Motion of both stars and gas about the center of the Milky Way galaxy has provided strong evidence of the existence of a black hole. We'll concentrate on the investigation of the orbits of stars. Underway since about 1980, this effort is now led by Andrea Ghez's group at UCLA, which uses the Keck telescope in Hawaii . Key to the group's success is adaptive optics (see sidebar), a technique that compensates effectively for distortions due to Earth's atmosphere.

The twin Keck telescopes-- one images and infrared and any other in visible light—atop Mauna Kea , Hawaii . The infrared images of the close-in stellar orbits at the center of the Milky Way were made with the Keck. (photo credit: W. M. Keck Observatory

The image, made with infrared light, shows the region within about a tenth of a light-year of the galactic center, with the observed positions of stars for the past nine years superimposed on the most recent image (for comparison, the diameter of the Milky Way is about 100,000 light years). A yellow five-pointed star marks the center of the galaxy, which coincides with the intense, compact radio source mentioned above. This point appears to be at rest—its motion against the background of extremely distant objects (quasars) is accounted for by the Sun's rotation around the center of the galaxy.

The closest-in star, shown in various shades of red, has completed more than three-quarters of an orbit during the time of observation. At the distance of closest approach—slightly larger than the radius of Pluto's orbit—it is moving at 4% of the speed of light. To bend an object with such a huge velocity into this tight orbit requires an extraordinary central mass, which the Ghez group has determined to be 3.7 million suns.

Infrared image of stellar orbits around the object at the center of the Milky Way. The image shows approximately the innermost three lights years of our galaxy. Note the short periods of these stars, which indicate a central mass of 3.7 x 10 6 suns. (This image was created by Professor Andrea Ghez and her research team at UCLA and are from data sets obtained with the W. M. Keck Telescopes.)

A radiotelescope at the National Radio Astronomy Observatory in Green Bank, West Virginia. (photo credit: NRAO/NSF)

As for the size of the black hole, results from recent radio astronomy indicate that this object is about the size of about one Earth-orbit, ruling out alternatives and establishing that it is a black hole. The data in this study came from combining the observations of ten different radio telescopes, at locations ranging from Hawaii , across the continental U.S. , and the U.S. Virgin Islands. This system, known as the Very Long Baseline Array (VLBA), provides the resolution—the ability to see fine detail—of one extremely large telescope.

Although the black hole at the center of the Milky Way is now well-established, a question remains about the stars in this region. The Ghez group has identified young stars nearby, but the strong tidal forces near a black hole make it seem an unlikely place for star formation, leading to the hypothesis that these young stars formed further out. On the other hand, x-ray astronomers, working with the satellite observatory Chandra, determined the size distribution of these young stars and ruled out the possibility that they were formed away from the center. This question is still unresolved.

Another question is how a compact object with a mass of 3.7 million suns would form. But it seems to have happened frequently, since astronomers have concluded that a supermassive black hole resides at the center of most if not all galaxies.