Andromeda: Our Sister Galaxy
In Greek mythology, Andromeda, the daughter of Cepheus and Cassiopeia, was stripped and chained to a rock, only to be saved from certain death in the claws of a sea monster by Perseus (Figure 1 shows a wonderful depiction of the myth by Lord Frederic Leighton).
Figure 1. ‘Perseus and Andromeda’ by Lord Frederic Leighton. Walker Art Gallery, Liverpool. At Google Cultural Institute.
In the northern sky, a constellation is named Andromeda, and it contains the galaxy M31 (so cataloged by astronomer Charles Messier on August 3rd, 1764), commonly known as the Andromeda galaxy. At a distance of 2.5 million light years, the Andromeda galaxy is next door in astronomical terms. Its mass is only about twice that of the Milky Way, making the two galaxies if not quite twins, then close sisters.
By measuring very precisely the motion of Andromeda relative to the Milky Way, astronomers using the Hubble Space Telescope were able to determine in 2012 that the Milky Way and Andromeda are destined for a head-on collision in about 4 billion years. About two billion years later, the two sister spiral galaxies will completely merge, most probably producing an elliptical galaxy. While solar system will not —> Read More Here