Black holes seen swallowing stars and burping energy back out

Supermassive black holes are voracious beasts. Their tremendous gravitational pull sucks in everything that gets too close, including stars.

For the first time, astronomers have clearly observed at infrared wavelengths what happens after a black hole eats a star: It burps back up a brilliant flare of light that echoes through space.

Two studies published this week – one by scientists at NASA, the other by researchers at the University of Science and Technology of China – describe these "tidal disruption flares" using data from NASA's Wide-field Infrared Survey Explorer (WISE), a space telescope that has photographed the entire sky in infrared light.

"This is the first time we have clearly seen the infrared light echoes from multiple tidal disruption events," Sjoert van Velzen, a postdoctoral fellow at Johns Hopkins University and lead author of the NASA study, said in a statement. Van Velzen's study caught three black holes in the act of star swallowing; researchers in China documented a fourth.

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The technical term for these celestial phenomena is "stellar tidal disruption events." When a star gets too close to a black hole's event horizon (the "point of no return," at which not even light can escape), it gets stretched and torn apart by variations in the black hole's gravitational pull. Scientists call the process "spaghettification" for the way that it elongates everything that has the misfortune of enduring it.

As it devours the star, the black hole emits an enormous amount of energy, including ultraviolet and X-ray light, that destroys everything in its immediate neighborhood.

"It's as though the black hole has cleaned its room by throwing flames," van Velzen said.

But beyond the reach of the most intense radiation, a patchy web of dust swirls. At this distance – a few trillion miles from the black hole – the dust particles can absorb the light released during the death of the star without being destroyed by it. The particles then re-emit the light at longer, infrared wavelengths. Scientists recently detected several X-ray emissions from black holes that seemed to be signatures of this phenomenon, but the new studies are the first to catch the event in infrared.

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The WISE telescope, which is attuned to infrared radiation, can capture these "echoes" of the star's destruction; by measuring the delay between the original light flare and the subsequent echoes, scientists on the ground can figure out how much energy was released as the star got consumed.

The studies also let astronomers figure out the exact location of the dust web and understand some of its most basic characteristics. This material isn't only the outskirts of black hole – it represents the nucleus of the galaxy for which the black hole forms the center. That makes observations of tidal disruption flares doubly interesting: They can help scientists understand not just the dark, dense mysteries of black holes, but also the bright, swirling places that surround them.