Astronomers move a step closer to glimpsing a black hole

Astronomers are reporting Wednesday that they have moved a notch closer to seeing the unseeable.

Using a worldwide array of radio telescopes to obtain the most detailed look yet at the center of the Milky Way, they said they had determined that the diameter of a mysterious fountain of energy there was less than half that of Earth's orbit about the sun.

The result strengthens the case that the energy is generated by a black hole that is gobbling stars and gas, they said. It also leaves astronomers on the verge of seeing the black hole itself as a small dark shadow ringed with light, in the blaze of radiation that marks the galaxy's center.

Until now, the existence of black holes — objects so dense that not even light can escape them— has been surmised by indirect measurements, say of stars or gas swirling in their grip. Seeing the black hole's shadow would require the ability to see about twice as much detail as can now be discerned. Such an observation could provide an important test of Albert Einstein's theory of general relativity, which predicts that black holes can exist.

"We're getting tantalizingly close to being able to see an unmistakable signature that would provide the first concrete proof of a supermassive black hole at a galaxy's center," Shen Zhiqiang of the Shanghai Astronomical Observatory, a leader of an international team of radio astronomers, said in a news release. Their report appears Thursday in the journal Nature.

Another member of the team, Fred K.Y. Lo, director of the National Radio Astronomy Observatory in Charlottesville, Va., said that achieving the extra resolution could take several years and would probably require new radio telescopes.

"We're not there yet," he said, "but in time, no question, we will get there."

He added that seeing the shadow would be "proof of the pudding" that Einstein was right and that black holes exist.

In an accompanying commentary, Christopher Reynolds of the University of Maryland wrote that such observations would "herald a new era in probing the structure and properties of some of the most enigmatic objects in the universe." Other experts, however, said it might be difficult, even if the extra resolution could be achieved, to untangle the detailed properties of the black hole from its blazing surroundings.