The center of the Milky Way is a crowded, violent place, where stars and gas clouds whirl at high speed around a gigantic black hole. Now astronomers have spotted a haze of high-energy X-rays in this maelstrom that might be coming from dead stars—enough of them to make up a starry mass graveyard.
They X-rays are bunched tightly around the Milky Way's central black hole and come from a region about 13 light-years across and 26 light-years thick. “The mystery,” says Kerstin Perez of Haverford College and Columbia University, “is where does that haze come from?”
The answer, says Perez, lead author of a report published Wednesday in Nature on the X-ray haze: “We don’t know.”
When stars venture too close to a black hole, it shreds them, gulps them down and releases a violent blast of electromagnetic energy to mark the occasion. It’s not a friendly neighborhood.
And even when the black hole is between meals, the Milky Way’s core crackles with energy—notably, a constant sizzle of X-rays.
Perez and her co-authors aren't at a loss for explanations of where the X-rays could come from. “We have several plausible ideas,” she says. “We can’t rule any of them out, but each has its own complications.”
The first three ideas fall into a single category: dead stars that are pulling in matter from an orbiting companion star.
The dead stars could be white dwarfs—the glowing embers left behind when a sun-like star collapses into a small, dense mass. If gas from an adjacent star falls onto a white dwarf, it vaporizes in a burst of energy that includes X-rays.
But the original sun-like stars that produced the white dwarfs would have had to be relatively massive to create the type of high-energy X-rays Perez and her colleagues see with the NuSTAR telescope. And there aren't enough of these white dwarfs in the galaxy to explain the strength of the signal.
Or the dead stars could be neutron stars or star-sized black holes, which would also emit X-rays when sucking in matter from a companion. The complication here, says Perez: “These objects generally give off bright flares every now and again,” she says. “We’ve been looking at the galactic center for ten years, and don’t see enough of these flares to explain the population you’d need.”
The other possibility is that the X-rays don’t come from stars at all, but rather from gas clouds whirling around the galaxy’s giant central black hole. The black hole emits small burps of energy about once a day, but truly noteworthy blasts happen far less often.
This idea has its own complication, she says. “The shape of the X-ray-emitting region doesn’t match up with our maps of where the gas clouds are.”
Neither Perez nor her colleagues venture to guess which explanation might be right, although Avi Loeb, who heads Harvard’s astrophysics department and was not involved with this research, has a favorite. Most likely, he says, is a huge graveyard of white dwarfs that is gobbling up matter.
Whatever the explanation, however, it almost certainly has to do with how ultra-dense objects, whether they’re white dwarfs or neutron stars or black holes, rip apart and destroy matter. It’s happening all the time at our galaxy’s core, as the resulting X-rays make clear. “It’s really a fun place to look with NuSTAR,” Perez says.