Appeared in the September, 21, 2000 issue of NATURE

The Accelerations of Stars Orbiting the Milky Way's Central Black Hole

A. M. Ghez, M. Morris, E. E. Becklin, A. Tanner, & T. Kremenek

High-resolution image of the center of the Galaxy at a wavelength of 2 microns, made with the Keck Telescope. The region outlined at the center immediately surrounds the location of the Galaxy's central, supermassive black hole, and is displayed to an order of magnitude greater depth than the rest of the image. The star symbol marks the location of SgrA*, the supposed radio counterpart of the black hole. The stars located closest to the black hole display the largest motions on the plane of the sky, and the accelerations of three of them are reported in this issue.

Infrared image of the Sgr A* star cluster obtained by A.~Ghez in May, 1999 using adaptive optics and the Keck telescope. Filled circles show the positions of six stars at the same and earlier epochs as indicated in the key. The yellow ellipses are orbits for S0-1 and S0-2 calculated assuming that Sgr A* is a supermassive black hole with mass 2.6X10^6 Msun. The periods are 63 and 17 years, respectively; they are near the short end of the optimal range allowed by the acceleration measurements. (From Nature News & Views)


Recent measurements, of the velocities of stars near the center of the Milky Way have provided the strongest evidence for the presence of a supermassive black hole in a galaxy, but the observational uncertainties poorly constrain many of the properties of the black hole. Determining the accelerations of stars in their orbits around the center provides much more precise information about the position and mass of the black hole. Here we report measurements of the accelerations for three stars located ~0.005 pc from the central radio source Sgr A*; these accelerations are comparable to those experienced by the Earth as it orbits the Sun. These data increase the inferred minimum mass density in the central region of the Galaxy by an order of magnitude relative to previous results and localized the dark mass to within 0.05 +- 0.04 arcsec of the nominal position of Sgr A*. In addition, the orbital period of one of the observed stars could be as short as 15 years, allowing us the opportunity in the near future to observe an entire period.