Once in a while, a supermassive black hole gets a sumptuous treat. A passing star wanders too close and gets caught in the black hole's gravitational pull, like a fly trapped in a spider's web. The star then becomes an easy meal for the black hole, which tears its prey to bits and ingests a good portion of it. 

Astronomers have witnessed several such disruptions before in distant galaxies, but usually only toward the end of the process. (These feedings are far too rare, however, to have been witnessed in our own Milky Way anytime in recent human history; they occur only once every 10,000 years or so per galaxy.) Now researchers have documented a black hole's feasting in such detail that they were able to infer its size as well as the type of star that fell prey to its gluttony. 

Astronomers cannot peer inside a black hole itself; beyond the event horizon, a black hole's point of no return, even light cannot escape into the outside world. But material falling into a black hole gives off intense flares of radiation as it compresses and heats up outside the event horizon. 

Suvi Gezari, an astronomer at Johns Hopkins University, and her colleagues used a number of different telescopes to track the outburst from a supermassive black hole in a galaxy more than two billion light-years away as the black hole consumed a star that ventured too close.

"While there has been evidence of these types of flares before, there's never been enough information to say what kind of star fell victim to the black hole, and what was the mass of the black hole that destroyed the star," Gezari says. She and her colleagues published their findings online May 2 in Nature. (Scientific American is part of Nature Publishing Group.)

They first discovered the flare using the Pan-STARRS 1 telescope in Hawaii, a relatively new observatory that can scan the sky rapidly and repeatedly to identify changes in a celestial object's appearance. In May 2010 the telescope picked up a transient feature in the sky, dubbed PS1-10jh, that brightened rapidly in visible light. Around the same time, NASA's spaceborne Galaxy Evolution Explorer registered a flare in the ultraviolet at the same location, near the center of an otherwise unremarkable galaxy. Both telescopes followed PS1-10jh into 2011 as it peaked and finally faded; the researchers also commandeered the larger MMT telescope in Arizona to take spectroscopic readings of the flaring object.

"Because we [saw] the flare of radiation at so many different wavelengths and in so much detail over time," Gezari explains, the researchers were able to closely compare their observations with theoretical predictions for a star's infall into a black hole. The match between observation and theory is quite convincing, says Giuseppe Lodato, an astrophysicist at the University of Milan who wrote a commentary accompanying the research in Nature. "The shape of the light curve is very close to what is expected for such events—it's actually one of the best examples," he says. 

And with the help of the spectra from MMT, which can pinpoint individual chemical species by the wavelength of light they emit, Gezari and her colleagues were able to parse the makeup of the debris from the disrupted star. That material is notably bereft of hydrogen, the most common element in the universe. "We see no evidence of hydrogen in the system," she says. "This is very peculiar gas. It's mostly helium gas." 

The composition of the stellar debris indicates that the devoured star was the helium-rich core of a red giant—a swollen type of star that the sun will evolve into some five billion years from now. "When [a red giant] balloons up, its hydrogen envelope is very vulnerable to being stripped by the gravitational forces of the black hole," Gezari says. "What we're seeing is this sort of stripped core." 

The tracking of the flaring black hole over more than a year, with multiple telescopes, allowed for an unprecedented level of forensic reconstruction, Lodato notes. "This is the first time that such accurate determination of the star's properties have been done," he says. 

It also allowed the researchers to draw some conclusions about the black hole and how much of the star it ultimately consumed. Gezari and her colleagues estimated, based on the properties of the flare and the physical attributes of red-giant cores, that the black hole has a mass of roughly three million suns. (The Milky Way's central supermassive black hole, for comparison, is about four million solar masses.) And by summing up the radiated light from the flaring black hole, the researchers concluded that possibly 10 percent, and perhaps as much as half, of the star ended up being consumed. "A large fraction of the star gets ejected away," Gezari says. "It's a very messy process."