Black hole–shredded megastars power a new class of cosmic explosions

Massive stars know how to go out in style.

Some of the brightest, longest-lived flares probably come from megastars getting shredded by supermassive black holes, researchers report in the June 6 Science Advances. This new class of cosmic explosions — dubbed extreme nuclear transients, or ENTs — release more energy than any other known transient, an event that changes brightness in the night sky.

“These extreme nuclear transients, in the timescale of a year, emit about the same energy as our sun does in its entire 10-billion-year lifetime,” says astronomer Jason Hinkle of the University of Hawaii at Mānoa in Honolulu.

Similar transients usually involve a star less than twice the mass of the sun veering too close to a black hole, whose gravity is stronger than that holding the star together. The black hole’s powerful pull overwhelms the star, which stretches around the black hole. During this tidal disruption event, or TDE, the spewed star matter — some of which gets swallowed by the central celestial body — heats up and produces X-rays, ultraviolet radiation and other wavelengths of light, typically for days to months.

Curious about rarer instances of stars getting gutted, Hinkle and colleagues searched telescope data for cosmic events with especially radiant glows that continued for more than one year.

They identified three events, each of which took over 100 days to reach peak light production and another 150 days or more to dim to half of that. Their maximum brightnesses were 30 to 1,000 times that of various kinds of exploding stars, or supernovas. ENTs are “very bright and very long-lived,” Hinkle says. “But it’s really the combination that doesn’t exist in any other type of transient.”

These showy affairs emitted flares with two to 10 times the energy of those from the most energetic supernovas known. That means the events couldn’t be exploding stars or various other transient types. Instead, the researchers determined that the events were probably supercharged TDEs, where stars three to 10 times the mass of the sun got ripped apart by black holes around 250 million times the sun’s mass.

Moreover, the observed ENTs show evidence that hot gas and dust lingers near their supermassive black holes, hinting these celestial bodies are actively shaping their host galaxies. Previously studied TDEs have been in dormant galaxies, so researchers haven’t explored the links between the star-shredding events and awake black holes beyond theoretical models, says astrophysicist Taeho Ryu of the Max Planck Institute for Astrophysics in Garching, Germany, who was not involved in the study.

“These extreme nuclear transients seem to provide an avenue to make a connection between these two different [phenomena],” he says. “That means we can have a better understanding of TDEs and [galaxy-shaping black holes], both of them, at the same time.”

ENTs are quite rare, with one estimated to occur for every 10,000 typical TDEs. But, Hinkle says, “they’re still, by far, our best probes of these very massive, otherwise quiet, supermassive black holes.”