M 31 Andromeda Galaxy

The Andromeda Galaxy (sometimes known as the Great Andromeda Nebula or by the catalog designations M 31 and NGC 224) is a large barred spiral galaxy about 2.538 million light-years from Earth in the direction of the constellation Andromeda, from which it takes its name. It is the closest large galaxy to ours, the Milky Way; it is visible even to the naked eye and is among the most distant objects visible without the aid of tools.

The Andromeda Galaxy is the largest of the Local Group, a group of galaxies that also includes the Milky Way and the Triangulum Galaxy, plus about fifty other minor galaxies, many of which are satellites of the main ones.

According to studies published in the 2000s, derived from observations by the Spitzer Space Telescope, it would contain about a billion stars (one thousand billion), a number greater than that of the Milky Way, estimated between 200 and 400 billion stars. However, there are conflicting opinions on mass: some studies indicate a mass value for the Milky Way equal to 80% of that of Andromeda, while, according to others, the two galaxies would have similar mass sizes. However, some studies suggest that the Milky Way contains more dark matter and may therefore be the one with the largest mass.

With an apparent magnitude of 3.4; The Andromeda Galaxy is one of the brightest objects in Messier's catalog.