Weird ancient burst of light in the sky turns out to be a nova

A 25-year hunt for a bright spot in the sky first seen almost six centuries ago has turned up a nova – and answered a long-standing question about this unique type of exploding star. We now have a better idea of what they do between more intense periods of activity.

Royal astronomers working in the court of King Sejong the Great of Korea noticed something unusual shining in the sky on 11 March 1437.

“A guest star began to be seen between the second and third stars of Wei,” an astronomer recorded – Wei being a collection of nine stars in the tail of the constellation Scorpius. “It lasted for 14 days.”

Now, Michael Shara at the American Museum of Natural History in New York has discovered the origin of this “guest star”. It was previously a classical nova, a unique type of stellar explosion, and now exhibits dwarf nova behaviour.

Repeating explosions

Shara found that binary star systems with classical nova and dwarf nova eruptions are in fact the same systems, just seen at different times in their lives.

Classical novae are not the same as supernovae, the death gasp of stars. They are more like a thermonuclear bomb going off on a star’s surface, says Shara, and happen in binary star systems that include a white dwarf accompanied by a nearby red giant or red dwarf.

The white dwarf pulls hydrogen away from its neighbour, and when enough hydrogen piles up, the gas can get hot enough to trigger nuclear fusion. Eventually, the gas layer explodes and is ejected from the star, resulting in a nova. This cycle repeats itself over a period of several months to several thousand years.

Classical novae can shine a million times brighter than the sun, but do not persist for long. Because the Korean astronomers noted that the ancient nova disappeared in just two weeks, Shara suspected this must have been what they saw.

He has been searching for evidence of it for more than 25 years. “We absolutely scoured that part of Scorpius, and came up with nothing,” he says.

Widening the search

About a year and a half ago, Shara expanded his search using digitised sky survey records and data from telescopes that observe the universe in different parts of the light spectrum. He included two additional Scorpius stars in his search, and found what he was looking for in a matter of minutes: a star with a nearby shell of hot gas.

“It was just one of those moments where you take the palm of your hand and whack it against your forehead,” he says.

Shara and his colleagues then used archival photographic plates from the Digital Access to a Sky Century at Harvard, a collection of astronomical survey photos dating back more than 100 years. The team saw that the nova had erupted again in 1923 and 1942, and noticed other smaller explosions in the 1930s that were dwarf novae rather than classical ones.

This suggests that these nova-producing systems generate smaller explosions between their repeating classical novae. The finding settles a debate about what these stars are doing in the intervals between novae, says Steven Shore at the University of Pisa in Italy.

“Astronomy is concerned with both the large scale and the long term,” he writes in an editorial, “and historical observations are often important for resolving evolutionary questions.”

Journal reference: Nature, DOI: 10.1038/nature23644

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