Video of the day -- the great collision
This NASA animation shows events that will take place over the next several billion years (it's a silent video, so don't adjust your volume control). Over this huge span of time, the Andromeda galaxy and our own galaxy will approach, collide, and pass through each other twice -- the distances between stars within each galaxy are so vast that there will be few or no star-on-star collisions, but both galaxies' spiral structure will be totally disrupted by each other's gravity -- and then merge into a single cloud of stars. The counter at the lower right shows billions of years into the future, so that last third-decimal-place digit -- the one changing too fast to follow -- is counting millions of years. Found via Faye Kane (NSFW blog).