Ghez and colleagues collected infrared images of the nucleus of the Milky Way over a four-year period using the 10-metre Keck telescope on Mauna Kea in Hawaii. The positions of the stars nearest Sagittarius A* change significantly in only a few years, so the team were able to measure the stars' velocities in the plane of the sky. The velocity vectors implied that the position of the black hole coincides with Sagittarius A*. The team went on to deduce the acceleration of the stars. The acceleration vector of an orbiting body points towards its centre of rotation, so the three stars' acceleration vectors should intersect at the location of the black hole. Ghez and co-workers established that they intersect within 0.002±0.0016 parsecs of Sagittarius A* (1 parsec = 3.26 light years).

The orbital periods of the stars may be as short as just a few decades. "There is something quite grand in the realization that we can expect, with a little luck, to see the Galactic centre rotate at least once in our lifetimes", says John Kormendy of the University of Texas at Austin. Other studies have indicated that the black hole has a mass 2.6 million times greater than the Sun, and a similar radius to the orbit of Mars around the Sun.