Andromeda Galaxy

A galaxy in the Local Group

Andromeda, designated M31 or NGC 224, is a spiral galaxy in the constellation Andromeda. It is the Milky Way’s nearest major galaxy, not the nearest galaxy of any size. Together with the Milky Way, the Triangulum Galaxy and numerous smaller systems, it belongs to the gravitationally bound Local Group.

Distance, size and brightness

NASA places M31 approximately 2.5 million light-years away. ESA/Hubble uses a diameter of about 200,000 light-years in its 2019 comparison: an approximate scale, not a sharp physical boundary. NASA gives an apparent magnitude of 3.1; the consulted OpenNGC version lists 3.44 in the V band. These values retain their separate provenance rather than being treated as one measurement. A dark sky allows naked-eye viewing, but not the detail seen in telescope images.

What the observations reveal

Hubble images distinguish a densely populated central bulge, dust lanes and arms containing young blue stars. The panorama presented in January 2025 combined more than a decade of observations and resolves about 200 million stars: this is not a census of all stars in the galaxy. That mosaic must also be distinguished from the earlier partial PHAT survey image. Comparing stellar populations helps reconstruct the disk’s formation and interactions with smaller galaxies.

Interpreting the catalogue

OpenNGC identifies NGC0224 with M31 and lists a heliocentric radial velocity of −300 km/s; this is neither its transverse velocity nor a complete prediction of its future orbit. These catalogue parameters are historical. The version’s parallax field of 6 mas is incompatible with a distance of millions of light-years and is not adopted as a physical measurement of M31. Approximate distance and illustrative size describe different quantities. Images in different wavelength bands highlight different components; an ultraviolet image is not a natural-colour view.