The Andromeda Galaxy is the largest galaxy of the Local Group, which also contains the Milky Way, the Triangulum Galaxy, and about 30 other smaller galaxies. Although the Andromeda Galaxy is the largest, as recent findings suggest that the Milky Way contains more dark matter and could be the most massive in the grouping.
The 2006 observations by the Spitzer Space Telescope revealed that M31 contains one trillion stars, at least twice of the Milky Way, with an estimation of 200-400 billion stars.
The Andromeda Galaxy is estimated to be 7.1×1011 solar masses, In comparison a 2009 study estimated that the Milky Way and M31 are about equal in mass, while a 2006 study put the mass of the Milky Way at ~80% of the mass of the Andromeda Galaxy.
The Milky Way and Andromeda are are expected to collide with each other in approximately 3.75 billion years, eventually merging to form a giant elliptical galaxy.
The Andromeda Galaxy has an apparent magnitude of 3.4, making it 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 or when viewed using binoculars or a small telescope.