1. 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.
2. 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.
3. It’s huge! With an enormous diameter of around 220,000 light years across, Andromeda is over 2.5 times longer than the entire Milky Way and appears longer than the full Moon in the night sky. Despite it being further away than any visible star, you can see it with the naked eye and it’s still the closest known galaxy to our own.
4. You can see it with the naked eye Andromeda has been known since ancient times because, at just 2.5 million light years away, this large, bright galaxy is visible with the naked eye. On a clear night with little light pollution, it can be seen as a diffuse blur, with the central region clearly visible through a good pair of binoculars. Larger telescopes provide even more-spectacular views of this impressive galaxy.
5. We know its entire life history The major benefit of Andromeda being such a conspicuous night-sky object is that it’s been studied and scrutinised by astronomers for decades. It was born 10 billion years ago out of the merger of many smaller protogalaxies and then, around 8 billion years ago, it ran head-on into another galaxy to form a giant that became the M31 galaxy we see today.
6. Andromeda holds a grave secret To most telescopes, Andromeda’s giant stellar stream will look like a wispy cloud that orbits the galaxy like a huge celestial ring. In fact, this is the remains of an ancient companion that was ripped up and consumed by M31 billions of years ago. The clouds are actually the remains of the stars that once formed this galaxy and are easily resolved by space telescopes such as Hubble.
7. 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.
8. 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.
9. 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.
10. It prompted the Great Debate In 1920, the Andromeda galaxy prompted a fierce debate between two prominent astronomers, Harlow Shapley and Heber Curtis. Shapley believed that Andromeda and the Pinwheel galaxy were nebulas found within the Milky Way, based on the fact that if they weren’t, it would put Andromeda an ‘impossible’ multi-million-light-year distance from us. Curtis argued that this was in fact, the case. Work by Edwin Hubble, Henrietta Leavitt et al eventually proved Curtis right.
Image Credit: NASA