NUMBER 3
Our Milky Way's Black Hole

At the center of the Milky Way, a black hole 2.6 million times as massive as our Sun gobbles gas and stars. This "food" is thought to swirl into the center, like muck going down a bathtub drain.

But all this swirling should create lots of friction, which should generate enormous energy. The black hole should, therefore, be very, very bright -- in visible light, X-rays and other wavelengths.

"Instead it is very faint," says Tom Geballe of the Gemini Observatory.

Why? Is there not much stuff falling in? Or is the stuff falling directly in instead of swirling, thereby creating less friction? Or is some unknown effect preventing us from seeing the radiation?

"Nobody is sure," Geballe says, but he suspects we may learn the answer in the next 5 to 10 years.

"One might argue that all black holes always will be strange," he said. "So perhaps I should simply predict that the galactic center black hole will no longer be considered stranger than other black holes by 2010."

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More About Our Milky Way's Black Hole

Since no one has ever actually seen a black hole, how do astronomers even know there's one at the center of the Milky Way?

Geballe offers up an analogy. Around our Sun, the inner planets move more rapidly than the outer planets. By measuring those speeds, astronomers can calculate the mass of the Sun.

Likewise, astronomers observe that stars and gas near the center of our galaxy move faster than the stuff farther out. So some object has to be at the center, exerting a certain gravity that causes such speeds. This technique unambiguously shows a mass of 2.6 million Suns in a very small volume of space, Geballe says.

"From the small volume that this mass must inhabit, we know that the only physically realistic object that can have that mass is a black hole," he says.

Astronomers see all kinds of radiation emitted from the black holes in other galaxies -- X-ray, infrared, ultraviolet, visible light and radio waves. The only signature of the Milky Way's black hole, however, is some radio radiation.

Ultraviolet and visible wavelengths are blocked by interstellar dust between Earth and the center of the galaxy. But the other wavelengths can penetrate this dust. Geballe says better instruments may be needed to solve the mystery.

Meanwhile, theoretical astrophysicists are on the case, trying to come up with a theory that would account for the weird faintness of the black hole at the center of the Milky Way.

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