There have been
nine supernovae in the Milky Way bright enough to be seen by the unaided eye. The last such supernova was seen in
AD 1604 (and is commonly named for
Johannes Kepler, one of its more renowned eyewitnesses). Supernovae typically occur every 50-100 years, so we're a bit overdue for one of these titanic stellar explosions (keep an eye on
Eta Carinae, a good candidate for the next blast). Five of these nine have been identified with hot, expanding blast waves (and sometimes the compact objects left behind by the blast have also been identified). A well-known example is the
Crab Nebula (and the
Crab pulsar, a
rapidly-spinning neutron star at the heart of the nebula), which was observed in AD 1054. The remnants of some of these other supernovae are a bit more difficult to identify, given the imprecisions of estimating positions of these blasts seen centuries ago. One Milky Way supernova was recorded in
AD 1181 by sky watchers in China and Japan, and evidently remained visible for about 3 months before fading away. Now, after nine centuries, astronomers believe they have finally found the remnant of this supernova, and have even identified the compact object left behind by the explosion. In 2013, an astronomer working with the "Deep Sky Hunters" group of amateur astronomers, discovered a rather innocuous extended nebula in some infrared images obtained by NASA's
WISE space telescope (shown above left). This object is within a few degrees of the reported position of
SN 1181. Deep optical observations (above right) revealed a blast wave speeding through space at more than 2 million miles per hour, suggesting an origin in an explosion that happened about 1000 years ago. X-ray observations by the
XMM-Newton X-ray telescope (middle, shown as contours superimposed on an infrared image) revealed a strong point-like source of X-rays from near the center of the nebula. This X-ray source is an unusual object, a rare, extremely hot, mass-losing star. Astronomers believe that this weird star is the result of the collision and merger of two
white dwarf stars, which would have produced an unusual type of supernova. If so, then this would be the first recorded double-white-dwarf merger known in the Milky Way, and the only one whose remnants can be studied in detail.