Enjoy these galaxy collisions. It's interesting that countless years pass as the collisions happen. Because galaxies colliding are so giant, all the stars and objects in the galaxies are light years apart. Light travels at the speed of light, however; galactic and stellar orbits don't. The collision from far distant Earth is so slow, it doesn't even look like anything is happening, even though the galaxies are experiencing radical change. There are millions of years for a transition that from a macro perspective is the snap of a finger.

Accuracy

Celestia is reasonably reliable as its space simulations actually seem to be almost accurate (for example IC 1987).