“What happens when you throw a star at a black hole?” We cannot answer this question physically here on Earth.
Fortunately, real black holes and stars can’t be smashed together in the lab! However, scientists can use modern supercomputer models to simulate how a black hole rips apart and devours a star in what’s called a tidal disruption event (TDE). A team of researchers led by Danel Price of Monash University has done just that and found that the answer to our initial question is: “It gets messy.”
“Black holes can’t eat that much,” Price told Space.com. “Like me after a bad curry, not much goes into the black hole but comes back in the form of violent outflows. We see this in tidal currents – strong outflows, relatively low and constant temperature of the material, and large emission distances.”
And if that wasn’t disgusting enough, similar to a Saturday night mishap involving alcohol and a dodgy bhuna, black holes wake up surrounded by the regurgitated remains of their meals in a structure called an “Eddington envelope.”
“We found that the black hole is smothered by material during the disturbance. This is new,” explained Price. “It is an old idea that this should happen, but we were able to show How This is done by simulating gas dynamics.”
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Pasta and curry? No wonder black holes get sick!
TDEs occur when stars venture too close to the supermassive black holes that lurk at the heart of all large galaxies.
“Stars are nudged by each other as they travel through the galaxy, so their orbits are slightly perturbed. Only occasionally, once every 100,000 years, does a star get nudged hard enough to bind to the black hole and crash toward it,” Price explained. “The key is that stars are only nudged a little, so like comets crashing toward the Sun, they tend to end up in parabolic orbits. Simulating these is difficult.”
If the star comes too close to the supermassive black hole, the enormous gravitational force of this cosmic titan creates strong tidal forces on the star, compressing it horizontally and stretching it vertically.
This process, called spaghettification (we know we’ve changed cuisine here, but let’s stick with it), turns the star into bright noodles of stellar material, or “plasma.” This wraps around the destructive black hole like spaghetti around a fork. From this swirling, flattened cloud of superheated plasma, called the “accretion disk,” the supermassive is gradually fed.
The TDE process and the swirling disk of stellar debris around the black hole produce strong electromagnetic emissions that allow astronomers to study these events.
However, there are still mysteries surrounding TDEs that need to be solved.
To understand the intricacies of TDEs, Price and team performed the first self-consistent simulation of a star being torn apart by the tidal forces of a supermassive black hole. Using an advanced smoothed particle hydrodynamics code called Phantom, they were able to track the evolution of the resulting debris over an entire year.
“Essentially, we threw a star at a black hole in the computer,” Price said. “In particular, we have long tried to correctly implement the effects of Einstein’s general theory of relativity, which describes space-time near a black hole.”
“Our simulations offer a new perspective on the final moments of stars near supermassive black holes.”
The Phantom simulation showed that stellar debris created during a TDE forms an asymmetric bubble around the black hole, leading to energy recycling and producing light emissions with lower temperatures and weaker luminosity.
The team also discovered that this gas moves around the supermassive black hole at speeds of 10,000 to 20,000 kilometers per second, which is about 60,000 times the speed of sound at sea level, or about 7 percent of the speed of light.
“The study helps explain several puzzling properties of the observed TDEs,” Price said. “A good analogy is the human body: When we eat lunch, our body temperature doesn’t change much. That’s because we convert the energy from lunch into infrared wavelengths.”
“It’s similar with a TDE. We usually don’t 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 occurs.”
Energy recycling and the suppression of black holes in the simulation explain one of the biggest observational puzzles of TDEs: They help understand why they emit mainly optical or visible wavelengths of light rather than X-rays, Price added.
The results also explain many other TDE mysteries, such as why stellar destruction is visible in dimmer light than expected and why this material appears to be moving toward us at a fraction of the speed of light.
Looking to the future, the team’s simulations gave us a lot to think about.
“There is a lot to explore here. Once the Vera Rubin Observatory starts operating, 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 is applicable to different observed events.”
The team’s research results were published in Astrophysical Journal Letters.
