A step closer to identifying that monster in the Milky Way

Astronomers have taken the measure of a monster lurking at the center of the Milky Way. It's not as big as astronomers once thought. But its incredible density throws more weight behind the idea that it's a supermassive black hole, not some oddball collection of other exotic objects.

The research brings astronomers a step closer to capturing images of the edge, or "event horizon," of a black hole - which scientists say would be the "smoking gun" that proves such entities exist. Once that horizon is crossed, anything falling in - including light - will never come out.

Supermassive black holes are enormous concentrations of matter confined to relatively tiny spaces in the centers of most galaxies. They are the oversized cousins of stellar black holes, which can form after stars at least 10 times as massive as the sun burn out and collapse. These smaller black holes tend to be 18 or 19 miles across.

To produce the new results, astronomers aimed a continentwide network of radio telescopes at a source of radio emissions at the center of the galaxy, in the constellation Sagittarius. The source is an object 4 million times more massive than the sun.

But the object apparently does not take up much space as previous measurements had indicated, the new results show. Instead of filling a patch of space as wide as the solar system, or even as wide as Earth's full orbit around the sun, the object is smaller than the distance between the Earth and sun, or 1 astronomical unit. Given estimates of its mass and its incredible shrinking volume, calculations of its density are going through the roof.

By closing in on the object's true size, "we're getting tantalizingly close to being able to see an unmistakable signature ... of a supermassive black hole," notes Zhi-Qiang Shen of the Shanghai Astronomical Observatory and the Chinese Academy of Sciences, who led a team from China, the US, and Taiwan. Its results appear in the current issue of the journal Nature.

What would that signature look like?

Most likely, it would appear as a black circle, surrounded by a thin, bright line, astronomers say. This thin line of radiation actually would come from objects behind the black hole. But a process called gravitational lensing would focus the radiation around the black hole. Moreover, the black hole would likely appear off-center inside its thin halo - an effect traced to its rotation.

Solving the mystery at the heart of the Milky Way is important, astronomers say, because supermassive black holes are thought to lie at the center of most galaxies. Knowing more about the one in the Milky Way - if that's really what it is - will help astronomers understand the role these objects play in other galactic cores.

Until the smoking gun is spotted, notes University of Maryland astrophysicist Christopher Reynolds, the object could be something more bizarre and still fall within the confines of standard physics. One possibility, although remote, is what he calls a "boson star," made up of particles associated with the fundamental forces of nature. Such a star could have the size and mass characteristics of the object at the heart of the Milky Way.

"There are other families of particles out there that can form massive compact objects," says Dr. Reynolds. New instruments expected to be developed over the next decade should help astronomers sift the right answers from the wrong ones, he says.