Cassiopeia A: A supernova remnant about 11,000 light years away.

This image presents a beautiful composite of X-rays from Chandra (red, green, and blue) and optical data from Hubble (gold) of Cassiopeia A, the remains of a massiv star that exploded in a supernova. Evidence for a bizarre state of matter has been found in the dense core of the star left behind, a so-called neutron star, based on cooling observed over a decade of Chandra observations.

(Credits: X-day: NASA/CXC/UNAM/loffe/D.Page, P/Shternin et al; Optica: NASA/STScI)