It was as jarring as meeting a 600-year-old human -- one who looked no older than 60, Arcavi said.
On top of that, iPTF14hls did not follow the normal path for a supernova, Woosley pointed out. It appeared to vary in brightness by as much as 50 percent, implying that it might be exploding and fading over and over again. It was far more luminous than your typical type II-P supernova, and it had clearly let off far more energy than its peers simply by outlasting them.
"It means there's definitely another way for stars to explode that doesn't destroy them completely so they can do it multiple times," Arcavi said.
What was feeding this mysterious series of explosions? Arcavi and his colleagues narrowed down the possibilities. Perhaps a fast-spinning neutron star, a magnetar, was behind the repeated explosions -- but that would mean the supernova should have started out far brighter than this one did. Perhaps the colliding shells of high-speed matter seen in another kind of supernova, known as type IIn, were causing this supernova to shine -- but iPTF14hls wasn't producing the X-rays and radio waves that would be expected if that were happening.
Instead, scientists think this supernova may be a pulsational-pair instability supernova. In this scenario, the inside of a monster star with the mass of 100 suns becomes so hot that some of its energy turns into matter and antimatter. If that happens, the core becomes unstable, triggering an explosion that blows off the star's outer layers, Arcavi said. If true, iPTF14hls would be the first example of this kind of supernova ever seen.
The researchers looked back in their data from 1954 and found a luminous object right where iPTF14hls sits in the sky. In their data from 1993, that object was gone. It seems iPTF14hls has been slowly flickering for more than half a century.
Even with this theory, not everything lines up, Arcavi said. For example, the star should have blown off all of its hydrogen in its mid-century episode, but the star clearly has plenty of hydrogen left.
"I've never seen anything like this," Arcavi said. "Usually when you see a weird supernova or something for the first time there's some explanation and then you find more and you work it out. But this one is just defying all of the explanations we have. There's no model or theory that can fully explain this."
The original star itself is something of a surprise, he pointed out. Jumbo-sized stars were more common in the universe's past, when they were made of mostly hydrogen; the more traces of heavy elements in a star's body, the smaller it tends to be. And those massive stars did not live long -- on the order of 100 million years or so, unlike our smaller sun, which has already lived 4.6 billion years.
"That also tells us something about how stars are formed," Arcavi said. "Also, if these things are common, then whatever kind of elemental abundances they're ejecting into their galaxy could have a significant effect on the overall abundances of elements that we should be seeing."