The arrival of fall signals the return of the Andromeda Galaxy to our early evening skies. This popular astronomical feature is one of the most distant objects visible with the naked eye and played a significant role in unraveling the expanding nature of the universe.
The Andromeda Galaxy, also known as M31, is a spiral galaxy like our Milky Way. This type of galaxy features a bulge of stars that forms the central portion of a disc of stars, dust, and gas. This disc is arranged into a pattern of spiral arms, locations of star formation.
The age of the Andromeda Galaxy is uncertain, with recent estimates ranging from five to 10 billion years old. It contains perhaps one trillion stars, well more than the estimated 300 billion of the Milky Way, and has at least 19 satellite galaxies orbiting it.
The Andromeda Galaxy is the largest member of the Local Group, a cluster of about 30 galaxies that are gravitationally bound to one another. The Local Group also includes the Milky Way but consists mostly of small, faint galaxies known as dwarfs. At a distance of 2.5 million light years, M31 is the 33rd known closest galaxy ...