Casting light on a mystery 600 years later

Star seemed to vanish in 1437 after sighting by astronomers

Nearly six centuries ago, Korean astronomers scanning the night sky for omens of the future spotted a new star in the cluster of stars they called Wei, and what today’s star-watchers consider the tail of the Scorpius constellation. Fourteen nights later, it vanished. Astronomers have now identified the source of that brief brightening — a binary star system a couple of thousand light-years away.

Michael Shara, an astrophysicist at the American Museum of Natural History in New York, has been seeking to understand what happens following explosions in violent star systems known as “cataclysmic variables.” He has searched for the remnants of this particular event for a long time.

“Now, about 25 years later, we’ve finally come up with it,” Shara said. The researchers report their findings Wednesday in the journal Nature.

In these systems, one of the stars is a white dwarf, the burned out but still hot remnant of a star. The powerful gravity of the white dwarf pulls hydrogen away from its companion star and onto its surface. “You accumulate about a Pacific Ocean’s worth of hydrogen,” Shara said. With more and more hydrogen, the pressure builds until it sets off a thermonuclear explosion, a burst of light known as a nova that is up to a million times as bright as the sun.

“You get a giant fusion bomb, a hydrogen bomb going off on top of this white dwarf star,” Shara said.

That is what the Koreans saw on March 11, 1437.

As powerful as nova explosions are, they do not destroy either star. The white dwarf fades, and the cycle repeats until the next explosion, which could be up to 100,000 years later.

In recent decades, astronomers have observed the fading of novas over decades. They have also spotted binary star systems that appear quite stable and others belching only small eruptions known as dwarf novas.

Three decades ago, Shara proposed that novas, dwarf novas and the quiescent binaries were variants of the same type of system but at different stages. The idea was hard to test because astronomers have not been observing them for very long.

Thus, historical novas such as the one observed in Korea could provide important clues to the life cycle between explosions.

“You get a giant fusion bomb, a hydrogen bomb going off on top of this white dwarf star.” MICHAEL SHARA ASTROPHYSICIST