The Andromeda Galaxy is a spiral galaxy approximately 2.5 million
light-years (2.4×1019 km) from Earth in the Andromeda constellation.
Also known as Messier 31, M31, or NGC 224, it is often referred to
as the Great Andromeda Nebula in older texts.
The Andromeda Galaxy is the nearest spiral galaxy to our Milky Way
galaxy, but not the closest galaxy overall.
It gets its name from the area of the sky in which it appears, the
constellation of Andromeda, which was named after the mythological
princess Andromeda.
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 largest, the Andromeda Galaxy may not be the most massive,
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 (1012) stars: at least twice the number of stars
in the Milky Way galaxy, which is estimated to be 200–400 billion.

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,[13] while a 2006 study put the mass of
the Milky Way at ~80% of the mass of the Andromeda Galaxy.
The two galaxies are expected to collide in 3.75 billion years,
eventually merging to form a giant elliptical galaxy.

At an apparent magnitude of 3.4, the Andromeda Galaxy is 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.