Galactic Pileup 
Hydra, the water snake, is wriggling into the evening sky this month. Its brightest star, Alphard, climbs into view in the east-southeast by around 8:30 or 9 o'clock. It's not all that bright, but it's in a relatively barren region of the sky, so it's not that difficult to find.
Not far from Alphard, research telescopes reveal the final stages of a titanic cosmic collision. Two clusters of galaxies have rammed together, scattering debris and heating up their environment.
Such collisions are fairly common. Our own Milky Way and its family of nearby galaxies will collide with the Virgo Cluster of galaxies in a few billion years.
Individual stars don't hit each other during these events, though -- there's just too much space between them. Instead, the powerful gravity of all those galaxies can toss the galaxies around, sometimes pulling out great streamers of stars. Clouds of gas around the galaxies slam together and heat up to tens of millions of degrees.
The merging clusters in Hydra are known collectively as Abell 754. They're about 800 million light-years away. The smaller cluster has passed through the larger one, and is now falling back toward its larger partner. Researchers say the clusters will complete the merger over the next few billion years.
Astronomers continue to study the merger in hopes of learning more about the interactions between galaxy clusters, and about mysterious dark matter and dark energy.
Script by Damond Benningfield, Copyright 2004, 2010