Cassiopeia A

Cassiopeia A (or Cas A) is a supernova remnant about 11000 light-years away, left over from a supernova that would have first been visible to Earth at some time near 1670-1680; there are records of observations in that era that might indicate it, but no unambiguous record of a new source, and various supernova designators have been used for the supernova, including SN 1667, SN 1671, and SN 1680. The remnant was first recorded in 1947 and has been much studied since. Reflections of the supernova off gas clouds (a light echo) have recently provided some look at the actual event. An X-ray source, CXOU+J232327.8+584842, detected by Chandra X-ray Observatory in 1999 is presumed to be the resulting neutron star.

This supernova was not either of the 1500s/1600s two that were observed by the naked eye, i.e., neither that noted by Tycho Brahe in 1572 in Cassiopeia (SN 1572 aka Tycho's supernova), nor that noted by Johannes Kepler in 1604 in Ophiuchus (SN 1604 aka Kepler's supernova).