Cassiopeia A is  expanding debris cloud from a stellar explosion, a supernova. This picture was the result of a one million second exposure using the Chandra X-Ray Observatory.  In the false-color image, Cas A’s outer green ring, around 10 light-years across, marks the location of the expanding shock from the original supernova explosion. A structure extends beyond the ring (at about 10 o’clock), evidence that the initial explosion may have also produced energetic jets. The tiny point source near the center of Cas A is a neutron star, the collapsed remains of the stellar core. Cas A is about 10,000 light-years away, but light from the supernova explosion first reached Earth just over 300 years ago.

Image Credit: NASA