Supernova remnant G21.5-0.9, Chandra X-ray telescope image. The bright inner shell is a cloud of high-energy electrons. The fainter outer shell is seen as material ejected by the supernova explosion collides with gas in interstellar space. The shock wave caused by this collision heats the gas and causes it to emit X-rays. It was thought that G21.5-0.9 did not possess this second shell, but its discovery in this 150-hour exposure implies that all supernovae produce a shock shell like this. The star that produced this remnant is thought to have been at least ten times as massive as the Sun, and exploded a few thousand years ago. The remnant lies around 20,000 light years from Earth in the constellation Scutum.