Great Observatories portrait of Cas A This stunning portrait of supernova remnant Cassiopeia A (Cas A) is made from images taken by three of NASA's Great Observatories. Warm dust seen by the Spitzer Space Telescope is red; visible filaments of hot gas seen by the Hubble Space Telescope are yellow; and extremely hot, X-ray-emitting gas seen by the Chandra X-ray Observatory is shown in green and blue. Located 11,000 light-years away in the northern constellation Cassiopeia, Cas A is the remnant of a once massive star that died in a violent explosion 325 years ago. It consists of a central remnant called a neutron star and a surrounding shell of material blasted off the star as it died. Cas A, site of our galaxy's most recent supernova, is one of the most studied objects in the sky.