M31

The Andromeda Galaxy is the nearest large spiral galaxy to the Milky Way, located approximately 2.5 million light-years away. It is the largest galaxy in the Local Group, containing roughly one trillion stars and spanning about 220,000 light-years in diameter — larger than our own Milky Way. M31 has at least two prominent satellite galaxies visible in amateur telescopes: M32 (a compact elliptical) and M110 (a dwarf elliptical).

Visible to the naked eye from dark sites as an elongated fuzzy patch in the constellation Andromeda. Binoculars show the bright central core and a hint of the disk's extent. A wide-field telescope at low power (around 30-50x) is ideal to frame the galaxy, which spans over 3 degrees — six times the width of the full Moon. Dark skies and averted vision reveal the dust lanes on the near side of the disk. The two satellite galaxies M32 and M110 are easy to spot in the same field of view.

First recorded by the Persian astronomer Abd al-Rahman al-Sufi in his Book of Fixed Stars in 964 AD, where he described it as a 'small cloud.' Charles Messier cataloged it as M31 in 1764. In 1912, Vesto Slipher measured its blueshift, showing it was approaching us. Edwin Hubble resolved individual Cepheid variable stars in M31 in 1924-25, proving it was a separate galaxy far beyond the Milky Way — settling the Great Debate about the nature of 'spiral nebulae.'