in the sky will have a positive magnitude number. So the brightest objects in the sky…the Sun and the Moon have a magnitude of -26.7, and about -12 (the full moon), Venus at its brightest is -4.7 and the bright star Sirius is -1.5. The faintest stars visible through the largest telescopes have an apparent magnitude of 20. So Andromeda, with a 3.4 magnitude, is a faint object in the sky.
Number of Stars
Observations by the Spitzer Space Telescope in 2006 revealed that Andromeda contains one trillion stars, a number at least twice more than the number of stars in our own galaxy, where that estimate puts the number between 200 and 400 billion stars.
Distance and Visibility
The distance to the Andromeda Galaxy is immense; it is 2,300,000 light years, nevertheless its vast size and luminosity mean that it is still visible to the naked eye. Remarkably, it is the most distant object that can been seen without a telescope.
However, much of the structure in its spiral arms is too faint to be seen, so that it appears smaller than it actually is. In fact, if we could see the entire galaxy, it would occupy an area of the sky almost six times bigger than the size of the Moon.
So when is the best time to see it? Or is it viewable anytime? Actually, it is most visible in the northern sky towards the end of autumn and beginning of winter. Look for the Andromeda Constellation. So draw an imaginary line from the Pole Star through the ‘W’ of Cassiopeia; this leads directly to it.
Origins
A collision of two smaller galaxies may have created the Andromeda galaxy. A team of astronomers reported their findings in 2010. That collision took place between 5 and 9 billion years ago.
Similarities to Milky Way
Though the Andromeda Galaxy is considerably larger than our own, the two share many common features. Both have a clearly recognizable spiral structure, and both have various attendant dwarf galaxies associated with them. For the Milky Way, the most prominent of these Galaxies are the Magellanic Clouds, but for the Andromeda Galaxy this role is taken up by several small galaxies; the two most prominent are M32 and NGC 205.
Collision with Milky Way
While the two galaxies are far apart, they are moving towards each other at the rate of about 100 to 140 kilometers per second (or 62 to 87 mi/s), the equivalent of 400 light years every million years. Thus a potential collision may be coming up in about 3 billion years between the two galaxies.
