The accelerations of stars orbiting the Milky Way's central black hole

Recent measurements(1-4) of the velocities of stars near the centre of the Milky Way have provided the strongest evidence for the presence of a superm assive black hole in a galaxy(5), but the observational uncertainties poorl y constrain many of the black hole's properties. Determining the accelerati ons of stars in their orbits around the centre provides much more precise i nformation about the position and mass of the black hole. Here we report me asurements of the accelerations of three stars located similar to 0.005 pc (projected on the sky) from the central radio source Sagittarius A(star) (S gr A(star)); 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 relati ve to previous results, and localize the dark mass to within 0.05 +/- 0.04 arcsec of the nominal position of Sgr A(star). In addition, the orbital per iod 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.