Andromeda Galaxy

M31 has been part of the human sky for as long as humans have been looking up. The 10th-century Persian astronomer al-Sufi catalogued it as a 'small cloud' in his Book of the Fixed Stars long before the invention of the telescope. Edwin Hubble's 1925 measurement of Cepheid variables in its arms became one of the most consequential observations in the history of science, finally settling the 'Great Debate' by proving that the spiral nebulae lie far outside our own galaxy. At roughly 2.5 million light-years away it spans nearly three degrees of sky — six full Moons across — yet its outer halo is so faint that most casual observers only ever see the bright central bulge. Two prominent dwarf companions (M32 and M110) lie close by and are routinely captured in the same frame. The galaxy is on a collision course with the Milky Way and will merge over the next several billion years to form a single elliptical galaxy informally nicknamed 'Milkomeda'. From a dark site M31 is among the easiest deep-sky objects to photograph; even a few minutes through a 200 mm lens reveals its dust lanes and star-forming regions.