The Milky Way's Hungry Black Hole

By Jamie Murphy.

No hypothetical creatures of physics appeal to the imagination quite as vividly as black holes -- those insatiable gravitational maws that gobble up everything within reach, not even allowing light to escape. Now it seems more likely than ever that one of these bizarre celestial objects is lurking at the center of the Milky Way galaxy, the island of stars inhabited by the sun and the earth.

In a paper published this month in the British journal Nature, six astronomers reported that they had made the most precise measurement yet of an object at the galactic center within Sgr A*, which for two decades has been known to be radiating large amounts of energy. The size and shape of Sgr A*, and its energetic output, says Caltech Astronomer Kwok-yung Lo, are strong evidence that "there may be a black hole in the center of our galaxy" devouring matter at a rapid rate.

To make their measurements, the astronomers used radio telescopes, which can peer through the clouds of interstellar dust that hide the galactic center from the glassy-eyed view of optical telescopes. Still, the enigmatic source of radiation was an enormous distance away -- 30,000 light-years, or 180 quadrillion miles. The only way to discern its size and shape accurately was to employ a technique called VLBI (very long baseline interferometry). In 1983 the astronomers observed Sgr A* with six giant dish-shaped radio telescopes, one each in Massachusetts, West Virginia and Texas, and three in California. "In effect, the configuration of the telescopes gave us a 'lens' 3,000 miles in diameter," explains Astronomer Mark Reid of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

After months of painstaking measurements, some of the mystery was stripped from Sgr A*. It proved to be shaped like an ellipsoid no more than 1.9 billion miles long and almost a billion miles wide -- impressive by earthly standards but diminutive on a galactic scale and in relation to the tremendous amount of energy it emits. Concludes Astronomer Donald Backer of the University of California, Berkeley: "What we observe is a very bright, compact object that appears to be rather small by stellar dimensions. Yet it's radiating a lot of luminosity. There are many stellar objects in the galaxy that radiate this amount of energy, but this one is peculiar. None of the others are as compact or as steady." In other words, it behaves like a black hole.

Scientists have suspected since the early 1970s that a black hole might be the source of the intense radiation emanating from the galactic center. They speculate that when the galaxy was young, so much material (in the form of stars, dust and gas) collected at its center that it collapsed under the force of its own gravity. Crushing together until all of its mass was concentrated in an incredibly small volume, the matter continued to exert tremendous gravitational force. Like the Cheshire cat in Alice's Adventures in Wonderland that vanished, leaving only a smile, the great mass of galactic material in effect disappeared. Only its gravity remained, continuing to pull in nearby stars and dust.

As galactic material swirls into the abyss, it forms a giant disk around the black hole, heats up and begins radiating energy. It is probably this so- called accretion disk that the astronomers measured. Nonetheless, to prove that Sgr A* is indeed a black hole, scientists must conclusively determine its mass. Then, knowing its size, they will be able to calculate its density. Says Lo: "When you find that the density is extremely high and the gravity is so strong that it will overcome all known mechanisms of pressure support, then that would be proof that there is a gravitationally collapsed object."

The best way to calculate the mass of Sgr A*, says Lo, is to study the motion of stars orbiting near it; their movement is dependent upon Sgr A*'s gravity, which in turn is related to its mass. But, Lo explains, "measuring the stellar motion is very difficult because it's highly obscured."

What are the implications of confirming that Sgr A* is a black hole? For one thing, along with recent findings that indicate black holes are in the centers of two other galaxies, it lends credence to the idea that the phenomenon is common. Also, says Reid, Sgr A* could serve "as a model for quasars," the distant, bright dynamos that most astronomers think are galaxies in their earliest stages of formation.

Physicist John Wheeler, a leading black-hole theorist, sees another implication -- ominous, but rather far off. If Sgr A* is a black hole, he says, it is continuously consuming bits and pieces of the galaxy, becoming gravitationally more powerful as it does. "The more it eats, the more it wants," explains Wheeler. "In that sense," he says, "it is eating the galaxy, which means, it is conceivable that the Milky Way could one day be consumed."

With reporting by John D. Hull/San Francisco and William Sonzski/Boston