G21.5-0.9: A Supernova s Cosmic Shell . ... Manitoba ), CXC , NASA Explanation: The picture is lovely, but this pretty cosmic shell was produced by almost unbelievable violence - created when a star with nearly 20 times the mass of the sun blasted away its outer layers in a spectacular supernova explosion . ... Cataloged as G21.5-0.9, this shell supernova remnant is relatively faint, requiring about 150 hours of x-ray data from the orbiting Chandra Observatory to create this false-color image. ...