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“What happens if you throw a star at a black hole?” This is a question we cannot answer with certainty here on Earth.
Fortunately, real black holes and stars can’t be smashed together in the lab! However, scientists can use advanced supercomputer models to simulate a black hole ripping apart and devouring a star in a so-called “tidal disruption event,” or “TDE.” By doing just that, a team of researchers led by Danel Price of Monash University has discovered that the answer to our opening question is “things get messy.”
“Black holes can only eat so much,” Price told Space.com. “Like me, after a bad curry, not much goes through the black hole, and most of it comes back as violent outflows. We see this in tidal disruptions — strong outflows, relatively low and constant temperature of material, and large emission distances.”
As if this wasn’t nauseating enough, black holes wake up, like a failed Saturday night adventure involving alcohol and a seedy bhuna, surrounded by the regurgitated remains of their meal in a structure called an ‘Eddington envelope’.
“We found that during the disruption, the black hole is engulfed in material. That’s new,” Price explained. “It’s an old idea that this should happen, but we were able to show How “This is done by simulating the gas dynamics.”
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Pasta and curry? No wonder black holes get sick!
TDEs occur when stars pass too close to the supermassive black holes that lie at the hearts of all large galaxies.
“Stars bump into each other as they travel through the galaxy, so their orbits are slightly perturbed. Only occasionally, about once every 100,000 years, does a star bump enough to become bound to the black hole and dive toward it,” Price explained. “The key is that stars are only slightly bumped, so like comets diving toward the sun, they tend to end up on parabolic orbits. These are hard to simulate.”
Once the star gets too close to the supermassive black hole, the immense gravity of this cosmic giant creates strong tidal forces on the star, compressing it horizontally and stretching it vertically.
This process, called “spaghettification” (we know we’ve switched cuisines here, but stick with me), turns the star into bright noodles of stellar material, or “plasma,” which wraps around the destructive black hole like spaghetti around a fork. From this swirling, flattened cloud of superheated plasma, called an “accretion disk,” the supermassive material is gradually fed.
The TDE process and the rotating disk of stellar debris around the black hole generate powerful electromagnetic emissions that allow astronomers to study these events.
Yet there are still mysteries surrounding TDEs that need to be solved.
To understand the complexity of TDEs, Price and his team performed the first self-consistent simulation of a star tidally disrupted by a supermassive black hole. They followed the evolution of the resulting debris for an entire year using a sophisticated, fluid particle hydrodynamics code called “Phantom.”
“Essentially, we threw a star at a black hole in the computer,” Price said. “More specifically, we spent a long time trying to correctly implement the effects of Einstein’s general theory of relativity, which describes spacetime near a black hole.
“Our simulations provide a new perspective on the last moments of stars near supermassive black holes.”
The Phantom simulation revealed that stellar debris created during a TDE forms an asymmetric bubble around the black hole. This leads to the reprocessing of energy, producing light emissions with lower temperatures and weaker luminosities.
The team also found that this gas is moving around the supermassive black hole at a speed of 22 million to 45 million miles per second (10,000 to 20,000 kilometers per second), which is about 60,000 times the speed of sound at sea level, or about 7% of the speed of light.
“The study helps explain several puzzling properties of observed TDEs,” Price said. “A good analogy is the human body: When we eat lunch, our body temperature doesn’t change much; this is because we convert the energy from lunch into infrared wavelengths.
“A TDE is similar; we don’t usually see the black hole’s stomach eating gas, because it is suffocated by material that re-emits at optical wavelengths. Our simulations show how this suffocation happens.”
The reprocessing of energy and the smothering of black holes in the simulation explains one of the biggest observational mysteries about TDEs: understanding why they emit light primarily in optical or visible wavelengths rather than X-rays, Price added.
The findings also explain many other TDE mysteries, including why star fragmentation is observed in fainter light than expected and why this material appears to be moving toward us at a fraction of the speed of light.
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— NASA X-ray observatory reveals how black holes swallow stars and spit out matter
Looking ahead, the team’s simulations have provided much food for thought.
“There are a lot of things to explore here. Once the Vera Rubin Observatory comes online, we expect to see thousands of observed transients over the next decade,” Price concluded. “We need to try the same kind of simulation for all kinds of stars and black holes of different masses, so that it can be applied to different observed events.”
The team’s research is published in the Astrophysical Journal Letters.