It's one of astronomy's oldest mysteries—Chinese astronomers in AD 185 discovered a "guest star" that appeared in the sky for about eight months. Modern scientists have for decades believed it to be the first documented sighting of a supernova, but what kind of supernova was it? At long last, astronomers know. The ancient astral explosion was in fact a Type Ia supernova—a star about the same size as our sun that shriveled into a white dwarf, then emptied all the matter around it in space, siphoned off a nearby star, and exploded in an unusually large and rapid nova, reports Space.com.
Now called RCW 86, the star once sat about 8,000 light years from Earth. Researchers used two NASA telescopes to measure the temperature of the exploded star's "guts" still in space—which would appear larger than a full moon if they were visible to the naked eye—and determined how much gas had to be in the star that created the supernova. "This supernova remnant got really big, really fast," said the astronomer who led the study of the supernova. "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."