It was once classed as a nebula. Before the true scale of the universe was realised, what is now known to be the rim of our own Milky Way was thought to be the edge of space. Within that the Andromeda galaxy (M31), little more than a fuzzy blur in the sky to all but the most powerful telescopes of the earliest 20th century, was considered to be a mere collection of forming stars and cosmic dust clouds. This meant it was originally called the Great Andromeda nebula.
A trillion stars can be found here. Although the Milky Way is probably the most massive in the galactic Local Group, Andromeda is the biggest by volume. Our neighbouring galaxy also contains around twice the number of stars as our own galaxy, according to observations made by the Spitzer Space Telescope.
Dozens of black holes lie here. The centre of M31 is home to 26 known black hole candidates and many more have been picked out by the Chandra X-ray Observatory. Like our own galaxy, there’s also a supermassive black hole at the centre, with two others possibly orbiting as a binary, with a mass around 140 million times that of the Sun.
Andromeda is a galactic bully. Andromeda has numerous satellite galaxies, including 14 dwarf galaxies that it regularly bullies. Both M32 and M110 have had encounters with M31 that they’ve come off worse from: a stream of stars was ripped away from M110 while Andromeda robbed M32 of a large part of its stellar disk at some point in the distant past.
It’s on a collision course. Whereas most of the rest of the universe is accelerating away from our galaxy, Andromeda is blue-shifted, meaning it’s moving towards us. Both the Milky Way and Andromeda are moving towards each other at a rate of 120 kilometres (75 miles) a second, putting them on course for a galactic smash- up in around 4 billion years.