An illustration of a black hole at the center of a galaxy. Photo: NASA

For the first time ever, astronomers have seen stars accelerate around a supermassive black hole. UCLA astronomers report that three stars have accelerated by more than 250 thousand miles per hour per year as they orbit the monstrous black hole at the center of our Milky Way galaxy.

The astronomers, led by Andrea Ghez, professor of physics and astronomy at UCLA, report that five years of measurements reveal that the star closest to the black hole has turned a corner in its orbit around the black hole. The research is published in the Sept. 21 issue of the journal Nature.

"We are actually seeing stars begin to curve in their orbits," Ghez said. "One of these stars may complete its orbit around the supermassive black hole in as little as 15 years. The light from these stars takes 24,000 light years to get to us, and we're talking about a complete orbit in perhaps as few as 15 years!"

The two closest stars are only 10 light days from the black hole, but Ghez predicts they will orbit the enormous black hole and not be swallowed by it. In 1995 the three stars were moving at two million miles per hour, and by 1999 they had changed their velocities by more than one million miles per hour.

Writing about Ghez's work in the same issue of Nature, John Kormendy, who holds an endowed chair in astronomy at the University of Texas, Austin, observes that astronomers who study the center of a galaxy "generally observe a snapshot of objects that change only on time scales of millions or billions of years. There is something quite grand in the realization that we can expect, with good health and a little luck, to see the galactic center rotate once in our lifetimes."

In 1998 Ghez reported that the black hole, with a mass 2.6 million times that of our sun, lies at the center of our galaxy, 24,000 light years away. This discovery resolved a raging debate among astronomers that lasted for more than a quarter-century. Ghez is now able to pinpoint the location of the black hole, which is in the constellation of Sagittarius, much more accurately. Black holes are collapsed stars so dense that nothing can escape their gravitational pull, not even light.

"We now know the location of the black hole so precisely," Ghez said, "that's it's like someone in Los Angeles who can identify where someone in Boston is standing to within just over a yard, if you scale it out to 24,000 light years."

Ghez uses the W.M. Keck Observatory's 10-meter Keck I Telescope atop Mauna Kea in Hawaii -- the world's largest optical and infrared telescope -- to study the movement of 200 stars that are close to the galactic center. In the Nature paper, she reports on the acceleration of three of the stars that are among the closest to the black hole; others may also be accelerating, but she has not been able to measure that yet. She has made nine measurements over five years, using a technique she refined called infrared speckle interferometry.

One reason why astronomers had been unable to determine for so long whether a black hole is at the galactic center is that our atmosphere distorts the images of stars. Ghez's speckle interferometry involves taking thousands of very quick, high-resolution snapshots that correct for the distortions produced by the Earth's atmosphere. She has developed algorithms -- specific computer commands based on sophisticated mathematics -- and software for analyzing the data. Using traditional imaging techniques at the center of the galaxy would cause the stars closest to the galactic center to look fuzzy and indecipherable. Ghez's technique, however, improves the resolution by a factor of at least 20.

"The atmosphere blurs your vision," Ghez said, "but speckle interferometry clears the picture up; it's like putting on glasses. Think of seeing a coin that looks distorted at the bottom of a pond. We take thousands of freeze frames, and then can determine what is distorted and what is really at the bottom of the pond."

An artist's impression of a supermassive black hole. Photo: CXC/A. Hobart.

For future images of the region, she will use an even more sophisticated technique, called adaptive optics, which should enable her to see more of the densely packed stars in this region, and perhaps stars that are even closer to the black hole.

"Speckle is a poor woman's adaptive optics," she said.

In the Nature paper, the UCLA astronomers report that the density of dark matter is substantially greater than estimated from previous measurements, which further establishes that it is indeed a supermassive black hole, and not a more exotic form of matter, Ghez said.

"Our galaxy is rather mild-mannered and quiet, and was one of the least likely galaxies to have a black hole at its center," Ghez said. "Black holes cannot be seen directly, but their influence on nearby stars is very visible, and provides a signature."

The Milky Way is one of approximately 100 billion galaxies containing at least 100 billion stars each.

Ghez's co-authors on the Nature paper are UCLA physics and astronomy professors Mark Morris and Eric Becklin, UCLA graduate student Angelle Tanner and UCLA undergraduate Ted Kremenek. Becklin identified the center of the Milky Way in 1968. The galactic center is located due south in the summer sky.

The black hole at the center of our galaxy came into existence billions of years ago, perhaps as very massive stars collapsed at the end of their life cycles and coalesced into a single, supermassive object.

Ghez, who has the highest resolution images of the galactic center ever obtained, said. "The Keck Observatory is the best facility in the world for this research. The Keck Telescope enables us to track stars very precisely." The telescope's resolution is so high, she said, that it could detect two flies in Japan that are less than 10 feet away from each other.

Ghez's research is supported by the National Science Foundation and the Packard Foundation.