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What is dead may never die

09 November 2017, 07:35 | Justin Tyler

The fluctuating light curve of the 600-day supernova compared to a standard 100-day supernova. Image LCO S. Wilkinson

The fluctuating light curve of the 600-day supernova compared to a standard 100-day supernova.    
   Image LCO  S. Wilkinson

The Las Cumbres group was aiming to track 500 supernovas over three years as part of a broader mission by astronomers to develop a taxonomy of star deaths.

A shocking supernova refuses to die. But the supernova, known as iPTF14hls, keeps on burning.

In a galaxy far, far away lies a puzzling star, one that seems to be dying multiple deaths, leaving astronomers to rethink the way these beacons of the sky end their lives. This behaviour has never been seen in previous supernovae, which typically remain bright for approximately 100 days and then fade. "They don't know a damn thing about it".

So what's going on with this continual massive explosion? These events can repeat until eventually the star collapses into a black hole. The leading idea is that this may have been something of an imposter - an event that looks like a supernova, but doesn't ultimately lead to the destruction of a star.

A normal supernova rises to peak brightness and fades over 100 days.

Usually, when something explodes, that's it-it's done. Whatever this was looked like a Type II-P supernova in composition but didn't behave like one.

In September 2014, when astronomer Iair Arcavi found a new supernova in the night sky, he didn't think much of it.

"For now, the supernova offers astronomers their greatest thrill: something they do not understand". "That would cause the star to go violently unstable, and undergo repeated bright eruptions over periods of years". It then contracts, then explodes, in a halting heave-ho. It was a phenomenon they had never seen before. They analyzed the light of the explosion to study the material ejected and its speed.

In Garching, Nugent thought to check the historical record for evidence of precursor explosions from iPTF14hls's progenitor star.

"Piecing it all together, from our observations at Palomar Transient Factory, Keck Observatory, LCOGT, and even the images from 1954 in the Palomar Sky Survey, has started to shed light on what this could be".

An image of the supernova in 1954, compared to 1993 when it was dark. But when he looked at an older, grainier photographic plate from the 1954 survey, amazingly, a point of light shone at its location. It's hard to estimate the statistical significance of a signal in a digitized image of a photographic plate, Nugent said.

Which means either that supernova iPTF14hls is a really, really weird pulsational pair instability supernova - or something completely new.

It soon became clear this exploding star wasn't conforming to expectations. However, even the PPI theory doesn't exactly match what Arcavi and his team saw.

Writing in a news and views article published in Nature, Prof Stan Woosley, from the University of California, Santa Cruz, said that in the Pulsational Pair Instability theory, a massive star may lose about half its mass before the series of violent pulses begins.

But the star may not be an example of a pulsational pair instability as the energy it released is more than the theory predicts.

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

The PPISN idea isn't a ideal match for observations, but that might just mean that researchers need to improve the theory.

RG: Why do you suggest that current models of star evolution and explosion might have to be rewritten? . 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.

But there are a few problems with that theory: for one, this was only thought to happen in the early days of the universe. The fresh raw ingredients in dwarf galaxies allow huge stars to form, like pumpkins in fertile soil. According to that model, which has been only theoretical until (maybe) now, a very massive star (about 100 times the mass of the sun) will get so hot in its core that some of its energy will actually turn into matter and antimatter through Einstein's famous e=mc^2. We saw that it looked like a spectrum of the most common type of supernova - Type IIP. "So it's a dinosaur or something".

After all, iPTF14hls is not the only odd supernova to have appeared in recent years. Sure enough, that "ordinary" supernova wasn't so normal anymore. "So they're not just wrinkles on an old theme; they are beasts".

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

This star's size could have a lot to do with its mysterious behavior, scientists said.

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