Long before Kim Jong-un, on a cold March night in Seoul almost 600 years ago, Korean astrologers spotted a bright new star in the tail of the constellation Scorpius. The date was March 11, 1437; two weeks later the “guest star” had faded from view. Based on its behavior, modern-day astronomers determined that the Royal Imperial Astrologers saw a nova explosion.
All novas, except the Chevy type, occur in close binary star systems, where a tiny, dense white dwarf — the dead remnant of a sun-like star — is paired up with a normal star similar to our sun. The dwarf cannibalizes the companion, stealing its tasty hydrogen gases and funneling them down to its surface. It takes about 100,000 years for the dwarf to build up a layer of hydrogen dense and hot enough to detonate in a colossal explosion. When it does, the blast blows the envelope off to produce a burst of light up to 300,000 times brighter than the sun.
For years, Michael Shara, a curator in the American Museum of Natural History’s Department of Astrophysics and lead author of a new study published this week in the journal Nature, has tried to pinpoint the location of the binary star that produced the nova eruption. Recently, his team expanded the search field and found the ejected shell of still-hot hydrogen gas expanding into space. They confirmed the finding with a photographic plate from 1923 taken at the Harvard Observatory station in Peru that showed the old nova undergoing yet another smaller burst called a dwarf nova eruption.
Finding old supernovas is difficult enough, but a smaller nova explosion is even more of a needle in a haystack. Shara and team did it by figuring out how much the star had moved in the century since the photo was taken and then extrapolating that motion back six centuries to what then would have been the center of the shell and site of the eruption.
Shara’s study pinpoints the nova and shows that it now undergoes smaller-scale dwarf nova eruptions. This jibes with with the long-held idea that novas go through a long-term life cycle: erupting, fading to obscurity for thousands of years, and then re-erupting in smaller scale explosions called dwarf novae after grabbing fresh material from their companions.
“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 Shara.
More plates from the 1940s helped confirm that the old nova is now a dwarf nova, proving that novae, so-called “nova-like” stars and dwarf novae fall along a continuum. After a nova eruption, a nova becomes “nova-like,” then a dwarf nova, and then, after a possible hibernation, returns to nova-hood. Over and over again up to 100,000 times over billions of years.
“In the same way that an egg, a caterpillar, a pupa, and a butterfly are all life stages of the same organism, we now have strong support for the idea that these binaries are all the same thing seen in different phases of their lives,” Shara said. What a wonderful comparison. I suppose it’s no surprise to see that nature operates in similar ways whether it comes to remote stars or the butterflies that delight us on warm afternoons.