The mystery of 900-year-old Chinese supernova points to an unusual nebula.

In 1181 AD, a new bright point of light as luminous as the planet Saturn appeared to Chinese and Japanese sky gazers for a little more than six months before disappearing. Hundreds of years later, researchers believe they have finally figured out the source of this mysterious apparition.

Like the famous Crab Nebula-forming stellar explosion of 1054, the event is one of just a handful of bright nearby flashes noted in historical records. Still, unlike the Crab Nebula, the 1181 spectacle was tricky to pin down.

The historical archives leave some clues which have served modern astronomers well. First, timing: this "guest star" has shone for 185 days, from August 6, 1181, to February 6, 1182. The disc also shows its place in the sky, which was between two Chinese constellations, Chuanshe and Huagai, near the modern Cassiopaeus.

These cosmic puzzle pieces led a search team to the likely culprit of the former flash: a supernova whose remnants now form a rapidly expanding nebula called Pa30. The nebula's clouds move so quickly that, in the new research, scientists from Hong Kong, the U.K., Spain, Hungary, and France found that Pa30's dust and gas could travel the distance from Earth to the moon in a whopping five minutes. Using this speed and calculating backward, the researchers determined that the nebula would correspond to a supernova that blew up around 1181.

The team found that Pa30 formed from a rare and relatively faint type of supernova called a 'Type Iax supernova.' "Only around 10% of supernovae are of this type, and they are not well understood. The fact that SN1181 was faint but faded very slowly fits this type," Albert Zijlstra, an astrophysicist at the University of Manchester in the U.K., said in a statement about the new research.

The scientists also discovered that the Parker star, one of the hottest stars on the Milky Way, is probably a supernova's counterpart. The nebula and star result from a massive collision and subsequent fusion of two weakened star bodies known as white dwarves.

"It is the only Iax-type supernova in which detailed studies of the remaining star and nebula are possible," Mr. Ziljlstra said. "It's good to be able to solve both the historical mystery and the astronomical mystery."