I remember sitting spellbound, watching the movie When Worlds Collide. Two planets hurled through space toward Earth while scientists and engineers frantically raced to complete a rocket­ ship that would take them to safety. In the final moments the spaceship lifted off as the occupants watched the Earth bulge, crack, then literally explode as one of the planets struck it. As I left the theater I wondered if it was really possible for another world to collide with Earth. Later I learned that while many catastrophic collisions no doubt occurred early in the his­ tory of the solar system, today they are exceedingly rare. I was relieved, but in another sense I was disappointed (not that I hoped a collision of this type would actually occur). A collision of two objects in space, say, two stars, I was sure would be a spectacular event. It is quite unlikely, however, that we will ever witness the collision of two stars. The event is just too rare. But collisions of systems of stars-galaxies-oddly enough, are relatively com­ mon. In fact, we see evidence of several in the sky right now.