What is dead may never die

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Under this scenario, a star about 100 times as large as our sun could become so hot deep in its core that energy could turn into matter and antimatter, which would make the star unstable.

Arcavi and his colleagues aren't positive yet what might be happening inside the star to cause such a weird pattern.

So what's going on with this continual massive explosion? Such a supernova forms when a massive star can no longer sustain nuclear reactions in its core, which then collapses to form a neutron star or black hole. Knowing the full spectrum of stellar fates is crucial for understanding galactic evolution.

RG: You suggest that the star might have been 95-130 times more massive than the sun. The observations showed that the supernova remained bright for 600 days, which had never been seen in a Type II-P event.

"A supernova is supposed to be a one-time thing - the star explodes, it's dead, it's done, it can't explode again", says astrophysicist Iair Arcavi of the University of California, Santa Barbara. "They don't know a damn thing about it". Do we know of any other star this massive? That's true even in space, where stars routinely blow themselves up. "Ordinary supernovae don't do that". For one thing, it didn't fade, but shone brightly for 600 days - almost two years. "We'd never seen a supernova do that before". Whatever this was looked like a Type II-P supernova in composition but didn't behave like one.

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But the mystery only deepened when astronomers looked at archival data gathered by the Palomar Observatory Sky Survey. "For now, the supernova offers astronomers their greatest thrill: something they do not understand", Woosley says.

An artist's depiction shows a supernova explosion. But after the explosion, the electrons and positrons can recombine into gamma rays and hold up the remaining stellar core.

Arcavi explained to The Register that, "according to Einstein's famous equation, E=mc, energy can be turned into matter, but you need a lot of energy to make a little matter". It then contracts, then explodes, in a halting heave-ho.

Washington D.C. [USA], Nov 9: A star that refuses to stop shining has been discovered.

In Garching, Nugent thought to check the historical record for evidence of precursor explosions from iPTF14hls's progenitor star. So after 600 days, the supernova looked as if it was only 60 days old. This supernova, for example, will affect the entire galaxy it's in! It's hard to estimate the statistical significance of a signal in a digitized image of a photographic plate, Nugent said. The only model that comes close to explaining 14hls is one called "pulsational pair instability". If so, the discovery would prove the bold PPISN hypothesis and add a major branch to the star-death taxonomy. However, even the PPI theory doesn't exactly match what Arcavi and his team saw. "We can't tell for sure that it's the same star exploding", Arcavi said.

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For one thing, the enormous energy released in the course of iPTF14hls's known explosions (and there may have been more) already surpasses Woosley's predictions for how much energy the pair-instability mechanism can muster. "It got faint and then bright", he said.

The PPISN idea isn't a ideal match for observations, but that might just mean that researchers need to improve the theory. Often theorists can only do computer simulations of two-dimensional slices of stars and cautiously extrapolate to three dimensions. They provide the energy to power the supernova. "What we saw was the spectrum of the most typical supernova observed".

"These explosions were only expected to be seen in the early universe and should be extinct today", says Andy Howell, co-author of the study. "So it's a dinosaur or something". Continuous monitoring by Las Cumbres Observatory's global telescope network has opened up a temporal view of supernovas, which could yield many more surprises. But the supernova, known as iPTF14hls, keeps on burning. Astronomers have also spotted extremely bright so-called "superluminous" supernovas, whose cause and origin are also unknown. "So they're not just wrinkles on an old theme; they are beasts".

But this one, called iPTF14hls, just kept going and going.

"It remains bright but it is fading, though it depends what one means by "bright". It was like it was erupting over and over again, as if the star just refused to die.

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