G21.5-0.9: A supernova remnant about 20,000 light years from Earth
Caption: This image, made by combining 150 hours of archived Chandra data, shows the remnant of a supernova explosion. The central bright cloud of high-energy electrons is surrounded by a distinctive shell of hot gas. The shell is due to a shock wave generated as the material ejected by the supernova plows into interstellar matter. Although many supernovas leave behind bright shells, others do not. This supernova remnant was long considered to be one without a shell until it was revealed by Chandra.
Scale: X-ray image is 6.3 arcmin per side.
Chandra X-ray Observatory ACIS Image