Einstein's Probably Right (Again)

Astronomers said Wednesday they have pinpointed a tiny area at the Milky Way's heart where a black hole with the mass of 2.6 million suns is likely to lurk.

Scientists generally hold that almost every galaxy revolves around a black hole. Previous studies have estimated that the center of our galaxy, the Milky Way, contains something very dense and massive, which most scientists already believed was a black hole.

Black holes are extremely dense celestial objects. Their gravity is so powerful that not even light can escape them, making them invisible to conventional telescopes.

This does not absolutely confirm the existence of the mysterious matter-sucking drains, but it appears to rule out any other explanations scientists can imagine for the weird cosmic behavior at our galaxy's center.

"I always have to worry about what could nature do that I'm not intelligent enough to figure out," lead researcher Frederick Baganoff said at a briefing on the discovery. "While I wouldn't put my life on it, I would put my career on it, and that's what I'm doing right now."

The small area that may hold the putative black hole - just 93 million miles across, or the distance between Earth and the sun - is in line with what theorists believe would be appropriate, Baganoff said.

"The theoretical prediction of what size it should be and what we're seeing are so good that it's either a cosmic coincidence or there's something really right about that model," said Baganoff, who is based at the Massachusetts Institute of Technology.

The distance between the Earth and the sun, which astronomers call one Astronomical Unit, may seem vast to earthlings but it is minuscule on the cosmic scale, where distance is measured in light years. That is how far light travels in a year, about 6 trillion miles.

The Milky Way is 100,000 light years in diameter, and the galactic center is about 26,000 light years from Earth, Baganoff said. The area where the black hole might be is about 10 light minutes across.

This finding, made with the unprecedented high resolution of NASA's orbiting Chandra X-ray Observatory, means Albert Einstein was probably right again, said Baganoff, whose research was published in this week's edition of Nature.

"The prediction seems to be right on the nose," Baganoff said in an interview, referring to the great 20th century physicist's General Theory of Relativity, which theorized that black holes exist at the middle of most big galaxies.

"What we are doing is just piling on the evidence, we keep making it so strong a case that we can exclude more and more options," Baganoff said in a telephone interview.

Baganoff and his colleagues looked at the galaxy's center, peering through dust clouds and a cluster of stars in the constellation Sagittarius. They looked once in September 1999 and saw little of interest, but when they looked again in October 2000, they saw an X-ray flare that intrigued them.

The flare was locatd just at the edge of the possible black hole's event horizon, the theoretical point of no return. Anything that crosses a black hole's event horizon - even light - never comes out again.

Baganoff's team used fluctuations in the X-ray flare to figure out that super-massive object at the Milky Way's center was packed into an area no bigger than one Astronomical Unit, bolstering the argument that this is indeed a black hole.

The flare lasted about three hours, but dropped in intensity for a period of about 10 minutes right at its apparent peak, Baganoff said.

This drop in intensity by a factor of five gave the researchers the evidence they needed to compute the size of the black hole. Because light travels at a known speed - 186,000 miles per second - scientists could figure out how big an object is by the time it takes light to get across it, in this case 10 minutes.

"Because this dip occurred in just 10 minutes, we can multiply by the speed of light and we can get an estimate of the size of the region," Baganoff said.

If the intensity dip had been smaller, say 10 percent or so, the results would have been less convincing. But because it dropped by a factor of five, that means the whole area of X-ray emission was involved, he said.

Richard Mushotzky, an astronomer with NASA's Goddard Spaceflight Center in Greenbelt, Md., said the new findings push previous evidence of a black hole in the center of the Milky Way "one step further."

"It's gone from a reasonable supposition to very hard to believe it's not true," he said.

Earlier research had shown that the mass of the Milky Way's probable black hole was 2.6 million times the mass of the Sun. That research was based on the speedy swirl of stars around the galactic center. Only something very massive, like a black hole, could be pulling them in that fast, scientists believe.

Scientists believe there are billions of black holes in the universe, including many that are thousands of times more massive and vastly more luminous than the object at the center of the Milky Way.

Because scientists did not have an image of an X-ray flare before, some suggested that the dense object in the center of our galaxy was a clump of dark stars rather than a black hole.

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