Remarkable 'zombie star' exploded, survived - and then kept on exploding

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

That is, until now.For the first time, astronomers have discovered a star that has gone supernova more than once. "You think of a supernova as a death of a star, and you think of death as something that only happens once".

In this kind of stellar death, a star that has about 105 solar masses (one solar mass is equal to the mass of our sun) dies in an explosion, but instabilities produce several outbursts over an extended period of time, which would fit with iPTF14hls.

Harvard University's astronomy chairman, Avi Loeb, who was not involved in the study, speculates a black hole or magnetar - a neutron star with a strong magnetic field - might be at the centre of this never-before-seen behaviour. During this time, the star repeatedly grew brighter and fainter up to five times. "It's the biggest puzzle I've encountered in nearly a decade of studying stellar explosions". "They don't know a damn thing about it". And while a typical supernova gets cooler as time goes on, this one stayed about the same temperature. When those two meet they annihilate each other, creating an explosion that blasts away some of the outer layers of the star without destroying the core. Typically, they continue to shine for roughly 100 days and over time they begin to fade.

According to one theory, some stars with a mass between 95 and 130 times that of the Sun can explode several times in cyclic deaths. These types of supernovas occur when a star eight to 15 times that of our own sun exhausts its energy and explodes. "But it predicts that the star should lose all of hydrogen in the first explosion". Rather than undergoing core-collapse, these huge stars burn so hot that their radiation spontaneously converts into electron-positron pairs, calculations suggest.

The radiation pressure from gamma rays stops a star from collapsing under gravity. It then contracts, then explodes, in a halting heave-ho. This was thought to be end awaiting all stars - until now. He searched a vast archive of old photographic plates from the Palomar Optical Sky Survey that he knew had been digitized and put online.

But a subsequent image of the galaxy in 1993 showed no sign of the supernova. But the supernova, known as iPTF14hls, keeps on burning. It's hard to estimate the statistical significance of a signal in a digitized image of a photographic plate, Nugent said.

"So, if iPTF14hls is the first pulsational pair instability supernova then the theory needs to be revised". But although Arcavi, Howell and their Las Cumbres colleagues offer the PPISN idea as the leading explanation for iPTF14hls in their paper, the theory is not without problems. "This one just really blew away everything we thought we understood about them", said astrophysicist Iair Arcavi of the University of California, Santa Barbara who led the study.

For one thing, the huge 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. The fresh raw ingredients in dwarf galaxies allow huge stars to form, like pumpkins in fertile soil.

"It begs the question: how many have we missed in the past?" he said. "So it's a dinosaur or something". "Otherwise, it's something completely new", Arcavi added.

"We could not have kept tabs on iPTF14hls for this long and collected data that challenges all existing supernova theories if it weren't for the global telescope network", said Arcavi.

"As of now, no detailed model has been published that can explain the observed emission and constant temperature of iPTF14hls, let alone the possible eruption 60 years ago", he wrote.

The fact that astronomers has spotted a supernova that has apparently "risen from the dead" is truly puzzling to science.

The odd supernova is still bright today, more than three years after it was first discovered. "So they're not just wrinkles on an old theme; they are beasts". Its light has attenuated to one-twentieth its peak brightness. It would also result in multiple shells of expanding material like the one we've already seen. Further monitoring may better explain what's going on, he said.

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