When a black hole swallows a star
HubbleSite published this story on January 12, 2023. It’s based on scientific results, reported during a press conference on January 12 at the 241st meeting of the American Astronomical Society in Seattle, Washington. EarthSky made small edits.
Black hole swallows a star
Black holes have such strong gravity that they even swallow light. There’s no escape if you stumble across one in the inky blackness of space. And that’s no worry for astronauts who have yet to travel farther than the moon. But entire stars can face that peril from a lurking black hole.
Hubble Space Telescope astronomers got a front row seat to such interstellar demolition when they were alerted to a flash of high-energy radiation from the core of a galaxy 300 million light-years away. Astronomers used the Hubble Space Telescope to view the mayhem before the collision was over.
Hubble is too far away to see the doomed star getting sucked in. Instead, Hubble astronomers took the fingerprints of starlight coming from the mishap. These spectra tell a forensic story of a star falling into a cosmic blender. It was shredded and pulled toward the black hole like a piece of stretched taffy. This process formed a donut-shaped ring of gas around the black hole with superheated gas bleeding out in every direction. Astronomers have observed about 100 insatiable black holes to date.
They’re called ‘tidal disruption events’
Black holes are gatherers, not hunters. They lie in wait until a hapless star wanders by. When the star gets close enough, the black hole’s gravitational grasp violently rips it apart and sloppily devours its gases while belching out intense radiation.
Astronomers using NASA’s Hubble Space Telescope have recorded a star’s final moments in detail as a black hole gobbled it up.
They’re called tidal disruption events. But this wording belies the complex, raw violence of a black hole encounter. There is a balance between the black hole’s gravity pulling in star stuff, and radiation blowing material out.
In other words, black holes are messy eaters. Astronomers are using Hubble to find out the details of what happens when a wayward star plunges into the gravitational abyss.
Hubble can’t photograph the AT2022dsb tidal event’s mayhem up close, since the munched-up star is nearly 300 million light-years away at the core of the galaxy ESO 583-G004. But astronomers used Hubble’s powerful ultraviolet sensitivity to study the light from the shredded star, which include hydrogen, carbon, and more. The spectroscopy provides forensic clues to the black hole homicide.
100 hungry black holes
Astronomers using various telescopes have detected about 100 tidal disruption events around black holes. NASA recently reported that several of its high-energy space observatories spotted another black hole tidal disruption event on March 1, 2021, and it happened in another galaxy. Unlike Hubble observations, these observations detected X-ray light from an extremely hot corona around the black hole that formed after the star was already torn apart. Emily Engelthaler of the Harvard Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, explained:
However, there are still very few tidal events that are observed in ultraviolet light given the observing time. This is really unfortunate because there’s a lot of information that you can get from the ultraviolet spectra. We’re excited because we can get these details about what the debris is doing. The tidal event can tell us a lot about a black hole.
Changes in the doomed star’s condition are taking place on the order of days or months.
For any given galaxy with a quiescent supermassive black hole at the center, it’s estimated that the stellar shredding happens only a few times in every 100,000 years.
Here’s how we know
The All-Sky Automated Survey for Supernovae (ASAS-SN or “Assassin”) AT2022dsb first caught this stellar snacking event on March 1, 2022. ASAS-SN is a network of ground-based telescopes that surveys the extragalactic sky roughly once a week for violent, variable, and transient events that are shaping our universe. This energetic collision was close enough to Earth and bright enough for the Hubble astronomers to do ultraviolet spectroscopy over a longer than normal period of time. Peter Maksym of CfA commented:
Typically, these events are hard to observe. You get maybe a few observations at the beginning of the disruption when it’s really bright. Our program is different in that it is designed to look at a few tidal events over a year to see what happens. We saw this early enough that we could observe it at these very intense black hole accretion stages. We saw the accretion rate drop as it turned to a trickle over time.
The Hubble spectroscopic data are interpreted as coming from a very bright, hot, donut-shaped area of gas that was once the star. This area, known as a torus, is the size of our solar system and is swirling around a black hole in the middle. Maksym said:
We’re looking somewhere on the edge of that donut. We’re seeing a stellar wind from the black hole sweeping over the surface that’s being projected towards us at speeds of 20 million miles per hour (32 million kph or 3% the speed of light). We really are still getting our heads around the event. You shred the star and then it’s got this material that’s making its way into the black hole. And so you’ve got models where you think you know what is going on, and then you’ve got what you actually see.
This is an exciting place for scientists to be: right at the interface of the known and the unknown.
Bottom line: Hubblesite reports on what happens when a black hole swallows a star. The hungry black hole twists the star into a donut shape.