On a spring night in 1437 AD, something unusual happened — Korean astronomers spotted a new star in the sky above Seoul. Two weeks later, they reported, it vanished again.
Now scientists have finally tracked down the object behind that temporary sparkle in the sky, offering a new glimpse into the hidden lives of stars, and how they evolve through different stages of their lives.
The new star was a nova (a word that literally means new star) or stellar eruption that appeared in the tail of the constellation known in the western world as Scorpio.
Canadian astronomer Michael Shara began looking for the source of the ancient nova more than 30 years ago. He has finally found it, he reports in a paper published Wednesday in the journal Nature.
Shara, an astrophysics curator at the American Museum of Natural History who was born and raised in Montreal, became interested in the story of the mysterious Korean star back in 1986. The fact that it appeared and then disappeared in 14 days suggested it was a classical nova, sudden brightening of a star that isn’t as intense or long lasting as a supernova or star explosion.
What excited Shara was the Korean astronomers had provided a lot of details that should allow astronomers to figure exactly where in the sky that nova had appeared.
It was spotted above Seoul on March 11, 1437, they reported, between the second and third star of a part of the sky that eastern astronomers call the sixth lunar mansion. That would have been very near the horizon.
The problem was, Shara said, “there’s no good map from the Koreans which points at the sky or shows you the constellations and tells you which is second star and which is the third.”
Shara enlisted the help of Richard Stephenson, a historian of ancient Asian astronomical records at Durham University in England. By looking at Chinese maps, which also divide the sky into lunar mansions, Stephenson managed to pinpoint where he thought the star that caused the nova should be located.
For decades, on and off, Shara, Stephenson and other collaborators used telescopes around the world to search the area that Stephenson pointpointed.
“It became a bit of an obsession,” he admitted.
Then about a year and a half ago, after a long break, he decided to try again. But this time, he widened his search area a little bit.
“And in 90 minutes, I found it,” he recalled. “It was the moment both of exaltation and a little bit of, ‘Oh my God, all that wasted time and effort!'”
It turns out the star wasn’t where it was expected because it had “moved” over the past 580 years, owing to the fact the star is relatively close by — just a few hundred light years away — compared to most other stars in the sky. So it appears to move more quickly, just as nearby objects go by more quickly than those that are farther away when you look out the window of a moving car.
That inconvenient quirk made it hard to find the star, but now represents a new way to measure time in astronomy, Shara said. “That’s a cool thing.”
Butterflies and caterpillars
But an even more exciting discovery was what got Shara interested in the star in the first place — he thought it could help him observe the way certain stars evolve through different stages of their life cycle. It would help prove his theory that two types of binary stars — systems of two stars that orbit each other — were actually two different life stages of the same star.
“They’re like butterflies and caterpillars,” he told CBC News. But while caterpillars turn into butterflies within weeks, stars transform very slowly. Often, even very “fast” changes would take hundreds of years to observe, so you would have to compare a star today to what it looked like hundreds of years ago.
That’s what Shara did.
Sure enough, the data seemed to show what he predicted.
A nova like the one spotted by the Korean astronomers is typically caused by a binary star consisting of a white dwarf and a younger, active star such as a red dwarf. A white dwarf is a tiny, dense dead star, while a red dwarf is a star similar to our own sun, but a little bit smaller and cooler.
A nova comes from a nuclear eruption from the surface of the white dwarf “in kind of a giant hydrogen bomb,” Shara said. The eruption throws a giant shell or nebula of material from the surface into space. It also heats everything up into a “nova-like binary star,” an object that glows 100 to 1000 times as bright as the sun for hundreds of years, Shara said.
Shara had predicted that nova-like binary stars eventually cool down and become much dimmer objects called dwarf novas, which brighten to a tenth the brightness of the sun every few months. But no one had ever observed that — until now.
By looking at images over the past century of the star that caused the 1437 nova, Shara spotted the shell thrown off by the eruption and multiple dwarf novas.
Interestingly, Shara likely first spotted the shell at the beginning of his search in the 1980s, when he was searching with University of Montreal astronomer Anthony Moffat at an Australian telescope. “And we saw, ‘Oh, there’s a smudge — that could be the nova,'” recalled Moffat, who co-authored the paper.
But the star, which later turned out to have moved more than the shell, was nowhere to be found.
Moffat finally heard the news about the discovery of the star itself last year.
“I couldn’t believe it,” he said, “after all these years.”