The Andromeda Galaxy, M31, is a large spiral galaxy, the nearest one to the Milky Way, and situated within the same galactic cluster called the Local Group. It appears to be within the constellation Andromeda.
The distance of 2.51 million light years has been reached through three separate methods for determining the distance to an astronomical object.
- Cepheid variables
- Eclipsing binary star
- Measurement of brightness of individual stars and comparing to the distance to the Magellanic Clouds
The name "M31" is an abbreviation for "Messier 31," the thirty-first object in Charles Messier's (1730-1817) pioneering catalog of nebulae.
The Andromeda galaxy is sometimes described as being visible with the naked eye, but this is not possible in the light-polluted skies of a typical U. S. city or suburb. However, it is easily seen with binoculars even from a suburban back yard once you know where to look and know what to look for. Through binoculars, the galaxy is a small, dim, fuzzy spot, unlike the dramatic, bright detailed photographic images shown in astronomy books.