Galactic Center
By imaging the Galactic Center at infrared wavelengths, Ghez and her colleagues have been able to peer through heavy dust that blocks visible light, to reveal images of the center of the Milky Way. Thanks to the 10-meter aperture of the W.M. Keck Telescope and the use of adaptive optics to correct for the turbulence of the atmosphere, these images of the Galactic Center are at very high spatial resolution and have made it possible to follow the orbits of stars around the black hole Sagittarius A* (Sgr A*). The partial orbits of many stars orbiting the black hole at the Galactic Center have been observed. One of the stars, S2, has made a complete elliptical orbit since detailed observations began in 1995. Several decades more will be required to completely document the orbits of some of these stars. These measurements may provide a test of the theory of general relativity. In October 2012, a second star, S0-102, was identified by her team at UCLA, orbiting the Galactic Center. Using Kepler's third law, Ghez's team used the orbital motion to show that the mass of Sgr A* is 4.1±0.6 million solar masses. Because the Galactic Center where Sgr A* is located, is one hundred times closer than M31 where the next nearest supermassive black hole (M31*) is, it is one of the best demonstrated cases for a supermassive black hole.