M31

M31, or the Andromeda Galaxy, is a spiral galaxy and a member of the Local Group, located about 2.5 million light-years from Earth. It is the closest spiral galaxy to the Milky Way and the largest galaxy in the Local Group by mass and size. The galaxy contains roughly one trillion stars, more than the Milky Way, and features a large, inclined disk with a prominent central bulge. The disk hosts spiral arms, gas, dust, and regions of active star formation, including rings at roughly 10 kiloparsecs from the center. It also contains a substantial globular cluster system and a diffuse stellar halo.

At the core lies a supermassive black hole, estimated at tens to hundreds of millions of solar masses, commonly cited around 100–150 million. The nucleus appears double, a feature explained by a dense cluster of stars surrounding the black hole.

Andromeda is approaching the Milky Way, with a radial velocity of about -300 km/s. It is expected to collide and merge with the Milky Way in roughly 4 billion years to form a larger galaxy often nicknamed Milkomeda. The galaxy can be observed from the Northern Hemisphere with the naked eye under dark skies, appearing as a faint smudge and best seen in autumn.

Historically, M31 has been known since antiquity and was later organized into the Messier catalog. It is a major subject of modern extragalactic astronomy and possesses several satellite dwarf galaxies, including M32 (NGC 221) and M110 (NGC 205).