“Living Dead” Star Is Like Nothing Ever Seen Before

An artist’s impression of a supernova involving a massive star. (Credit: NASA, ESA, G. Bacon (STScI))

Ordinarily, a supernova marks the death of a mammoth star, which then briefly outshines an entire galaxy before fading away. Not so for a baffling supernova that went off in a nearby galaxy in 2014. Instead of being the end of the story, the stellar explosion inexplicably began to brighten and has since dimmed, then brightened up again four more times.

If that weren’t odd enough, it turns out a supernova blew up in the same place in the sky more than 60 years ago. Somehow, a star that apparently died around the time Elvis Presley released his first record endured only to die again—truly a “living dead” star.

Astrophysicists suspect this apparent stellar zombie was a rare, colossal type of star with 50 to 100 times the mass of our Sun. The universe’s first stars were similarly huge, they think, though these distant objects lie beyond the reach of even our most powerful telescopes. The re-exploding star could, therefore, be a cosmic anachronism, offering scientists an unprecedented glimpse into the primeval universe.

To discuss the major scientific potential of this supernova, The Kavli Foundation reached out to two scientists key to its discovery, as well as an astrophysicist who specializes in massive stars.

The participants were:

• IAIR ARCAVI – is an observational astronomer and NASA Einstein Postdoctoral Fellow at the University of California, Santa Barbara (UCSB) and the Las Cumbres Observatory, as well as a former researcher at the Kavli Institute for Theoretical Physics (KITP). He is the lead author of a Nature paper describing the strange supernova.
• LARS BILDSTEN – is the Director of KITP and a Professor in the Physics Department at UCSB. A co-author of the new paper, Bildsten’s specialty is developing theories that explain supernovae.
• EMILY LEVESQUE – is an Assistant Professor of Astronomy at the University of Washington. Her research focuses on massive stars, such as the one implicated in this re-exploding supernova.

THE KAVLI FOUNDATION: Iair, you’ve called this supernova the biggest puzzle you’ve seen in your career studying stellar explosions. What makes it so puzzling?

IAIR ARCAVI: Most supernovae get bright over a few days or weeks, then fainter over a few weeks and months, and then they disappear and we never see them again. This supernova has gone bright-faint, bright-faint about five times over the course of three years! We’ve never seen that before.

Another weird thing is when we took a spectrum, or fingerprint, of this supernova, which is useful for identifying what type it is, we got very ordinary results. Despite the supernovae’s strange behavior, the spectrum matched that of the most typical supernovae, which have a lot of hydrogen in them. We’ve seen hundreds of those. A normal spectrum was the last thing we were expecting to see.

The third puzzling thing: We found this photographic plate from 1954 with an image of this supernova’s host galaxy, and we can see a supernova going off at the same position. We’ve never seen the same place in the sky explode twice before—let alone 60 years apart.

TKF: How exactly did you locate that crucial piece of evidence?

ARCAVI: It was paper co-author Peter Nugent, a professor at the University of California, Berkeley and at Berkeley Lab, who thought to look back to see if there was any information in the historical surveys. Interestingly, the 1954 image was taken by the same telescope that discovered the supernova in 2014! In 1954, the telescope surveyed the entire sky, and it did so again in 1993. All of the photographic plates from both of those surveys were scanned, digitized, and put online. You can go to a website, type in any position in the sky, and it’ll show you the images from those past surveys. When Peter punched in the coordinates of this unusual supernovae, he discovered in the 1954 survey a bright source of light at that same position. We were all very surprised when we saw that!

TKF: Emily and Lars, what were your reactions to the discovery of this supernova?

EMILY LEVESQUE: I learned about it when the paper came out. Students in my research group immediately started pitching around ideas about what this supernova could be. We study massive stars and are really interested in their death throes, their final evolutionary states before they become supernovae. The more we read about this new supernova, the weirder it got.

LARS BILDSTEN: Over the past few years, Iair had shared with me and some colleagues the news about the strange rebrightening of this supernova. We had started trying to do theoretical work to understand it, but the fun part was, none of our ideas were working!

The big question remains, what is the supernova’s source of energy? Typical supernovae, from a collapsing, massive star, are gone after about 100 days. That’s because the explosion leaves only a certain amount of energy in the stellar material that gets blown out into space. The temperature of that material, and the light it gives off, drops over time. But that didn’t happen for this supernova. Its material is getting re-energized, and it’s a pretty open question as to how.

TKF: Well, what could be causing this supernova to behave that way?

BILDSTEN: It could be related to something called “pair instability,” which is a phenomenon we think can happen in the most massive stars in the universe, those that have more than 100 times the mass of our Sun. These stars get so hot that they start making electron-positron pairs. Electrons are the familiar negatively charged particles that swirl around nuclei in every atom, whereas positrons are their positively charged counterpart usually only created when nuclei undergo radioactive decays. However, once the center of the star reaches conditions where the radiation is able to create these pairs, the pressure support in the star declines. That forces a partial collapse, igniting a runaway thermonuclear reaction that triggers an explosion.

Smaller stars can go in and out of producing electron-positron pairs. During one of these episodes, the star falls in on itself somewhat and ejects a large part of its mass. But the star survives, only to then have another contraction and a later ejection. So obviously seeing something that is bright in 1954 and then again in 2014 is suggestive of this mechanism. I would say it’s a good working hypothesis.

TKF: Emily, your research focuses on the massive stars Lars just mentioned, which are thought to have been much more common in the early universe. What can we learn about that bygone cosmic era by studying an event like this supernova?

LEVESQUE: Understanding how the very first stars in the universe behaved, how they were born, evolved and died, is really the key that we need to explain the chemical evolution of our universe. This goes back to the “we’re all made of star stuff” idea from Carl Sagan. To understand where the first “star stuff” came from, we have to understand the first stars. But they’re simply too far away to observe directly.

Instead, we have to look at stars that are closer and that might behave similarly to those first stars. This unusual supernova might help us understand how the very first, massive stars died. It’s the first, best and so far only example we have.

BILDSTEN: All of us would love to find the ‘first star.’ That’s a frontier we’ll keep probing with the next generation of major telescopes.

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