Shara et al. Nature 2017

Astronomers have identified the source of a stellar eruption observed by Korean astronomers in 1437 — and, as a result, new evidence for how certain types of stars evolve.

The history: In the 15th century, royal astronomers in Korea recorded a bright star that appeared in a constellation and, 14 days later, disappeared. Modern astronomers know that short appearance meant it wasn't a supernova but a classical nova.

Fast forward 550 years and, in 1986, Michael Shara from the American Museum of Natural History publishes a paper suggesting classical novas — unique stellar explosions in which the star ejects a layer from its surface — and dwarf novas were the same stars but at different stages of their evolution. Classical nova erupt every couple thousand of years so proof could only be found in centuries-old astronomical records. On and off for 20 years, Shara looked for the evidence in the constellation the Koreans noted but came up with nothing. "It just wasn't the right place," says Shara.

What they did: When Shara took another look about 18 months ago using new data from telescopes, he expanded his search to others stars in the Scorpius constellation — and found a star that supported his initial idea. When he put an image of it from 1923 on top of one from 2016, he could see how much it moved. He worked backward along its path over the past 580 years and "Bang! There it was!", he says. "We show that, almost 500 years a classical-nova event, the system exhibited dwarf-nova eruptions... We conclude that many old novae become dwarf novae for part of the millennia between successive nova eruptions," the authors wrote.

They also found the star continues to explode in a cycle of hibernation and eruption that could be seen in photographic plates from the 1930s and 40s.

Sound smart: Classical nova and supernova aren't the same. The latter undergoes a core-collapsing death while the outbursts of classical nova are eruptions from their surface that repeat — and the star lives on.

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