The star that keeps exploding

Well that's just cool.

Sure enough, that “ordinary” supernova wasn’t so normal anymore. It was getting brighter — almost as if it had exploded again. “We’ve never seen a supernova do that before,” Arcavi, an observational astronomer at UC Santa Barbara and Las Cumbres Observatory, tells The Verge.

Over the next two years, their data unveiled just how strange the supernova was: it stayed bright for a lengthy 600 days, instead of the typical 100 days supernovae usually shine before finally going dark. During this time, the star repeatedly grew brighter and fainter up to five times. It was like it was erupting over and over again, as if the star just refused to die.

Astronomers can tell how old a supernova is by measuring how fast the material is moving around it; usually the materials start to slow down a while after a supernova has exploded. With this star, however, the materials around the event remained at really high speeds. So after 600 days, the supernova looked as if it was only 60 days old.

Adding to the mystery is that this star may have even exploded before 2014 — actually, over half a century ago. When digging through archived telescope data, Arcavi and his team found another supernova that had exploded in essentially the same spot in the sky back in 1954. They’re pretty sure it’s the same star — or else a very unlikely coincidence. How did this star explode at least once in the ‘50s and then again three years ago?

So far, the best theory to explain what’s being seen is something known as pulsational pair-instability, or PPI. It’s an event that kind of masquerades as a supernova, occurring in stars around 100 times the mass of our Sun. When such a star reaches the end of its life, its core can get incredibly hot — upwards of billions of degrees — and become unstable. At this point, the oxygen inside the core ignites and the star will blow off an outer layer of material. The result is something that looks like a big explosion, but it ultimately leaves the core of the star intact. The core can then pulse like this again and again, shedding material until it eventually collapses into a black hole.

No one’s ever actually seen a PPI in action before, so this would be a big first. However, even the PPI theory doesn’t exactly match what Arcavi and his team saw. The fluctuations from a PPI are thought to be much more varied in brightness, with more time in between bursts. There are other less viable explanations for the zombie star, but nothing that fits quite right. “The bottom line is we don’t know what it is, but for scientists that’s the best news of all,” says Woosley. “We love a puzzle.”


https://www.theverge.com/2017/11/8/16618568/supernova-las-cumbres-observatory-multiple-explosions-pulsational-pair-instability