Stargazers ID imminent supernova

For a given value of imminent, anyway

Astronomers have spotted a star that is, in astronomical terms, just about to explode in a spectacular supernova.

RS Ophiuchi, a white dwarf located near the constellation of Libra, caught astronomers' attention last February when it flared up to 1,000 times its normal brightness.

Astronomers identified it as a very rare beast - a star on the brink of becoming a type Ia supernova. These supernovae are very rare, but incredibly useful to astronomers. When a white dwarf does explode, Type Ia style, the brightness of the explosion is remarkably consistent - around five billion times as bright as our sun.

Astronomers use these supernovae as so-called standard candles, to work out intergalactic distances that would otherwise be impossible to gauge.

The actual explosion might not take place for a while though. Jeno Sokoloski, the lead author of a paper on the star published in Nature says that while the star could go bang tomorrow, it might not change for another 100,000 years.

RS Ophiuchi is in a binary system and is swallowing matter from its companion star at a rate of around a millionth of the sun's mass per decade. It needs to reach 1.4 solar masses before it goes critical.

But before the star reaches that stage it will periodically flare up, becoming many times brighter than normal. Astonomers at Harvard University managed to spot a plume of material that had been thrown off RS Ophiuchi by such a flare.

Sokoloski told the BBC: "The explosion is so energetic it actually lifts an envelope of material off the surface of the star and throws it off into space.

"[The plume] started slowing down almost immediately, within just two days, and that tells us the white dwarf must be extremely massive, in fact almost massive enough to collapse."

The last known type Ia supernova in the Milky Way was back in 1572 and was witnessed by Danish astronomer Tycho Brahe. He interpreted the new star in the sky as being just that - a new star, and coined the term Nova to describe what he had seen. ®

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