At the end of its life, a star goes supernova, ejecting its outer layers in one of the biggest cosmic cataclysms in the universe. With most of its mass gone the star then "dies", collapsing in on itself to form a black hole or a neutron star. Thousands of these events have been observed over decades, but now one star has broken the rules and refused to die, somehow exploding several times over 60 years. The find may challenge everything we know about the death of stars.

Dubbed iPTF14hls, the supernova was first spotted in September 2014 by the Palomar Transient Factory astronomical survey, and was initially assumed to be a run-of-the-mill event. It brightened as the gas and matter was thrown away from the host star, then slowly faded from view over the course of about three months. A textbook supernova.

But a few months later, astronomers at Las Cumbres Observatory (LCO) noticed iPTF14hls was brightening again. In fact, it went on to brighten and fade away several times over the next few years, a pattern that had never been detected before.

"While the spectra bear a resemblance to normal hydrogen-rich core-collapse supernova explosions, they grew brighter and dimmer at least five times more slowly, stretching an event which normally lasts 100 days to over two years," says Peter Nugent, co-author of a new study describing the strange phenomenon.

But the mystery only deepened when astronomers looked at archival data gathered by the Palomar Observatory Sky Survey. One image from 1954 shows what looks like another supernova signature coming from the location of iPTF14hls, which wasn't visible in a shot from 1993 of the same patch of sky. Somehow, this star appears to have exploded back in 1954 and survived to repeat the process 60 years later.

"This supernova breaks everything we thought we knew about how they work," says Iair Arcavi, lead author of the study. "It's the biggest puzzle I've encountered in almost a decade of studying stellar explosions."

The study suggests that supernova iPTF14hls was the first observation of a previously-theorized event known as a "Pulsational Pair Instability Supernova." In these cases, stars more than 50 times more massive than the Sun burn with cores so hot that they convert energy into matter and antimatter. 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. Such a star might be able to repeat that process several times over the course of decades, slowly shrinking until it runs out of fuel and finally goes supernova in a more traditional manner.

But there are a few problems with that theory: for one, this was only thought to happen in the early days of the universe. Then there's the fact that this supernova released more energy than that model predicts, meaning this event could be a brand new type of supernova.

"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. "This is like finding a dinosaur still alive today. If you found one, you would question whether it truly was a dinosaur."

The strange supernova is still bright today, more than three years after it was first discovered. Teams are keeping an eye on it to learn whatever else they can, and the astronomers hope to spot more events like it in the future.

The study was published in the journal Nature.

Sources: Keck Observatory, Las Cumbres Observatory

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