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The Andromeda Galaxy is the nearest spiral galaxy to the Milky Way, our galaxy. Andromeda is sometimes called M31 or NGC 324 by astronomers. It is about 2.6 million light years away from us.
Andromeda is the largest galaxy of the Local Group, which consists of the Andromeda Galaxy, the Milky Way Galaxy, the Triangulum Galaxy, and about 30 other smaller galaxies. Although the largest, Andromeda may not be the most massive. Recent findings suggest that the Milky Way contains more dark matter and may be the most massive in the grouping.
The 2006 observations by the Spitzer Space Telescope revealed that M31 contains a trillion stars (1012). This is more than the number of stars in our own galaxy, which is estimated to be c. 200-400 billion.
Andromeda is estimated to be 7.1×1011 solar masses. In comparison, a 2009 study estimated that the Milky Way and Andromeda are about equal in mass, while a 2006 study put the mass of the Milky Way at ~80% of the mass of Andromeda.
At an apparent magnitude of 3.4, the Andromeda Galaxy is notable for being one of the brightest Messier objects, making it visible to the naked eye on moonless nights even when viewed from areas with moderate light pollution. Although it appears more than six times as wide as the full Moon when photographed through a larger telescope, only the brighter central region is visible to the naked eye. Being both large and bright, it is one of the farthest objects that can be seen without a telescope or binoculars.
The Andromeda Galaxy is approaching the Milky Way at about 100 to 140 kilometres per second (62 to 87 mi/s), so it is one of the few blue-shifted galaxies. The Andromeda Galaxy and the Milky Way are thus expected to collide in perhaps 4.5 billion years. A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy. Such events are frequent among the galaxies in galaxy groups.
The Persian astronomer Al Sufi was the first man to record seeing the Andromeda Galaxy. He called the Galaxy 'a small cloud' in his book, the Book of Fixed Stars, which was published in 964 AD. The first person to look at the galaxy with a telescope was Simon Marius in 1612. In 1764 Charles Messier put it into his catalogue of astronomical objects. He called it M31 and gave the credit for its discovery to Marius, as he did not know that Al Sufi had seen it hundreds of years previously.
In 1751 William Herschel estimated the distance to the Andromeda 'Nebula' as 'no more than 2000 times the distance to Sirius, or around 17,200 light years. The best modern estimate of Andromeda's distance is 2.54 million light years. In the 1920s astronomer Edwin Hubble proved that Andromeda was a galaxy, and not a gas cloud in the Milky Way as had been thought.
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