This is super cool.
Here’s the explanation: “This is an interactive java applet which allows you to model galaxy collisions on your own computer. With this applet you can study how galaxies collide and merge gravitationally and how the effects of the collision depend on the properties of the galaxies. You can also recreate collisions between real interacting galaxies observed in the sky.”
Via Mark SubbaRao (when can I get my own copy of the Black Hole Flight Simulator he discusses?).