Q&A: Galaxies, Galaxy Clusters, AGN, and Quasars

Q:
How many planet/planet collisions do you presume will come from the M31 incident to come--or do you think we will be smoothly assimilated?

A:
Galaxies are mostly empty space, so when two galaxies collide, the chance of stars or planets colliding is very small. About the only objects that actually bump into each other are huge clouds of gas and dust. These collisions can cause new stars to form a million or so years after the collision, so the colliding galaxies would brighten up at that point. Another effect is that some of the clouds will be bumped out of orbit and fall into the central black holes in one or another of the galaxies, and cause a dramatic outburst.