What would happen if another galaxy collided with our own?

Well, actually yes. Stars are at relatively low density in a galaxy, and so if two galaxies collide, the probability is that actually not one single star would collide in the process.There is, however, a lot of gas between the stars in a galaxy, and this gas would collide and be compressed, and that would, in itself, form a whole new generation of stars. The gravity from these stars would affect everything around them, and you would end up with a great big mess. Here on Earth, in the outer spiral arm of the Milky Way, we would probably survive.David Block, of the University of the Witwatersrand, South Africa, observed the Andromeda galaxy and a nearby dwarf galaxy and noticed a ripple effect in Andromeda. He worked out that if you wind back the clock on these ripples, it puts the origin at about 200 million years ago. The ripples are thought to be the result of Andromeda having previously collided with one of it's companion galaxies, so galaxies can definitely survive such a collision, but it's anybody's guess what the results of this ripple effect would be.Galaxies are not necessarily all moving in the same direction, some, found in clusters of galaxies, orbit each other. In all, it's a bit of a mess! This means that galaxy collisions do happen.We do know that the Andromeda galaxy is going to collide with our own. In about 3 billion years, even before our sun has died, Andromeda will collide with the Milky Way. Maybe future generations of humans, or whatever is living on Earth in 3 billion years, will be able to observe what does happen when two galaxies collide!