On the night of 11 March 1437, Korean astronomers recorded a strange light low in the sky, in the tail of the constellation Scorpius. It must have been at least as bright as the North Star, Polaris, maybe even as bright as the stars of the Big Dipper. Fourteen days later, the light disappeared.
Image: Shara et al (2017)
More than half a millennium has passed since that light flickered out, but after decades of searching, a team of astronomers has found it. A classical nova was the source; these explosions occur when one star in a binary sucks mass from a companion until it lets off a massive thermonuclear explosion on its surface. The team's hunt has yielded an important new kind of accurate astronomical clock to measure the ages of certain stars.
"You usually consider yourself lucky if you can age anything to 10 or 20 per cent," Michael Shara, Curator in the Department of Astrophysics at the American Museum of Natural History told Gizmodo. "Here we have it to the day, which is unprecedented accuracy."
Shara's team began looking for the source of this nova 30 years ago to support whether or not these pairs of stars go into hibernation after letting off their bright explosions. Eventually, he called off the search after identifying the incorrect pair of stars. But 18 months ago, he said, he began looking through new databases not available to him during his earlier search for cataclysmic variables — astronomical sources that repeatedly flash and then dim again.
He noticed a pair he'd initially ignored in the '80s because he thought it was the wrong one — the stars were off-centre from the shell of debris they could have created. Then he realised that the stars had just moved with respect to the other stars in the background. He located an image of the same patch of sky in 1923, and there was his source, just a smidgen closer to the centre of the shell. "If that plate had been lost or not digitised, all of those things would not have came together," he said.
The 1923 plate with the cataclysmic variable in between the red lines
Through "proper-motion age dating", accounting for the changing position of this stellar binary and dating backwards, this pair of stars would have been right in the middle of the shell in 1437. His team found the source of the nova, and published the results today in the journal Nature.
So, how did these stars leave this ball of gas behind? "Imagine you're diving in a convertible down the highway at 70 miles an hour," and throw confetti out the window, he explained. "The confetti will stop quickly because it's slowed down by the wind. That's the shell that's been thrown out of the central binary. The gas runs into the interstellar medium and gets slowed down, while the cataclysmic binary just keeps on going."
One researcher noted that this wasn't the first time astronomers have found the shell to accompany a cataclysmic variable. "For the first time, however, they could confirm the age of the shell from an independent measurement of the nova's proper motion," Przemek Mróz, a PhD student at the Warsaw University Observatory who knows a lot about these classical novae, explained to Gizmodo. He found the results compelling.
This discovery has broader implications for the study of these temperamental stellar binaries. Scientists have observed these binaries with a high level of mass transferred from the companion star to the white dwarf — all but one of the novae observed in the 20th and 21st century are in this state currently, said Mróz, including the nova he observed and published last year. But this binary is instead what's called a dwarf nova, flashing every so often with low levels of mass travelling between the stars. This implies that nova-producing pairs go into a slumber in the thousands of years between explosions.
Shara had previously told me that an important step in understanding these cataclysmic variables would be measuring a nova going into hybernation — and that's exactly what he's done. "It's something that was predicted 30 years ago: the hibernations scenario of cataclysmic binaries," he said. "This is the strongest evidence in support of that scenario."