‘Merge’ might be a clearer way to explain what happens when two galaxies interact. When we think of collisions we imagine two or more things smashing together, and this usually results in things breaking and there generally being a mess. This makes the prospect of a galaxy, a collection of billions of giant burning balls of highly volatile gas, colliding with another quite an exciting one.
Unfortunately this just isn’t the case, and this is all to do with the spacing of stars. Galaxies are teeming with billions of stars but they are also very, very large. This means these stars are spread out, very spread out. To give you an idea of how sparse they are, the average particle in air is 500 times its own width from another particle. Our Sun is 29 million times its own width from the next nearest star.
So when galaxies collide we certainly don’t see Hollywood style explosions; in fact, close up you probably wouldn’t notice much at all. The collisions are so tame that they are usually just referred to as ‘mergers’.
This is also to do with the fact that galaxies that have collided have a tendency to stick together. Because of the huge amount of stuff in a galaxy they have a strong gravitational influence. This can cause them to interact with each other as they get close together. In mergers we see galaxies pull and twist each other, stretching out huge bulks of material. This can change the structure of galaxies, destroying things like spiral arms, resulting in more globular types.
Answered by Josh Barker from the National Space Centre
Image courtesy of NASA