Black holes have ‘indigestion’ and ‘burp’ out bits of devoured stars, study finds

When it comes to devouring stars, black holes are not always in the clean plate club.

In fact, many of them belch up bits of their celestial meals years later, according to a study published on Aug. 25 by researchers at Harvard University and the University of Arizona. The study, which is a preprint and has not yet been peer reviewed, was published on arXiv, an open access online research platform.

“We looked at 24 different black holes, and out of those 24 we found that 9 of them showed this unexpected behavior at very late times,” Kate Alexander, an assistant professor of astronomy at the University of Arizona who contributed to the study, told McClatchy News.

Using radio telescopes in New Mexico and South Africa, researchers trained their sights on the two dozen black holes located between 100 million and one billion light years away from Earth, Alexander said.

Between 2014 and 2020, they watched in amazement as the cosmic sinkholes feasted on stars only to spit up stellar material later on.

Black holes, pits with intense gravitational pull that are found in the center of most galaxies, rarely swallow stars. Instead, they spend most of their time in a quiescent or inactive state, Alexander said.

But on the off chance that an unfortunate star wanders too close, a black hole will consume it in a process that typically lasts several months or years.

“You can imagine the star getting torn apart into a long string of debris, and the black hole is slurping up that debris,” Alexander said. “It’s kind of like slurping up spaghetti. And we thought most of the action happened very quickly.”

But, when checking back in on the black holes 3 to 5 years later using radio light, researchers noticed peculiar “burps” or signs of “indigestion,” according to Alexander.

“They haven’t managed to completely digest all this stellar material,” Alexander said. “It was really unexpected because we thought by then the fireworks show seemed to be over, but it turns out there’s still interesting things going on.”

To be clear, Alexander said, the emitted stellar material never became fully ingested by the black holes. Instead, it managed to break loose just before falling inside the event horizon, the threshold at which no light can escape.

In addition to being a strange phenomenon, the burps call into question the time table over which black holes consume stars.

When observing a black hole ripping a star apart, astronomers could just be watching a “pre light show,” Alexander said. The actual gas might be consumed years later than previously thought during the burping process.

Or, the black hole burps might just be — much like human burps — the final act following a big meal.

Either way, “this is a longer process than we thought,” Alexander said. “It’s hinting at more complex physics going on.”

This story was originally published September 7, 2023 at 6:00 AM with the headline "Black holes have ‘indigestion’ and ‘burp’ out bits of devoured stars, study finds."