A weird stellar explosion may have caused the brightest supernova yet seen

The brightest supernova ever seen may be the first known example of a rare type of stellar explosion.

The supernova, spotted in 2016 in a galaxy about 4.6 billion light-years away, radiated about 5 sexdecillion (5 followed by 51 zeros) ergs of energy. That’s about twice the amount of radiation emitted by the previous record-holder, and hundreds of times more energetic than normal supernovas. At its brightest, this supernova was as bright as all the stars in the Milky Way put together.

Such a bright blast could have been a pulsational pair-instability supernova — thought to occur when an extremely massive supernova collides with a shell of material cast off by the star before it exploded, researchers report online April 13 in Nature Astronomy.

“There’s no single, well-established case of such a supernova,” says Philipp Podsiadlowski, an astrophysicist at the University of Oxford not involved in the work. “This could be one.” Computer simulations of the event may help confirm the nature of the star’s demise.

After the supernova, dubbed SN2016aps, was identified in observations from the Pan-STARRS survey, astronomer Matt Nicholl and colleagues monitored its fading light for about two years. The amount of stellar debris left over from the supernova indicates that this star was at least 50 to 100 times as massive as the sun, whereas the stars behind ordinary supernovas are around 10 solar masses.

The telescope observations also revealed a surprising amount of hydrogen in the wreckage. More massive stars generally lose their hydrogen faster than smaller stars. “So, for stars in this 100-solar-mass regime, you expect that all the hydrogen is long gone well before it explodes,” says Nicholl, of the University of Birmingham in England. This finding suggests that two smaller stars still containing hydrogen merged into a supersized star that underwent a pulsational pair-instability supernova.

This exotic type of supernova is predicted to happen only to stellar juggernauts. Inside extremely massive stars, “the temperature in the core can get so high that photons, which are what keeps the star up and supports it from collapsing under its own gravity, get converted into pairs of particles — electrons and positrons,” Nicholl says. When these photons, or particles of light, disappear, “you lose some of the pressure in the core, and it starts to contract. This can lead to thermonuclear runaway, like an atom bomb going off.”

That explosive reaction can release enough energy to blow off the outer layers of the star into an enormous shell. When the star ultimately goes supernova, the explosion collides with the shell to release huge amounts of radiation. Nicholl’s team speculates that the stellar remnant forged during this type of supernova might be an intensely magnetic neutron star called a magnetar (SN: 11/8/17), which could pump energy into an explosion to make it as bright as the one seen in 2016.

This general scenario seems plausible to Stan Woosley, an astrophysicist at the University of California, Santa Cruz, not involved in the work. But the size of the star that underwent this explosion leads him to think that the 2016 supernova may have forged a black hole, instead of making a magnetar.