What happens when galaxies collide?
By Marcus Chown Two billion years from now, our galaxy is in for a shock. With every hour that passes, the Milky Way gets half a million kilometres closer to another large spiral galaxy called Andromeda, and it is only a matter of time before we collide. Over the course of 3 billion years or so, the galaxies will swing through each other, stretching out long, wispy streams of stars before settling down and merging into one. That much we know. Yet that picture is far from complete. Lying at the centre of our galaxy is a giant black hole more than 3 million times as massive as the sun. The black hole at the heart of Andromeda is believed to be 10 times the size. What will happen to these supermassive black holes during the encounter is anyone’s guess. Astronomers have recently started to find some clues, though. Most, if not all, galaxies have a supermassive black hole at their centres. Everyone thought that these hungry behemoths simply sat at the heart of their parent galaxies, vacuuming up gas clouds and ripped-apart stars. Now it seems they can go off sightseeing. A black hole can cut loose when two galaxies collide: their central supermassive black holes coalesce into a single object, and this can receive a tremendous kick in the process. Some supermassive black holes travel to the outskirts of their galaxy before returning home, others go into exile for good,