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“What happens when you throw a star into a black hole?” That’s not a question we can physically answer on Earth.
Fortunately, real black holes and stars can’t be smashed together in the lab! But using advanced supercomputer modelling, scientists can simulate a black hole ripping apart and swallowing a star in a so-called “tidal disruption event” or “TDE”. And by doing just that, a team of researchers led by Danel Price from Monash University discovered that the answer to our opening question is “things get messy”.
“Black holes can’t eat that much stuff,” Price told Space.com. “Like me, after a bad curry, not much goes into the black hole and most of it comes back out in the form of violent outflows. We see this in tidal disruption events — strong outflows, relatively low and stable temperature material, and large propagation distances.”
If that’s not nauseating enough, just like a Saturday night adventure involving alcohol and a questionable bhuna, black holes wake up surrounded by the regurgitated remains of their meals in a structure called an “Eddington envelope”.
“We found that during the decay, the black hole is being overwhelmed by matter. That’s new,” Price explained. “It was an old idea that this should happen, but we were able to show it. How “It’s done by simulating gas dynamics.”
Relating to: Massive star’s bloody ‘death by black hole’ is the biggest and brightest event of its kind
Pasta and curry? No wonder black holes get sick!
TDEs occur when stars pass too close to the supermassive black holes that lurk at the centre of all large galaxies.
“Stars bump into each other as they travel through the galaxy, so their orbits get slightly perturbed. Only occasionally, once every 100,000 years, will a star hit enough of a black hole that it connects to it and plunges toward it,” Price explained. “The important thing is that the stars get bumped just a little bit, so they tend to come out in parabolic orbits, like comets plunging toward the sun. These are hard to simulate.”
When the star comes too close to the supermassive black hole, the immense gravitational influence of this cosmic titan produces powerful tidal forces on the star, causing it to compress horizontally and stretch vertically.
This process, called “spaghettification” (we know we’re changing cuisines here, but let’s move on), turns the star into bright noodles of stellar material, or “plasma.” This wraps around the destructive black hole like spaghetti wrapped around a fork. From this swirling, flattened cloud of superheated plasma, called an “accretion disk,” the supermassive star is slowly fed.
The TDE process and the disk of stellar debris orbiting the black hole produce powerful electromagnetic emissions that allow astronomers to study these events.
But there are still mysteries about TDEs that need to be answered.
To understand the intricacies of TDEs, Price and his team used an advanced smoothed particle hydrodynamics code called “Phantom” to perform the first self-consistent simulation of a star being tidally torn apart by a supermassive black hole and tracked the evolution of the resulting debris over the course of a year.
“We essentially threw a star into a black hole inside the computer,” Price said. “In particular, we spent a long time trying to accurately apply the effects of Einstein’s theory of general relativity, which describes space-time near a black hole.
“Our simulations provide new insight into the final moments of stars around supermassive black holes.”
The Phantom simulation revealed that stellar debris created during the TDE forms an asymmetric bubble around the black hole. This leads to the reprocessing of energy, producing light emissions at lower temperatures and dimmer light intensities.
The team also discovered that this gas is moving around the supermassive black hole at 10,000-20,000 kilometers (22 million to 45 million miles) per second, which is about 60,000 times the speed of sound at sea level, or about 7% the speed of light.
“The study helps explain several puzzling features of 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 reprocess the energy from the lunch into infrared wavelengths.
“TDE is similar; we don’t see the gas that eats away at the black hole for the most part because it is drowned out by re-radiated matter at optical wavelengths. Our simulations show how this drowning occurs.”
Price added that the energy reprocessing and black hole choking in the simulation explains one of the biggest observational mysteries about TDEs, namely why they emit mostly in optical or visible light wavelengths rather than X-rays.
The findings also explain many of the TDE’s other mysteries, including why the stellar disruption is seen in fainter light than expected and why this material appears to be moving toward us at a fraction of the speed of light.
Related stories:
— A black hole makes itself known to astronomers by violently tearing apart a star
— Black hole ‘burps’ spaghetti-sacrifice star it swallowed years ago
— NASA X-ray observatory reveals how black holes swallow stars and expel matter
As for the future, the team’s simulations left much to think about.
“There’s a lot to explore here. Once the Vera Rubin Observatory is up and running, we expect to observe thousands of transient events over the next decade,” Price concluded. “We need to try the same type of simulation for all types of stars and black holes of different masses so it can be applied to different observed events.”
The team’s research was published in the journal: Astrophysics Journal Letters.