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


Recent measurements1,2,3,4 of the velocities of stars near the centre of the Milky Way have provided the strongest evidence for the presence of a supermassive black hole in a galaxy5, but the observational uncertainties poorly constrain many of the black hole's properties. Determining the accelerations of stars in their orbits around the centre provides much more precise information about the position and mass of the black hole. Here we report measurements of the accelerations of three stars located 0.005 pc (projected on the sky) from the central radio source Sagittarius A* (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 localize 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.

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Figure 1: East–west and north–south positional offsets from the nominal location of Sgr A* versus time for (a) S0-1, (b) S0-2, and (c) S0-4.
Figure 2: The acceleration uncertainty cones and their intersections.
Figure 3: The minimum enclosed mass implied by each star's acceleration measurement versus projected distance from the newly determined dynamical centre position.
Figure 4: The measured motion of S0-1, S0-2 and S0-4 and several allowed orbital solutions.

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This work was supported by the National Science Foundation and the Packard Foundation. We are grateful to J. Larkin for exchanging telescope time; O. Gerhard, M. Jura, and A. Weinberger for useful input; telescope observing assistants J. Aycock, T. Chelminiak, G. Puniwai, C. Sorenson, W. Wack, M. Whittle and software/instrument specialists A. Conrad and B. Goodrich for their help during the observations. The data presented here were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the financial support of the W. M. Keck Foundation.

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Correspondence to A. M. Ghez.

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Ghez, A., Morris, M., Becklin, E. et al. The accelerations of stars orbiting the Milky Way's central black hole . Nature 407, 349–351 (2000).

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