The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years (2.4×1019 km) from Earth in the Andromeda constellation, which was named after the mythological princess Andromeda. Also known as Messier 31 or NGC 224, it is often referred to as the Great Andromeda Nebula in older texts. 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. At 3.4, the apparent magnitude of Andromeda Galaxy is one of the brightest of any Messier objects, making it visible to the naked eye on moonless nights even when viewed from areas with moderate light pollution.
It has satellite galaxies just like the Milky Way. Orbiting M31 are at least 14 dwarf galaxies: the brightest and largest is M32. The second brightest and closest one to M32 is M110. The other galaxies are fainter, and were mostly discovered only starting from the 1970s.
The Milky Way and the Andromeda galaxies are expected to collide in 3.75 billion years, eventually merging to form a giant elliptical galaxy. It is approaching the Milky Way at approximately 100 to 140 kilometres per second.
Two European Space Agency observatories combined forces to show the Andromeda Galaxy in a new light. Herschel sees rings of star formation in this, the most detailed image of the Andromeda Galaxy ever taken at infrared wavelengths, and XMM-Newton shows dying stars shining X-rays into space. In this image, Herschel’s infrared image of the Andromeda Galaxy shows rings of dust that trace gaseous reservoirs where new stars are forming and XMM-Newton’s X-ray image shows stars approaching the ends of their lives.
A number of X-ray sources, likely X-ray binary stars, within Andromeda’s central region appear as yellowish dots. The blue source at the center is at the position of the supermassive black hole.