How do galaxies collide?

We put Rich's question to astronomer Carolin Crawford...

Carolin - This is a very good question. It's one that comes up quite often because as you say, we know the whole universe is expanding. What you have to be clear about here is that space is expanding and it's pulling the galaxies along for the ride. However, if two galaxies are close enough to each other, that gravity that they feel - they feel each other's gravity - that overrides the motion of the expanding space so they can move through the space towards each other. So for example our galaxy and the Andromeda galaxy, which is our nearest neighbouring major galaxy, are so close that they're only about 2.5 million light-years apart.

Kat - Hardly any distance at all.

Carolin - Well, believe me it isn't in space terms. That's near enough that they're feeling each other's gravity and they're getting pulled towards each other. In about 6 billion years, we're going to collide. So, if galaxies are close enough that their mutual gravity dominates over the expansion of space, that's when you get them colliding.