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'Zombie' star survived going supernova

From BBC - November 8, 2017

It's the celestial equivalent of a horror film adversary: a star that just would not stay dead.

When most stars go supernova, they die in a single blast, but astronomers have found a star that survived not one, but five separate explosions.

The "zombie" star kept erupting for nearly two years - six times longer than the duration of a typical supernova.

An international team details their results in the academic journal Nature.

"This supernova breaks everything we thought we knew about how they work. It's the biggest puzzle I have encountered in almost a decade of studying stellar explosions," said co-author Iair Arcavi, a postdoctoral fellow at Las Cumbres Observatory (LCO) who is based in California.

The mysterious object, iPTF14hls, was picked up in September 2014 by a wide-field camera astronomy survey.

Astronomers identified it as an exploding star in January 2015; everything about the discovery seemed normal at first.

In common types of supernova, a blast at the centre of the star ejects material at high speed into surrounding space. The expansion of this material releases energy, causing the object to shine brightly for up to 100 days (about four months) before it finally fades.

It soon became clear this exploding star was not conforming to expectations. For one thing, it did not fade, but shone brightly for 600 days - nearly two years.

What's more, the astronomers found that its brightness varied by as much as 50% on an irregular timescale, as if it was exploding over and over again.

And, rather than cooling down as expected, the object maintained a near-constant temperature of about 5,700C.

Intriguingly, by combing through archived data, scientists discovered an explosion that occurred in 1954 in exactly the same location. This could suggest that the star somehow survived that explosion, only to detonate again in 2014.

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