Cassiopeia A. Combined NuSTAR (Nuclear Spectroscopic Telescope Array) and Chandra X-ray Observatory (CXO) image of the supernova remnant Cassiopeia A. High-energy X-rays from radioactive material are shown in blue (NuSTAR). Lower-energy X-rays from non-radioactive material are shown in red, yellow and green (Chandra). This glowing high-energy mass of gas and dust has been expanding for 350 years after the star that formed it exploded in a supernova. It is 11,000 light years distant, in the constellation of Cassiopeia. NuSTAR detects radioactive elements like titanium-44, a direct tracer of the supernova blast. Image published in 2014.
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