Cassiopeia A, NuSTAR X-ray image. This object is a supernova remnant, the expanding shell from a massive star that exploded as a supernova some 11,000 years ago. The light from this supernova is thought to have first reached Earth around 300 years ago. The remnant is 11,000 light years from Earth, in the constellation of Cassiopeia. This image was obtained by NASA's Nuclear Spectroscopic Telescope Array (NuSTAR). The energy of the X-rays, formed as the expanding shells impact the interstellar medium, are colour-coded: blue (highest), and red and green (lowest). The starry background picture is from the Digitized Sky Survey. Image published in 2013.