Black holes mysteriously burp up fragments of stars they destroyed years earlier, and scientists have no idea why

• Black holes have been spotted spitting up remnants of stars years after gobbling them up.
• The finding is confusing astronomers, as it doesn't fit with what we know about black holes.
• This could change what we think happens when a black hole swallows a star.

Black holes can burp up remnants of stars years after destroying them, and no one knows why, a study has found.

Astronomers have long known that black holes can spew out a bright flash of energy after tearing apart an unfortunate star — this is called a tidal disruption event (TDE).

However, the expectation has been that such jets would be seen within months of the original TDE.

The new study, which has not yet been peer-reviewed, found that 10 out of 24 black holes observed started spewing matter between two to six years after the TDE.

The finding, which has been puzzling astronomers, could mean we've misunderstood what happens when black holes eat up a star, Yvette Cendes, a lead author on the research from the Harvard and Smithsonian Center for Astrophysics, said in a thread on X, formerly Twitter.

"The real short answer to what's causing this is 'we don't really know, but no one was predicting it,'" she said.

No one saw this coming because no one was looking

There's a good reason why we'd never seen this before: no one expected it to happen.

A TDE happens when a star gets too close to a supermassive black hole. Within a few hours, the star is shredded to bits.

At this point, astronomers typically think about half of the matter of the star will start swirling around the black hole, creating what is called the "accretion disk." The theory is then that the other half of the matter will be spewed out in a one-time jet of energy, which can be picked up from Earth.

Astronomers expect that flash of light to happen within a few weeks or months of the TDE. If nothing is picked up within that time, they turn their telescopes elsewhere.

"Radio telescope time is precious! And why look YEARS after the explosive event for something you didn't see right after said explosive event?" said Cendes on X.

But this view started to change in 2022, when Cendes and her co-workers spotted a black hole that woke up again two years after swallowing a star.

Since then, the collaborators have been turning their instruments to monitor 24 black holes for years on end. They've found that more than half of them woke up again years after the original star-swallowing event.

One black hole seemed to turn back six years later.

"If you know anything about physics, you know this time scale doesn't make sense!" Cendes said in a post on Reddit.

In another two of the cases, Cendes noticed the black holes peaking, then fading, then turning on again.

"That's completely new and unexpected," Cendes told Live Science. "People were thinking that you'd have one outflow, and then it's kind of done," she said.

Everything we know about accretion disks may be wrong

The findings could mean we need to rethink how black holes swallow up stars, Cendes said.

At this point, what we know is what is not happening.

Cendes has ruled out that this could be due to a second TDE, or that the jet started straight after the TDE, but was directed away from Earth at first. She's also ruled out some of the more fanciful explanations, such as the jet being delayed because the black hole was messing with time in the area.

What that leaves us with, however, is a big mystery.

"Our best guess is 'everything we assumed about accretion disc formation in TDEs is wrong''" Cendres said on X.

"What if the optical flash is NOT a disc forming, but streams of material interacting, and the disc doesn't form until YEARS later? We don't know the full details but def possible," she said.

The new findings suggest astronomers will have to rethink the relationship between stars and black holes.

"I've had great fun getting looks of incomprehension from theorists over recent months, believe you me!" Cendes said on X.

The findings were published on pre-print server arXiv on August 25.