What happens if two galaxies collide? The average distance between

bright galaxies is only about 20 times their size, so over the history

of the Universe (14 Billion years), we expect that most bright galaxies

will have had at least one close gravitational encounter with a

neighboring galaxy. This lecture explores what happens when two

galaxies undergo interactions ranging from passing tidal interactions to

head-on collisions, all the way to multiple collisions and galaxy

"cannibalism" in the centers of large clusters. While at first glance

galaxy interactions explain rare "peculiar" galaxies, on closer

examination we find that galaxy interactions and mergers are central to

understanding the assembly and evolution of galaxies. At the end, we

take a speculative look at the distant future 3-4 Billions years hence

if in fact Andromeda and the Milky Way are on a collision course.

Recorded 2006 February 15 in 1008 Evans Laboratory on the Columbus campus of

The Ohio State University.