2 Cool 2 Be 4-Gotten

The title is from a Lucinda Williams song, but I'm referring to a cool web site that simulates collisions between two galaxies. While searching for more interactive astronomy web stuff for the Cub Scouts (and myself!), I found Galaxy Crash, a hypnotic little Java applet by Chris Mihos (Case Western Reserve University) and several colleagues. You can set relative mass, closest approach distance, number of stars, and some orientation parameters relative to the orbital plane. Severe time acceleration allows hundreds of millions of years to pass in mere seconds. Although quite zippy in the simulation, these are soft, slow collisions - the stars in each galaxy are so far apart that they rarely collide - the galaxies pass through one another, and all the distortion that we see is caused by everything pulling on everything else gravitationally. Here's an unrelated animation that shows a two-galaxy collision in 3D (by Jeff Bryant). And here's a real example from Hubble.