The farthest thing most eyes can see is a whole other galaxy
On a moonless night you can spot a faint smudge in the constellation Andromeda without any telescope. That blur is a spiral galaxy about 2.5 million light-years away, larger across than our own. It took astronomers until 1925 to prove it was not a cloud inside the Milky Way.
The Persian astronomer al-Sufi described it around 964 as a small cloud in his Book of Fixed Stars, the earliest known mention of any galaxy beyond ours. Simon Marius gave a telescopic account in 1612, and Charles Messier listed it as M31 in 1764, wrongly crediting Marius with the discovery. Pierre Louis Maupertuis in 1745 and Immanuel Kant a decade later suggested such oval glows might be separate universes of stars, Kant reasoning that a disc like the Milky Way would look elliptical when tilted.
Clues piled up. William Herschel saw a reddish tint at its core in 1785 but guessed it lay only about 18,000 light-years off. Lord Rosse sketched its spiral in 1850, and in 1864 William Huggins found its light resembled that of stars rather than glowing gas. A supernova flared there in 1885, the only one ever recorded in Andromeda, though nobody then realised how extraordinarily bright it was. In 1912 Vesto Slipher clocked it moving at 300 kilometres per second relative to us, the fastest velocity measured up to then.
In 1917 Heber Curtis spotted a nova there, then found 11 more in old photographs, all about 10 magnitudes dimmer than novae elsewhere. That put Andromeda around 500,000 light-years off, five times too close but the first estimate within a factor of ten. Curtis argued for independent galaxies in his 1920 Great Debate with Harlow Shapley, and Ernst Öpik's 1922 estimate of roughly 1.5 million light-years supported him. Edwin Hubble ended the argument by picking out Cepheid variable stars in Andromeda with the 100-inch Hooker telescope, fixing its distance far outside our galaxy.
Andromeda spans roughly 152,000 light-years, the widest member of our Local Group, with a mass near a trillion Suns. It was long thought 25 to 50 percent heavier than the Milky Way, though newer studies question that. The two have roughly even odds of colliding within 10 billion years, possibly merging into one giant elliptical or lenticular galaxy.
Source: Andromeda Galaxy
