Stellar motions provide hole-y data

By tracking the speed of stars close to the center of our galaxy, astronomers over the past 5 years have established that a black hole with a mass equal to 2.6 million suns lurks at the Milky Way’s core (SN: 1/24/98, p. 59). Now, researchers have measured for the first time the acceleration of three stars near the core. By determining how much the orbits of these stars’ are bent by the tug of the black hole, the scientists have more precisely calculated the beast’s location and mass.

To measure acceleration, Andrea M. Ghez and her colleagues, all of the University of California, Los Angeles, used the Keck I telescope atop Hawaii’s Mauna Kea to take a series of infrared images of the Milky Way’s nucleus. The scientists periodically observed the stars from 1995 to 1999 and report their results in the Sept. 21 Nature.

The researchers took thousands of exposures—each about one-tenth of a second—during each observation. These freeze-frame images avoid capturing the jitter from turbulence in Earth’s atmosphere.

Although, the stars whiz around the galaxy’s core at speeds of about 1,350 kilometers per second, the black hole has a huge effect on the shape of their orbits, says John Kormendy of the University of Texas at Austin.

Each of the trio of stars accelerates at a rate similar to that experienced by Earth as it moves around the sun (3 to 5 millimeters per second per second). Moreover, this change in speed is directed toward a compact radio source, called Sagittarius A*, that lies at the presumed location of the black hole.

The new measurement “strengthens the association of Sagittarius A*” with the inferred black hole, Kormendy writes in a commentary accompanying the Nature report. Ghez’ team also finds that the unseen material exerting a gravitational tug at the Milky Way’s center has a minimum density nearly 10 times what previous observations had revealed. This result negates the idea that the core is merely a throng of dim, ordinary stars, rather than a black hole, Kormendy notes.

At least one of the three stars may have an orbit as short as 15 years, Ghez and her colleagues say. If so, the team could eventually track the star for an entire rotation about the galactic center. “There is something quite grand in the realization that we can expect, with good health and a little luck, to see the [stars at the] galactic center rotate at least once in our lifetimes,” Kormendy says.

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