A Supermassive Star Exploded And Left Nothing Behind

Artist’s impression of the SN 2016iet supernova as it exploded. Gemini Observatory/NSF/AURA/Joy Pollard

The life of a massive star usually ends in a dramatic explosion, releasing a huge amount of energy and leaving behind a black hole or a neutron star. But this doesn't happen every time. Researchers have long suspected that stars over a certain mass can instead explode in a way that leaves nothing behind.

Supernova SN 2016iet is suspected to be one of these. It is described by the researchers in The Astrophysical Journal as a pair-instability or a pulsational pair-instability supernova. In this type of event, the outer layer collapses with such energy that gamma rays are produced that then turn into pairs of electrons and positrons (the anti-electron). The matter and antimatter production from the gamma rays propagates through the entire star. Pairs of particle-antiparticles continue to form and annihilate throughout, leading to such a powerful explosion that nothing is left behind. The team was able to estimate the chemical composition and the expected mass range of the progenitor star, and it is consistent with such a supernova.

“There have been previous candidates of pulsational pair-instability supernovae such as SN2010mb and iPTF14hls, but neither of these supernovae had a measurement of the mass of the progenitor star, which is key to determining if the supernova was a pair-instability supernova,” lead author Sebastian Gomez, from the Harvard-Smithsonian Center for Astrophysics, told IFLScience. “So this is the first supernova in which we have a measurement of the mass and the metallicity that are consistent with it being a pulsational pair-instability supernova.”

SN 2016iet exploded in a dwarf galaxy roughly 1 billion light-years from us. It was spotted by the European Space Agency’s Gaia satellite on November 14, 2016. The spacecraft measures the position of billions of stars in the Milky Way, but it is also capable of spotting objects beyond our galaxy, like this one. Over the last three years, the team has followed the event with several other telescopes, including Gemini North.

The data paints a complex picture of the cosmic explosion. The star was in a region with little heavy elements and had a mass between 55 and 120 times the mass of our Sun at the time of the explosion. At its most massive point, it might have been 200 times the mass of our Sun, shedding mass over the decade preceding the explosion. So far, this is consistent with the models. However, there are other properties that are more baffling.

Other examples of stars this massive are not formed in isolation, they are usually formed in crowded areas. The progenitor star of SN 2016iet instead was located 54,000 light-years to its parent galaxy. That’s more than half the Milky Way away. And it is not like it could have traveled from elsewhere. These massive stars have an extremely brief life, living for millions of years instead of billions like the Sun.

It is also peculiar how long it has lasted and how much energy was emitted. Researchers suspect that SN 2016iet might be the first known example of a new class of objects. They will continue to study its emissions for as long as it stays visible, and will keep monitoring the sky for evidence of similar events elsewhere in the Universe.

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