As the brightest and closest spiral galaxy beyond the Milky Way, the Andromeda galaxy has been intensively studied, but it’s only in the last few years that astronomers have really got to grips with its origins. Galaxies come together in stages, and the properties of Andromeda’s oldest stars suggest that they originated in smaller ‘protogalaxies’ which began to coalesce into larger structures about 10 billion years ago. 2 billion years later, the young and rather shapeless Andromeda started to collide with another substantial protogalaxy. This merger process took 3 billion years and created Andromeda’s characteristic bright central bulge of stars and large extended disc – some stars flung off in the process still form distinctive streams wrapped around the main galaxy. Andromeda has continued to grow by cannibalising smaller galaxies until quite recently, and infrared images of the dust in its disc suggest its most recent victim was swallowed just 100 million years ago.
Answered by Giles Sparrow