Andromeda, 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 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 is the largest
Of the Local Group, which also contains the Milky Way, the Triangulum Galaxy, and about 30 other smaller galaxies.
Although the largest, the Andromeda 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 first photographs of M31 were taken in 1887 by Isaac Roberts from his private observatory in Sussex, England.The long-duration exposure allowed the spiral structure of the galaxy to be seen for the first time.
However, at the time this object was still commonly believed to be a nebula within our galaxy.
Roberts mistakenly believed that M31 and similar spiral nebulae were actually solar systems being formed, with the satellites nascent planets.
The radial velocity of this object with respect to our solar system was measured in 1912 by Vesto Slipher at the Lowell Observatory, using spectroscopy.
The result was the largest velocity recorded at that time, at 300 kilometres per second (190 mi/s), moving in the direction of the Sun.
The Andromeda Galaxy is approaching the Milky Way at about 100 to 140 kilometres per second (62 to 87 mi/s) (400 lightyears every million years), making it one of the few blueshifted galaxies.