Supernovae are some of the most energetic events in Universe. Most supernovae are thought to come from stars eight or more times more massive than the sun that have used up all of their nuclear fuel. Gravity causes the center of the star to collapse into a neutron star or a black hole, and the energy released by that collapse causes the star to blow itself apart.
There are many things about supernovae that we do not yet understand, and many of unknowns could be solved by looking at supernovae in our own galaxy. However, the last supernova seen in our galaxy was 402 years ago, on Oct. 9, 1604. Not very many living astronomers were around for that event, and the state-of-the-art observing tool (the eyeball) is primitive compared to today's instruments.
So, much of the work on supernova is done on explosions in distant galaxies, not in our own. The exceptions to this are studies of supernova remnants -- the shreds of gas that the explosions leave behind.
There are many such remnants, but for very few of them do we know the date when the star exploded. This is very useful, because knowing the time of the explosion allows us to study how these remnants evolve over time. Some famous supernova remnants are the Crab Nebula (seen by the Chinese in 1054), the supernova of 1572 (Seen by Danish astronomer Tycho Brahe), and the supernova of 1006, one of the brightest recorded supernova, being over 100 times brighter than the planet Venus!
From Chinese and Roman records, astronomers knew of a supernova that occurred in A.D. 185, but we weren't sure what, if any, remnant it left behind. Two supernova remnants are in the right general area of the sky, but astronomers guessed that these were many thousands of years old. Some astronomers even suggested that there was no supernova, but that the event was a bright comet.
Now the mystery appears to be solved. X-ray observations of one of the two suspect remnants, RCW 86, finds that it is not several thousand years old, but only a couple thousand years old, making it an almost perfect match to the observed supernova. How could we have had the age so wrong?
One way we get the age of a supernova remnant is to measure how fast it is growing and compare it with the remnant's size. For example, if it is 10 light-years across and growing at a rate of 0.01 light years per year, we know it is 1000 years old. But some parts of Supernova 185 seem to be plowing into dense clouds of dust and gas that are slowing it down faster than other parts, giving us errors in the derived age. The X-rays in the picture above allow us to see the dust and gas getting heated up by the expanding blast for the first time.
So, mystery solved, probably, and another supernova remnant with an age known to a few months.