All of us, not just astronomers, have an obvious fascination with what happens when matter gets sucked into the maw of a black hole. Usually, we've had to watch from the equivalent of partial-view bleacher seats as these cosmic garbage disposals do their thing in the Milky Way and in distant galaxies. For once, though, we may have procured primo seats behind the dugout.
In the next few years, the supermassive black hole at our galaxy's center could shred and consume a cloud of dust and gas with the mass of a small planet.
In about 18 months the newfound object will draw near the cosmic orifice at the center of our Milky Way galaxy. Its orbit will carry it to within about 36 light-hours of the black hole, roughly twice the distance now separating NASA's Voyager 1 spacecraft from the sun.
That's when things could get interesting. A group of astronomers reports in a study published online December 14 in Nature that the black hole ought to rip the gas cloud apart and pull it inward, producing a detectable surge in x-ray emissions as the gas compresses and heats up. Depending on how the cloud breaks up, the black hole may feed on it for years to come, significantly brightening the faint glow that emanates from just outside the edge of the black hole, its event horizon, and providing astrophysicists a unique view of black hole digestion. (Scientific American is part of Nature Publishing Group.)
"Essentially, we have never seen before how any black hole in the universe is being fed," says study co-author Stefan Gillessen, an astronomer at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. "Now we have the chance to watch something fall in." Gillessen and his colleagues identified the object and determined its trajectory using the European Southern Observatory's Very Large Telescope (VLT) in Chile.
A number of stars orbit the Milky Way's central black hole, which contains the mass of four million suns. But an object as weakly bound as a gas cloud would not survive the intense gravitational pull in the galactic center. Gillessen and his colleagues estimate that the newfound blob is three times as massive as Earth, and they say it is already being distorted by the black hole's overpowering pull as it streaks toward its closest approach in 2013. Sometimes called "spaghettification," extreme gravitational effects stretch objects falling toward a black hole into long strands. "We see that happening to this cloud," Gillessen says. "It's being disrupted right now in front of our eyes. We see that spaghetti effect happening."
The infall, which ought to be observable with large telescopes such as the VLT and Keck telescopes on Earth and with NASA's orbiting Chandra X-ray Observatory, could give new clues about the environs surrounding the Milky Way's black hole. It could also provide new information about the behavior of supermassive black holes, which are found at the centers of all large galaxies.
But the prediction of an infall rests on the conclusion that the object streaking toward a flyby of the Milky Way's central supermassive black hole is indeed a loose blob of gas and not something much sturdier. Andrea Ghez, an astronomer at the University of California, Los Angeles, who studies the region surrounding the black hole, suspects that the inbound object is in fact a star. If that were the case, the object would be able to survive such a close passage of the black hole without being torn apart—in fact, astronomers have watched stars cruise to within 11 light-hours of the Milky Way's black hole and live to see another orbit. The black hole, denied its dinner, in that case would continue its steady whisper rather than letting out a roar.
Because the object showed up in VLT observations at some infrared wavelengths and not others, the researchers were able to estimate its temperature. The cloud, they say, is only about 550 kelvins (275 degrees Celsius), roughly one tenth the sun's surface temperature, and hence too cool to be a star. But Ghez says that it could be a star shrouded in interstellar material.
"I wouldn't bet my house on their interpretation of what it is," Ghez says. "It's the difference between something extraordinary and something rather ordinary." Unlike most astronomical mysteries, this should actually be settled soon—with a roar or a whisper.