New type of supernova detected as black hole causes star to explode

By Will Dunham

WASHINGTON (Reuters) -Astronomers have observed the calamitous result of a star that picked the wrong dance partner. They have documented what appears to be a new type of supernova, as stellar explosions are known, that occurred when a massive star tried to swallow a black hole with which it had engaged in a lengthy pas de deux.

The star, which was at least 10 times as massive as our sun, and the black hole, which had a similar mass, were gravitationally bound to one another in what is called a binary system. But as the distance separating them gradually narrowed, the black hole's immense gravitational pull appears to have distorted the star - stretching it out from its spherical shape - and siphoned off material before causing it to explode.

"We caught a massive star locked in a fatal tango with a black hole," said astrophysicist Alexander Gagliano of the U.S. National Science Foundation's Institute for AI and Fundamental Interactions located at the Massachusetts Institute of Technology, a co-author of the study published this week in the Astrophysical Journal.

"After shedding mass for years in a death spiral with the black hole, the massive star met its finale by exploding. It released more energy in a second than the sun has across its entire lifetime," Gagliano added.

The explosion occurred about 700 million light-years from Earth. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

"The gravitational pulls of the two objects were actually similar because we think they had similar masses. But the star was much larger, so it was in the process of engulfing the black hole as the black hole pulled material off of it. The star was large but puffy, and the black hole was small but mighty. The black hole won out in the end," Gagliano said.

The researchers are not certain of the exact mechanism that caused the supernova.

"It's unclear if the distortion triggers an instability that drives the collapse of the star, and then the leftover stellar material gets rapidly eaten by the black hole, or if the black hole completely pulls the star apart before it goes supernova," said Harvard University astrophysicist and study lead author Ashley Villar.

"The star has been pulled and morphed by the black hole in complex ways," Villar added.

The binary system started out with two massive stars orbiting each other as cosmic companions. But one of the two stars reached the end of its natural life cycle and exploded in a supernova, and its core collapsed to form a black hole, an extraordinarily dense object with gravity so strong that not even light can escape.

"This event reveals that some supernovae can be triggered by black hole companions, giving us new insights into how some stars end their lives," Villar said.

Stars that are at least eight times as massive as the sun appear destined to end their lives with a supernova. Those with a mass at least 20 times that of the sun will form a black hole after the explosion.

An artificial intelligence algorithm designed to scan for unusual explosions in the cosmos in real time first detected the beginnings of the explosion, providing an alert that enabled astronomers to carry out follow-up observations immediately. By the time the explosion was completed, it had been observed by numerous ground-based and space-based telescopes.

"Our AI algorithm allowed us to launch a comprehensive observational study early enough to really see the full picture for the first time," Gagliano said.

Observations of the star dating to four years before the supernova revealed bright emissions that the astronomers believe were caused when the black hole guzzled material sucked off the star. For instance, the star's outer hydrogen layer appears to have been ripped off, exposing the helium layer below.

The researchers observed bright emissions in the explosion's aftermath as the black hole consumed leftover stellar debris. In the end, the black hole became more massive and more powerful.

Systems grouping two or more companions are quite common. Some of these multiples have a black hole as one of the companions.

"Our takeaway is that the fates of stars are incredibly impacted by their companion - or companions - in life. This event gives us an exciting window into how dramatically black holes can impact the deaths of massive stars," Gagliano said.

(Reporting by Will Dunham, Editing by Rosalba O'Brien)