It Took 600 Years to Figure Out This Mysterious, Exploding Ancient Star

On a cold, clear March night, in 1427 AD, Korean royal astrologers watching the sky witnessed a new star appear. It lingered in the sky, between what they called the "third and fourth star" in the constellation Scorpius, for two weeks. Then it faded from view.

Six hundred years later, modern astronomers finally figured out what the Koreans saw that night: a nova explosion.

"The fact that this was a new star, that it didn't have a bushy tail, means it wasn't a comet," Michael Shara, curator of astrophysics for the American Museum of Natural History and one of the researchers who solved the mystery,told Motherboard's podcast Science Solved It. "[The fact] that it was seen only for 14 days means that it couldn't have been a supernova. The only other transient stellar event that gets bright enough to be seen with the naked eye is a nova."

A nova explosion happens when a white dwarf star in a binary system sucks up so much gas from its companion star that it explodes and casts off all of that matter into the surrounding space. This causes the white dwarf to burn up to 100 times more brightly than normal, making it visible on Earth with the naked eye, and creating a new star in the sky as bright as Polaris—the north star.

Modern astronomers had long supposed that this was the explanation behind the ancient record, but they were unable to verify it because they couldn't see any evidence of what the Koreans had seen. The record at the time located the new star between Scorpius's "third and fourth" star, but ancient maps number the stars differently. Based on a Chinese constellation map that was carved into stone around the same sky, astronomers thought they had identified where the star appeared.

"It led to a pair of stars that we looked between over and over again with a wide variety of telescopes for more than 25 years with no success," Shara said. "Finally I and my colleagues threw up our hands in disgust and said, ‘Who knows what they were drinking that night!'"

But in 2016, when a new cache of astronomical images was digitized, Shara decided to take a second look. He used a computer to run an automatic search between the two stars, and again came up empty-handed, so he expanded his search zone slightly, to one star above and one star below the original pair. He found the nova in about three minutes.

By being able to identify exactly where the nova had occurred, researchers could use the thousands of images collected over hundreds of years from various telescopes to get a play-by-play of the cycles that occur before and after a nova explosion.The results were published in Nature in 2017, and provided greater insight into how these systems function.

It may have taken 600 years, but we finally have an explanation for what caused a new star to appear—and vanish—so many years ago, all thanks to humanity's enduring obsession with documenting the night sky.