A team of astronomers have pinpointed the location of a nova that was last seen almost 600 years ago.
In 1437, Korean astrologers spotted a bright new star in the tail of the constellation Scorpius and observed it for 14 days before it faded from view. Studying the ancient record made by the Royal Imperial Astrologers, modern astronomers determined that what they had seen was a nova explosion but, until now, they were unable to find the binary system that caused it.
“This is the first nova that’s ever been recovered with certainty based on the Chinese, Korean, and Japanese records of almost 2,500 years,” said the study’s lead author Michael Shara, a curator in the American Museum of Natural History’s Department of Astrophysics.
A nova is essentially a hydrogen bomb on a gigantic scale, produced in a binary system where a star is being devoured by a white dwarf (a dead star). Over roughly 100,000 years, the white dwarf builds up a critical layer of hydrogen that it then blows off, producing a burst of light that can make the star up to 300,000 times brighter than the sun for any period from a few days to a few months.
Shara has tried to locate the Korean nova for several years, teaming up with Durham University’s Richard Stephenson, a historian of ancient Asian astronomical records, and Liverpool John Moores University astrophysicist Mike Bode. Their recent success came after expanding the search field and discovering the ejected shell of the classical nova. The finding was confirmed with a photographic plate from 1923 taken at the Harvard Observatory station in Peru.
“With this plate, we could figure out how much the star has moved in the century since the photo was taken,” Shara said. “Then we traced it back six centuries, and bingo, there it was, right at the centre of our shell. That’s the clock, that’s what convinced us that it had to be right.”
The Peru plate is available online as part of the Digitizing a Sky Century at Harvard (DASCH) project and it was other such DASCH plates that helped reveal the system has now become a dwarf nova. The discovery supports the idea that novae go through an incredibly long-term life cycle, fading for thousands of years after eruption before slowly building back up to full-fledged nova once more.
It was previously believed that “cataclysmic binaries” – novae, novae-like variables and dwarf novae—were separate entities, but the rediscovery of this ancient nova as a dwarf suggests instead that they are one and the same, but at different stages in their lives. Following an eruption, a nova becomes nova-like, before taking the form of a dwarf nova. Then there may be a period of hibernation, after which it becomes nova-like again and then a fully fledged nova. This cycle repeats, potentially up to 100,000 times over the course of billions of years.
The study, which is published in the journal Nature, was based on observations from the Southern African Large Telescope (SALT), and the Las Campanas Observatories’ Swope and Dupont telescopes.