Astronomers using the Hubble Telescope have captured rare footage of a black hole tearing a star into a celestial donut. But this donut might give even Homer Simpson pause. The torus is the size of our entire solar system.
The astronomers first caught this “stellar snacking event” on March 1, 2022, with the All-Sky Automated Survey for Supernovae (ASAS-SN, pronounced “Assassin”). ASAS-SN is a network of ground-based telescopes that surveys the extragalactic sky about once a week for the “violent, variable, and transient events” that are shaping our universe. And this tidal disruption event, cataloged as AT2022dsb, certainly fits that description. The collision was close enough to Earth, and bright enough, that after the initial ASAS-SN report, the Hubble space telescope was able to take its sweet time making detailed follow-up observations in the UV band. The team of astronomers delivered their report at the 241st meeting of the American Astronomical Society in Seattle, Washington.
According to NASA, the Hubble spectroscopic data comes from a “donut-shaped area” of very hot, very bright gas that used to be the star. We can’t photograph the mayhem up close, since it’s all taking place nearly 300 million light-years away, at the heart of the galaxy ESO 583-G004. That means everything we’re seeing is 300 million years old. (Talk about nightmare lag!) But with Hubble, the astronomers could carefully study the spectral “fingerprint” of the shredded star. That’s how we know its nuclear furnace burned down through hydrogen, carbon, and more.
Changes in the torus are taking place on the order of days or months. But for any given galaxy with a supermassive black hole at its center, NASA estimates this kind of “stellar shredding” only happens a few times per hundred thousand years. That makes this a rarity among rarities.
“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,” said team member Peter Maksym of the Center for Astrophysics | Harvard & Smithsonian (CfA) in Cambridge, Massachusetts. “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.”
‘Somewhere on the Edge of That Donut’
It can be difficult to locate things in space unless they’re facing in just the right direction. When finding exoplanets, the ‘transit’ method is easiest to use when the planet’s orbit is facing us, edge-on. And as it happens, that appears to be what ASAS-SN saw when it pointed Hubble at this black hole and its late-night stellar snack. “We’re looking somewhere on the edge of that donut,” said Maksym.
But all this talk of midnight snacks belies what the Hubble project describes as the complex, raw violence of any encounter with a black hole. Not only is the star being torn and twisted into just a cloud of its former constituent atoms — that cloud is then lashed by the black hole’s stellar wind, which moves at relativistic speeds. “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 (three percent the speed of light),” said Maksym.
“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,” said another of the astronomers, Emily Engelthaler, also of the CfA. “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.”
In fact, there aren’t very many tidal events that we know of, period. According to NASA, astronomers have only identified about a hundred of these rare and extremely high-energy phenomena. Nevertheless, each one embodies a wealth of subtle detail about how galaxies like ours came to be.
“We really are still getting our heads around the event,” added Maksym. “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.”
The Hubble Space Telescope is a joint endeavor of NASA and the European Space Agency. Feature image: NASA/ESA/Leah Hustak (STScI)
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