Hubble telescope observes a hungry supermassive black hole devouring a star

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While some stars reach the end of their lives with a bang and explode in a massive supernova, others end in a whimper, swelling and shedding material before shrinking and cooling into a tiny core. But very rarely do some stars suffer a more drastic fate as they are ripped apart and devoured by a hungry black hole.

Such an event only happens a few times every 100,000 years in a galaxy with a dormant black hole at its center, but one such event was recently captured by the Hubble Space Telescope. Researchers observed the last few moments of a star’s life that wandered too close to a black hole nearly 300 million light-years away and was devoured, emitting a burst of light in an event called AT2022dsb.

“Black holes are very messy eaters.”

The shredding of a star by a black hole is called a tidal disturbance and is caused by the enormous gravity of a supermassive black hole. These massive black holes lurk in the centers of galaxies and can pull layers of gas from a star that gets too close. Eventually, the star is completely shredded and the remnants are pulled into a disk of matter around the black hole called an accretion disk – from which the black hole feeds.

“Black holes are very messy eaters,” said one of the researchers, Emily Engelthaler of the Harvard & Smithsonian Center for Astrophysics, at the American Astronomical Society meeting on Thursday, January 12. “They’re eating this accretion disk — this donut-shaped thing — from the inside out, and they’re eating too much and spewing out radiation in the process, making this accretion disk swell up into a nice, fat doughnut.”

Some of the radiation given off by these events zooms off in the form of jets, but this study focused on the radiation coming through the accretion disk itself. The researchers used Hubble to look at ultraviolet light emanating from the star, using a technique called spectroscopy to break that light into wavelengths to see which had been absorbed. That allows them to figure out what kind of elements were present and gather clues about what’s happening in the bright, hot chaos surrounding the black hole.

Such an event only happens a few times every 100,000 years in a galaxy with a dormant black hole at its center

It is not often that such events are observed in the ultraviolet because this wavelength is easily blocked and therefore difficult to collect data. A telescope outside the Earth’s atmosphere was needed. “Ultraviolet is very bad at penetrating atmospheres, which is great for us, but terrible for observation. So we have to use a space telescope,” Engelthaler said.

The researchers wanted to know how the star and black hole changed over time, so they made a series of observations over several months. They found that the temperature in the disk fell over time and that stellar winds were blowing away from the event and toward us, at tremendous speeds of 20 million miles per hour, or 3 percent of the speed of light.

However, the spectra the researchers collected were not stable – they varied significantly over time. It may be that this is simply because the studied source is very far away, so the signal became hidden in the noise. Or it could be that the ring of material surrounding the black hole actually thinned and the amount of material attracted by the black hole decreased.

The researchers are still working to unravel all the data they collected. “We are really still working on the event,” fellow researcher Peter Maksym said in a statement. “You shred the star and then it has this material that finds its way into the black hole. And so you have models where you think you know what’s going on, and then you have what you actually see. This is an exciting place for scientists to be: right at the intersection of the known and the unknown.”

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