'Zombie' Star Has Been Exploding For Years And Will Not Die

The aftermath of a typical, short-lived supernova as it fades. NASA's Goddard Space Flight Center/ESA/Hubble/L. Calcada

Usually, when something explodes, that's it—it's done. That's true even in space, where stars routinely blow themselves up. Now astronomers think they've spotted something remarkable: a star that has exploded twice and whose current explosion just keeps going. They reported their findings in a paper published Wednesday in the journal Nature.

Those findings are based on studying a star that scientists had written off as not being very interesting at first. Until, that is, an intern looked back at the data and saw something weird. "He saw it had faded and then gotten bright again, and that's what caught his attention," said first author Iair Arcavi, an astronomer at the University of California, Santa Barbara, and Las Cumbres Observatory. "That is very not normal. Supernovae are supposed to get bright and then fade."

But this one, called iPTF14hls, just kept going and going. First spotted in September 2014, it has brightened and dimmed five times since then, a phenomenon scientists had never seen before.

And when astronomers looked back at their records, they found something even weirder: An explosion in exactly the same part of the sky back in 1954. "We can't tell for sure that it's the same star exploding," Arcavi said. "But since supernovae are so rare and the galaxy is pretty small, we consider there's about a 95 to 99 percent chance that it's the same star."

Arcavi and his colleagues aren't positive yet what might be happening inside the star to cause such a weird pattern. But he pointed to a theory called pulsational pair instability as one possible explanation. 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.

The star would then act like someone shoveling snow and gradually shedding outerwear: As it became unstable, it would eject a layer of mass, which would make it stable again for a limited time before heat builds up again. Each successive layer would eventually collide with its predecessor, which could explain the current pattern astronomers are seeing.

11_08_iPTF14hls_light_curve The yellow line traces the unprecedented cycle of brightening and dimming that astronomers have watched unfold. ​LCO/S. ​Wilkinson.

But the pulsational pair instability theory doesn't perfectly match the data Arcavi and his colleagues had gathered. In particular, they found much more hydrogen in 2014 than they would have expected in a star that has already exploded, since hydrogen is the lightest element and therefore the easiest to lose.

"It's the theory that comes the closest to explaining this," Arcavi said of pulsational pair instability. "But it could also be something else. It could be something completely new, which is even more exciting."

It's unusual to even be able to keep watching a phenomenon like this every few days for three years, Arcavi added. Scientists have only caught the iPTF14hls antics thanks to a global network of robotic observatories that makes collecting data cheaper and easier. Fortunately, the same technique could increase astronomers' odds of seeing a similar event in the future—and coming one step closer to cracking the secret of the star that just doesn't want to die.