In 185 CE, Chinese astronomers reported the presence of an incredibly bright "guest star" that appeared suddenly in the sky and stayed there for months. This was the first recorded supernova...and astronomers are only now understanding what it really is.
By the 1960s, astronomers were pretty much certain that their ancient counterparts had seen a supernova. In fact, they were able to determine just where the remnants of the explosion would be now and rediscover this "first" supernova. Sure enough, they located RCW 86, the remnant of this ancient explosion, about 8,000 light-years from Earth.
This should have been a nice epilogue to the story of humanity's first known encounter with a supernova, but discovering RCW 86 only created a new mystery to replace the old one. The spherical remains of the supernova are absolutely gigantic - in fact, if we could see them in the infrared spectrum, they'd appear bigger than the full Moon in the night sky. That doesn't fit with what's created by your usual supernova, in which a giant star explodes at the end of its life.
North Carolina State astronomer Brian J. Williams explains the mystery:
"This supernova remnant got really big, really fast. It's two to three times bigger than we would expect for a supernova that was witnessed exploding nearly 2,000 years ago. Now, we've been able to finally pinpoint the cause."
As it turns out, this isn't your usual supernova. New observations by the Spitzer and WISE telescopes have revealed this is actually a Type Ia supernova, which is created not by the death of a giant star but by one much like our own Sun. Stars like ours don't generally explode - instead, they run out of fuel and temporarily swell to enormous size, then contract back into a super-dense white dwarf.
But if that white dwarf happens to be close enough to another star, its enormous gravitational pull can start siphoning off mass. Over time, an unstable situation builds up, and the bloated white dwarf explodes in a truly spectactular supernova - the Type Ia variety. This particular white dwarf created a huge cavity around it of mostly empty space. This is why the modern remnants are so gigantic. The gas and dust ejected by this explosion were able to spread out much further than those from normal supernovas.
WISE and Spitzer project astronomer Bill Danchi adds:
"Modern astronomers unveiled one secret of a two-millennia-old cosmic mystery only to reveal another. Now, with multiple observatories extending our senses in space, we can fully appreciate the remarkable physics behind this star's death throes, yet still be as in awe of the cosmos as the ancient astronomers."