When they first observed supernova iPTF14hls in September 2014, astronomers at Las Cumbres Observatory in California thought it was perfectly normal. They analyzed the light of the explosion to study the material ejected and its speed.
But Zheng Chuen Wong, an intern at the observatory from the University of California, Santa Barbara, noticed something strange about the supernova and showed it to Iair Arcavi, a NASA Einstein postdoctoral fellow at the university.
A supernova, the massive explosion of a star, usually signals the end. Typically, a supernova remains bright for 100 days before fading. But this one fluctuated by brightening and dimming over the course of 600 days, according to a study released in the journal Nature
The brightness of a supernova provides a luminosity equaling that of about 100 million suns.
"My first thought was that this must be some nearby star in our galaxy, just varying its brightness," Arcavi, lead author of the study, wrote in an email. "But when we got the first spectrum of it, we saw that it was in fact a supernova 500 million light-years away. My mind was blown. The fact that it got bright and dim five times was very unusual. We'd never seen a supernova do that before."
The astronomers decided to look over archival data and discovered a surprise: The same star was observed exploding in 1954 and somehow survived the massively catastrophic event, only to explode again -- and continue to survive.
"This means that we still have a lot to learn about how massive stars evolve and how they explode," Arcavi said. "Given that so much mass and energy were released, if such events are common, they would also have a big impact on their surroundings. This supernova, for example, will affect the entire galaxy it's in!"
The star was 50 times more massive than our sun, and the explosion itself is the longest-lived ever seen.
It may be even the most massive supernova ever observed, Arcavi said.
The sheer size of the explosion may explain why the current understanding of the deaths of stars doesn't work in this case.
The study suggests that the "pulsational pair instability supernova" theory may apply: a process in which the cores of a massive star reaches such a high temperature 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 wrote. "The star then stabilizes, and can go through this process multiple times every few years or decades. Eventually it will explode completely. We've never seen such a supernova before, so ours would be the first candidate."
But there are a couple of things about this theory that don't quite match up with what astronomers are seeing in iPTF14hls.
"These explosions were only expected to be seen in the early universe and should be extinct today," said Andy Howell, leader of the Las Cumbres Observatory supernova group and a co-author of the study, in a release. "This is like finding a dinosaur still alive today. If you found one, you would question whether it truly was a dinosaur."
The researchers said that the energy released by the supernova is greater than what the theory would predict. It also predicts that all of the hydrogen would have been lost during the explosion observed in 1954, but there was also a lot of hydrogen present after the 2014 explosion.
"If this is the first pulsational pair instability supernova, we need to figure out why it doesn't look exactly as predicted," Arcavi said. "Otherwise, it's something completely new. There are no existing theories that can fully explain this supernova, pulsational pair instability is our best guess, but it might be something completely new."
The supernova is still bright, and the team will continue to monitor it. As the light fades and the supernova expands, it becomes more transparent, which will enable the researchers to take a closer look.
"We might be able to see how much total mass exploded, and maybe catch a glimpse of any internal power source that has been providing the extra energy to power this supernova for so long," Arcavi said. "The Hubble Space Telescope is also planned to look at this supernova in about a month. Hubble's increased resolution could show us what the neighboring stars of this supernova look like, and that might shed some light on what kind of star exploded here.
"I'd like to know how common these explosions are. Was this a super rare beast, or will we find more now that we know what to look for?"
The team was able to make this discovery through the robotic telescopes at the observatory, as well as the SED Machine, designed and built by Nick Konidaris at Carnegie Observatories. These types of telescopes are playing a big role in pushing astronomy forward because they enable long-term monitoring.
The SED Machine is designed to be a simple point-and-shoot camera, and whenever a new object appears on the sky, it provides a spectrum of the object within a matter of minutes, Konidaris wrote in an email. Once copies of the machine are made, they can be attached to telescopes around the world.
"If we want to find more of these zombie supernova events, we'll need more robots to identify and study them," Konidaris said.
"This might be the first time we've seen a supernova like this simply because we couldn't have gotten observations like these before," Arcavi added. "It's hard to argue with the effectiveness of robotic telescopes -- they just open up a new window on the universe for us."