A “Zombie” Star Is Refusing to Die

The supernova has been flaring for more than 600 days—and it may be the second time that the star has exploded

ESO/M. Kornmesser

When large stars reach the end of their lives, they erupt into fantastic supernovas. These explosions typically fade out over the course of about 100 days, after which they collapse into a black hole or become an ultra-dense object known as a neutron star. But as Ashley Strickland reports for CNN, astronomers recently announced that they have observed a supernova that has been burning for more than 600 days—defying accepted theories about the way stars die.

When researchers first observed supernova iPTF14hls in 2014, they categorized it as a perfectly ordinary Type II-P supernova. At the time, the explosion appeared to be fading. But in early 2015, Zheng Chuen Wong, an intern at the Las Cumbres Observatory in California, noticed that the supernova was flaring up again, according to Marina Koren of the Atlantic. And this was highly unusual.

“A supernova gets bright and then fades,” Iair Arcavi, an astronomer at Las Cumbres, tells Koren.“It’s not supposed to get bright again.”

At first, Arcavi thought that iPTF14hls might be a living star in our galaxy, since stars can have variable brightness, notes Harrison Tasoff of Space.com. But when Arcavi and his fellow researchers used telescopes to assess factors like chemical composition and the speed of the material, they realized that iPTF14hls was indeed a supernova.

When the team looked through archival data, they found another surprise: the same star was recorded as exploding in 1954. In other words, the star seems to have survived the original explosion, only to repeat the process several decades later—and survive. Informally, the supernova has been dubbed a “zombie” star. But it is also possible, according to Taft, that the recently observed event has actually been ongoing for the past 60-odd years.

In a study published in Nature researchers suggest that this weird phenomenon could be the first-ever recorded example of a “pulsational pair instability supernova,” This process occurs when the core of a star get so hot that it converts energy into matter and antimatter.

"When that happens, the star becomes unstable and can partially explode, blowing off its outer parts, but leaving the core intact," Arcavi tells Strickland of CNN. "The star then stabilizes, and can go through this process multiple times every few years or decades. Eventually it will explode completely."

But this explanation doesn’t completely explain what astronomers have observed with iPTF14hls. For one thing, the supernova has produced much more energy than the pulsational pair instability theory predicts. And Andy Howell, a Las Cumbres Observatory scientist and a co-author of the study, tells Strickland that such explosions likely only occurred during the early years of the universe.

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

Ultimately, Arcavi noted in an interview with Space.com, “there is no existing model or theory that explains all of the observations we have.” Scientists will examine the supernova next month using the immensely powerful Hubble Space Telescope, in the hopes of better understanding this resilient and mysterious star.