The Andromeda Galaxy (), also known as Messier 31, M31, or NGC 224, is a spiral galaxy approximately 780 kiloparsecs (2.5 million light -years) from Earth, and the nearest major galaxy to the Milky Way. Its name stems from the area of the Earth 's sky in which it appears, the constellation of Andromeda.
The 2006 observations by the Spitzer Space Telescope revealed that the Andromeda Galaxy contains approximately one trillion stars, more than twice the number of the Milky Way 's estimated 200 to 400 billion stars. The Andromeda Galaxy 's mass is estimated to be around 1.76 times that of the Milky Way Galaxy (~0.8-1.5×1012 solar masses vs the Milky Way 's 8.5×1011 solar masses), though a 2018 study found that the Andromeda Galaxy 's mass is roughly the same as the Milky Way 's. The Andromeda Galaxy, spanning approximately 220,000 light -years, is the largest galaxy in the Local Group, which is also home to the Triangulum Galaxy and other minor galaxies.
The Milky Way and Andromeda galaxies are expected to collide in ~4.5 billion years, merging to form a giant elliptical galaxy or a large disc galaxy.
With an apparent magnitude of 3.4, the Andromeda Galaxy is among the brightest of the Messier objects making it visible to the naked eye from Earth on moonless nights, even when viewed from areas with moderate light pollution.
Observation history
Around the year 964, the Persian astronomer Abd al-Rahman al-Sufi described the Andromeda Galaxy, in his Book of Fixed Stars as a "nebulous smear". Star charts of that period labeled it as the Little Cloud. In 1612, the German astronomer Simon Marius gave an early description of the Andromeda Galaxy based on telescopic observations. The German philosopher Immanuel Kant in 1755 in his work Universal Natural History and Theory of the Heavens conjectured that the blurry spot was an island universe.
