How Dangerous are Galactic Collisions?

Galactic collisions pose little threat to life. Planetary systems ride out collisions; their central stars may be launched into tidal tails or scattered in random directions, but gravity acts so gradually that planetary orbits are not disturbed.

A collision takes about a billion years; during the last billion years, life on Earth evolved from single-celled organisms to amazingly primative apes who still think digital watches are pretty neat. Life forms evolving during the Milky Way's collison with Andromeda may enjoy the fireworks, but their worlds will not be demolished.

So galactic collisions are mostly harmless, unless...