Astronomers recently watched a supermassive black hole rip apart an unwary star much like our Sun.
Just before a star falls into a black hole, the tremendous gravity of the black hole pulls the star apart and stretches it into a thin stream of blazing hot gas. Astronomers call this act of cosmic destruction a tidal disruption event. The gas wraps around the black hole, forming what’s called an accretion disk; it’s as if the black hole is wearing the star’s skin as a battle trophy before finally swallowing it up. As the gas spirals inward, it heats up and emits enough light to be visible even against the bright light of an entire galaxy.
A team of astronomers led by University of Turku astronomer Yannis Liodakis recently caught a supermassive black hole in a nearby galaxy in the act of ripping apart a star. The spaghettified plasma that had once been a Sun-like star didn’t behave in quite the way Liodakis and his colleagues expected, though — which sheds some light on how much we still have to learn about the huge, violent things that happen at the edge of a black hole.
They published their findings in the journal Science.
Fishing Expedition
Liodakis and his colleagues were on a fishing expedition when they spotted the bright flare of light from a galaxy roughly a billion light years away. A “fishing expedition” is what Liodakis calls just looking into space to see what’s out there — it’s space, after all, so there’s bound to be something cool happening — and it’s a rare opportunity for astronomers.
“I think there is really value in being able to do that and not just having to have a plan,” Liodakis tells Inverse.
After some follow-up observations with other telescopes, it turned out that the bright flash of light had been the grisly demise of a star about the size of our own Sun that ventured too close to a supermassive black hole about 3.6 million times more massive than ill-fated star. The black hole’s gravity had ripped the star apart and draped itself in its entrails. But something weird happened next.
Destruction at the Edge of a Black Hole
Astronomers understand the basics of how tidal disruption events happen, but they’re still working to piece together some of the details. Sometimes the star's fiery plasma entrails immediately settle into a round, flat accretion disk around the black hole, just like something out of a physics textbook. Other times, they're less well-behaved, and sometimes the star actually survives. The shape of the star's original orbit, the chemical makeup and mass of the star, and other factors all help determine how the final act of destruction plays out.
“There's a lot of things going on that we don't fully understand yet,” says Liodakis. “In the past few years, we're getting more and more of these events that we can study, but I think we have a long way ahead until we truly understand what's going on very close to the black hole.”
In particular, astronomers like Liodakis and his colleagues want to understand how the spaghettified star forms an accretion disk, as well as how relativistic jets get kickstarted. Studying events like this one can help them unlock some of the answers.
A Stream of Plasma Collides with Itself
When Liodakis and his colleagues watched their tidal disruption event in June 2020, they used an instrument called a polarimeter, which measures whether the waves of light coming from a source are all tilted the same way.
The peaks and troughs of light waves are usually oriented in random directions; some of them might be waving up and down, while others might be wiggling left to right, or on a diagonal. Now picture a computer screen, which polarizes that light so that all the waves are rippling in the same left-to-right direction. The light from the spaghettified star was very polarized.
Normally, such polarized light would mean that the supermassive black hole had formed a relativistic jet: a stream of hot, electrically charged gas called plasma being hurled into space at nearly the speed of light. Supermassive black holes are messy eaters that occasionally regurgitate some of their food; you can think of relativistic jets as the astrophysical version of projectile vomit, only actually really awesome.
But this particular supermassive black hole wasn’t projectile vomiting at nearly light-speed after all. When Liodakis and his colleagues looked at the galaxy in X-ray wavelengths, and again in the much longer wavelengths of radio waves, they saw … nothing.
Liodakis and his colleagues ran computer simulations to see what kinds of processes could produce the type of light they saw when the star got ripped apart. Their answer turned out to be pretty dramatic, even by the admittedly high-drama standards of supermassive black holes and dying stars.
The remains of the star had wrapped around the supermassive black hole, but instead of swiftly settling into a spiraling accretion disk, the hot stream of plasma essentially circled the black hole and collided with its own tail.
“The star’s magnetic field doesn't magically go away. So when it's disrupted, been carried with the plasma,” explains Liodakis. And computer simulations show that powerful shock waves — like the ones created when a flowing stream of plasma curves around runs into itself — tend to amplify a magnetic field. When charged particles of plasma bounce around in a strong magnetic field, they emit what's called synchrotron radiation: very polarized light.
But the brilliant flare of light didn’t last long.
“After basically eight months, it was fainter than the host galaxy,” says Liodakis, who estimates that it probably took about a year for the last of the star's remains to vanish into the black hole.
How Weird Was This, Really?
Liodakis and his colleagues eventually hope to find out whether all accretion disks around supermassive black holes get such a turbulent start, or whether there was something strange about this particular star or this particular supermassive black hole.
“There have been a few more polarization measurements of sources, and we've not seen anything as highly polarized as this case,” says Liodakis.
The team has observed a few more tidal disruption events since 2020, but Liodakis says their total is still less than ten. He hopes to find and study a lot more. Future telescopes like the Vera Rubin Observatory could help with that.
“The goal is to actually measure enough of them to have a statistical sample and be able to see something about the polarized properties of the population,” says Liodakis.
