Also known as Messier 31, the Andromeda galaxy is our nearest large galactic neighbour – a spiral even larger than our own, just 2.5 million light years away. Although Andromeda’s diameter is larger than our own galaxy’s at about 140,000 light years, and it contains more than double the number of stars at about a trillion, it still weighs less than the Milky Way, which seems to contain more invisible dark matter.
The Milky Way and Andromeda are the largest members of the Local Group, a small galactic cluster that contains several dozen smaller galaxies. These two galactic heavyweights are also attracted to one another by gravity, and are moving towards each other at about 300 kilometres (186 miles) per second. This will cause them to collide in around 4 billion years from now – perhaps eventually merging together to become a single supergiant elliptical galaxy.