Black Holes Really Know How to Savor Their Meals

In 2005, astronomers detected a burst of infrared light coming from the heart of a galaxy nearly 150 million light-years from Earth. They had been studying the night sky for supernovae, the glittering explosions that mark the deaths of stars, but this seemed different. Intrigued, they decided to keep an eye on it.

After years of observations, the astronomers have determined the source of this burst was indeed the death of a star. But the star hasn’t exploded. It’s being eaten.

The star drifted too close to a supermassive black hole, the vacuum cleaner of the universe. The black hole’s fierce gravity dragged the star toward its invisible mouth, toward a point from which nothing, not even light, can return. The tugging stretched and shredded the star, producing a bright tail of light that could be detected by powerful telescopes on Earth.

An international team of astronomers—led by Seppo Mattila of the University of Turku in Finland and Miguel Pérez-Torres of the Astrophysical Institute of Andalusia in Spain—tracked this event over the course of a decade. Six years into their monitoring, the glow of light began to change shape and lengthen—a tell-tale sign that this wasn’t a one-off star explosion.

Their observations produced a ghostly GIF of this violent cosmic meal:

The discovery, described in a paper published Thursday in Science, marks the first time that astronomers have directly imaged the formation and shape-shifting expansion of the jet of stellar material that forms when black holes devour stars. The stellar debris grows brightly as it swirls around the black hole, emitting light across many different wavelengths. This interaction occurred at the site of another cosmic mashup: in one of the galaxies that makes up a pair of colliding galaxies known as Arp 299, shown below:

Matilla, Pérez-Torres, and their colleagues first spotted the light from the black hole’s dinner with the William Herschel Telescope in the Canary Islands, and followed up with observations with networks of radio telescopes in Europe, Asia, and North America, as well as NASA’s Spitzer space telescope.

Such bright streams of stellar debris are known to emit intense X-rays and visible light, but this particular case revealed itself to astronomers only through infrared and radio wavelengths. The astronomers say that could be explained by substantial amounts of interstellar gas and dust around the black hole and the crumbling star. The gas and dust absorbed the X-rays and visible light coming from the star, and then radiated it into the cosmos in these other wavelengths which, even at unfathomable distances, can still reach us here on Earth.

“This seems like a plausible explanation, as we know that this is a persistently accreting supermassive black hole, where only the high-energy X-ray emission can escape and most of the light at longer wavelengths is absorbed,” said Erin Kara, an astronomer at the University of Maryland and NASA’s Goddard Space Flight Center who studies black holes and was not involved in the study.

For black holes, consuming stars can be a drawn-out process. As the black hole spins, it takes the shredded star along for the ride, producing a hot, glowing disk of stellar material, like a merry-go-round that won’t stop. “In some cases … it takes a lot of time—years—for all the debris to fall into the black holes,” Kara said.

Astronomers hope to detect more cosmic meals like this one, and there’s a good chance they already have. Sky surveys are like fishing nets; a search for one astrophysical phenomenon often catches others. At first glance, a burst of radio emissions, or a pinprick of light, could be many things. For astronomers, few things beat the feeling of taking a closer look and finding something unexpected—and scientifically useful.

“This is a very exciting result,” Kara said. “What I love about this result is that it was serendipitous, as many of the most exciting astronomical results are.”