Video: Laura Brenneman on Black Holes
Black holes are completely described by three quantities: their mass, spin, and electric charge — and in real life, the charge is always zero. Sound simple? It's not!
Unlike stars, galaxies are fairly large compared to the space between them, so galaxy collisions are quite common. In fact, our own Milky Way Galaxy is thought to have devoured dozens of galaxies in the course of its life. When this happens, the black holes at the centers of the galaxies often collide. (See "Cosmic Cataclysms" in the April 2009 issue of Sky & Telescope.) And then, depending on the spins of the two black holes, all manner of different things can happen.
But don't worry about being devoured by a free-floating supermassive black hole. As Laura Brenneman explains in the video interview shown below, the chances of a black hole being ejected from its host galaxy are actually quite small.