A giant star’s death throes may offer the first evidence of a pair-instability supernova, and a glimpse of the first stars in the universe.

Quote Originally Posted by Quanta Magazine
billion years ago, something in the whirling darkness of space erupted with a fury that obscured the glow of entire galaxies. Eventually, the light from that cataclysm reached Earth, and in November 2016, it was captured by a group of intrepid humans at the European Space Agency’s Gaia satellite. They found that the conflagration wasn’t just unfathomably energetic, but, like a lonely bonfire, it kept on burning, dimming so slowly that its glow can still be seen years after it began.

This wasn’t a typical supernova, the fireworks at the end of a massive star’s life. This event came from a star so gigantic — at least 100 times the mass of our own sun — that its death was unlike almost anything scientists had ever seen. Stars this big were probably common in the early universe, but they have become exceedingly rare. Even its location was odd; it appeared 54,000 light-years away from the nexus of its dwarf galaxy, far from anywhere you might hope to find a mysterious flash.

As reported this August in The Astrophysical Journal, the phenomenon’s startling nature makes it a strong candidate for one of two types of highly elusive, still hypothetical stellar pyres. And because the supermassive star that exploded makes a great stand-in for the ephemeral stars that existed in the earliest chapters of the universe, the event could help astronomers better understand that long-ago chapter of our cosmic history.

At first, SN 2016iet — as the explosion was dubbed — was thought to be a super-bright supernova, nothing too unusual at a glance.

But this event was remarkably energetic. “We were kind of waiting and waiting and waiting for this thing to be gone,” said Edo Berger, an astronomer at Harvard University and co-author of the new study. “But every time we went to the telescope and observed it again, it was still there, fading so slowly.”

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In some of the most supermassive stars, with said furnaces firing at tremendously high temperatures, plenty of matter-antimatter pairs are created. Some of the energy that would otherwise contribute to the fight against gravity gets soaked up by the manufacture of these pairs. Outward pressure can’t keep up with gravity, which then dominates and makes the star shrink.

Collapse begets cataclysm. The star contracts so violently and the core burns so vigorously that “in one pulse, the nuclear burning blows the star entirely apart,” said Woosley, who was instrumental in the development of the theory of these “pair-instability supernovas.” It’s “probably the most violent thermonuclear explosion in the modern universe,” he said. The blasts are so complete that the entire star is obliterated, and nothing is left behind to form a black hole.

If a star has a lower total mass but is still massive enough for interference from those pesky pairs, it contracts and burns, but not aggressively enough to get torn apart. The star bounces back, jettisoning a giant shell of matter moving at thousands of miles per second out into the universe. The process repeats over time. Newly ejected shells collide with older shells, producing enormous bursts of light. Eventually, so much mass is lost that the creation of new pairs doesn’t significantly affect the star, and it dies in a classic black-hole-forming scenario.

This is known as a “pulsational pair-instability supernova.” To make one of these, the original star during its hydrogen-burning phase must have a mass at least 90 times that of the sun, Woosley said. A full-blast pair-instability supernova requires a star whose mass during its hydrogen-burning phase was 140 solar masses. With a minimum of 120 solar masses, SN 2016iet could fit one of these stories. Berger explained that the longer SN 2016iet continues to produce an afterglow, the higher the estimates of the star’s mass will become.

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