Cygnus Loop Supernova
Photograph courtesy J. J. Hester (Arizona State University)/NASA
This 1991 image shows a small portion of the Cygnus Loop supernova remnant. The formation shown here marks the outer edge of an expanding blast wave from a colossal stellar explosion that occurred about 15,000 years ago. The blast wave slams into clouds of interstellar gas, causing it to glow and revealing information about the composition of the gas.