February 20, 1995: This Hubble telescope image shows a small portion of a nebula called the "Cygnus Loop." This nebula is an expanding blast wave from a stellar cataclysm, a supernova explosion, which occurred about 15,000 years ago.
The supernova blast wave, which is moving from left to right across the picture, has recently hit a cloud of denser-than-average interstellar gas. This collision drives shock waves into the cloud that heats interstellar gas, causing it to glow.
Credit: Jeff Hester (Arizona State University) and NASA