In 1181 CE, Chinese and Japanese astronomers noticed a “guest star” as bright as Saturn briefly appearing in their night sky. In the thousand years since, astronomers have not been able to pinpoint the origins of that event. New observations have revealed that the “guest star” was a supernova, and a strange one at that. It was a supernova that did not destroy the star, but left behind a zombie that is still shining.
“Guest stars” are what modern astronomers now call novae or supernovae, and the brightness of the event in 1181 CE (described as being as bright as Saturn) and its longevity (visible to the naked eye for 185 days) means that it was almost certainly from a supernova. For decades, a pulsar wind nebula in the same region of the sky was thought to be the remnants of that supernova, but new estimates have placed the age of that nebula to be around 7,000 years old, far too old to account for the records from 1181.
Searching through the archives from NASA’s Wide-field Infrared Survey Explorer, a team led by astronomers at the University of Hong Kong have found an alternate, and much stranger, possible origin story. Their work recently appeared in the preprint journal arXiv.
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The astronomers found one of the hottest known Wolf-Rayet stars, which they call Parker’s star, named for one of the study leaders. Wolf-Rayet stars are massive stars surrounded by hot envelopes of gas, and are some of the brightest stars in the sky.
Surrounding Parker’s star is a nebula, dubbed Pa 30. The nebula has an expansion velocity of around 1,100 km/s, and given its current size it likely formed from a supernova event about 1,000 years ago – right in line with the “guest star” observations.
The expansion speed of 1,100 km/s is far slower than a typical supernova remnant, and is usually connected with a rare kind of supernovae that doesn’t completely detonate its star. That fact would also explain the existence of Parker’s star – it’s a zombie remnant that should have died a thousand years ago, but it still living.
These kinds of supernovae are extremely rare, and this observation could mean this is the only known such zombie remnant in the Milky Way. And we wouldn’t know if it weren’t for those astute astronomers a thousand years ago.