A group of astronomers, including U.Va.'s Craig Sarazin, have observed two groups of galaxies slamming into one another at a speed of about 4 million miles per hour. The colliding groups will eventually merge and form a single cluster of galaxies; these are the largest objects in the Universe. Clusters contain as much material as one million, billion stars. This cosmic train wreck was observed with a number of space and ground-based observatories, including by U.Va. astronomers using the Apache Point Observatory (APO) in New Mexico. U.Va. is a member of APO. The picture above shows a composite of the X-ray and optical images of the merging groups. These observations were the subject of an image press release by NASA's Chandra X-ray Observatory, which contains much more information and many more images. There is also a video about this collision on YouTube at https://youtu.be/PVFkc46CyYY.