Black holes belch stars years after they’ve destroyed them, baffling scientists

An artist’s impression of a tidal disturbance event. A new study indicates that, contrary to what was previously thought, black holes can spew out the remains of stars years after they’ve been swallowed.
NASA’s Goddard Space Flight Center/CI Laboratory

  • Black holes have been observed spewing out the remains of stars years after they devoured them.
  • This discovery baffles astronomers, because it does not fit with what we know about black holes.
  • This might change what we think happens when a black hole swallows a star.

A study finds that black holes can eject the remains of stars years after they’ve been destroyed, and no one knows why.

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 disturbance event (TDE).

However, such aircraft were expected to be seen within months of the original TDE.

The new study, which has not yet been peer-reviewed, found that 10 of the 24 observed black holes began ejecting matter between two and six years after TDE.

This discovery, which baffled astronomers, may mean we misunderstand what happens when black holes devour a star, said Yvette Sindis, lead author of the paper from the Harvard-Smithsonian Center for Astrophysics. topic on Xformerly Twitter.

“The real short answer to why that is, we don’t really know, but no one expected that,” she said.

No one saw this coming because no one was looking

There’s a good reason we haven’t seen this before: no one expected it to happen.

TDE occurs when a star gets too close to a supermassive black hole. Within a few hours, the star is shredded into small pieces.

At this point, astronomers usually think that half of the star’s matter will begin to swirl around the black hole, creating what’s called an “accretion disk.” The theory then is that the other half of matter will be ejected in a single burst of energy, which can be captured from Earth.

Artist’s impression of a rapidly spinning supermassive black hole surrounded by an accretion disk.
European Southern Observatory, European Space Agency/Hubble, M. Cornmisser

Astronomers expect this flash of light to occur within a few weeks or months of TDE. If nothing is caught during that time, they turn their telescopes elsewhere.

“Radio telescope time is precious! Why would you search years after the blast for something you didn’t see immediately after the blast?” Cendes said on X.

But that view began to change in 2022, when Sindis and her colleagues spotted a black hole that reawakened two years after it swallowed a star.

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

One of the black holes appears to have moved back six years later.

“If you know anything about physics, you know that this time scale doesn’t make sense!” Cendes said in a post on Reddit.

In two other cases, Sindis observed black holes peaking, fading, and then activating again.

“This is completely new and unexpected,” Sindis told Live Science. “People thought you would have one outflow, and then it just kind of went away,” she said.

Everything we know about accretion pills could be wrong

Sindis said the findings may mean we need to rethink how black holes swallow stars.

At this point, what we know is not happening.

Sindis ruled out that this was due to a second TDE, or that the aircraft started immediately after the TDE, but was pointed away from the ground at first. It also ruled out some of the more fanciful explanations, such as the jet lag because the black hole was messing with time in the region.

But what he leaves us with is a great mystery.

“Our best guess is that everything we’ve assumed about accretion disk formation in TDEs is wrong,” Cendres said at X.

“What if the optical flash wasn’t a disk forming, but rather streams of reactants, and the disk didn’t form until years later? We don’t know the full details but it’s definitely possible,” she said.

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

“I’ve had so much fun getting stares from theorists in recent months, believe me!” Cendes said on X.

The results were posted to the arXiv preprint server on August 25.

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