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.

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