You might have heard that in about 5 billion years, our Sun will burn up all its nuclear fuel and puff up into a red giant. If it doesn’t actually engulf the Earth, at the very least it will boil off the oceans and the atmosphere and turn the surface into a cinder.
Well, I’m afraid I have bad news. 5 billion years from now is also the timescale for an even more cataclysmic event. The Sun is one star among several hundred billion which form our galaxy, the Milky Way. The Milky Way is one of about 30 galaxies in the prosaically-named Local Group; and the second largest of them.
Our largest neighbour in the Local Group is the Andromeda galaxy, and in third position, quite a bit smaller, is Triangulum. Andromeda, Milky Way and Triangulum are the only galaxies in the Local Group to have the classic, spiral form. All the others are dwarfs of various shapes and sizes, and they tend to group in orbits around the biggest three.
The whole Local Group is bound by gravity though, engaged in a great, slow cosmic dance around each other. I think it’s staggering that understanding of gravity and dynamics means that the paths of the galaxies can be calculated far, far into the future. That was where the bad news I was talking about comes into the picture.
In 2.3 billion years, Andromeda and the Milky Way will sweep past each other, rather like when a spacecraft is swung round a planet to slingshot it onto a different course. The mutual gravitational attraction will probably rip some stars off the fringes of each galaxy, but no galaxy-wide effects are anticipated. No, those come later, when like a pendulum reaching the end of its swing, Andromeda reverses direction and comes back towards us, this time, 5 billion years from now, ploughing right into the Milky Way.
Galaxies are mostly empty space, so collisions between stars aren’t likely, but every star will feel the effect of the new combined gravity, as well as any local disturbances or near-misses. It will be like stirring a pot of paint. The two beautiful spirals will be smeared out into one much larger, less dense blob: an elliptical galaxy. Our Sun, already in the unfashionable suburbs, will be kicked double the distance out from the new galactic centre, where the two supermassive black holes from the galaxies will spin around each other and eventually coalesce in a blaze of radiation.
Of course, with the Sun already dying, any intelligent life from Earth will already have to have left it to survive (or has that already happened?). Even after the chaos of the galactic collision, there will still be younger Sun-like stars with life left in them, and presumably planets that could be made livable, but I suspect that once you’ve gone into space there would be little incentive to return to a flatland existence on a planet again.
Well, we can plot the paths of the galaxies 5 billion years into the future, but biology is a harder question. Will there be intelligent beings descended from us? From the cockroaches? From our machines? Five billion years is longer than the existence of any kind of life on Earth (longer than the life of the Earth, in fact). We don’t know what will happen that far into the future, and I can’t see how we ever will. But if there is something alive and thinking, one thing we can be sure of: that it will be nothing at all like us.