Fig 16.19.

(a) Measured positions for three observing years near the position of SgrA*. (b) Positions of the 90 stars whose proper motions were measured, with the size of the symbol scaled to the size of the motion. (c) Projected stellar velocity dispersion as a function of distance from SgrA*.The solid line is the value for Keplerian orbits (still outside all of the mass). (d) Enclosed mass as a function of projected distance for 30 stars.

With three observing runs, a year apart, Ghez's group were able to identify 90 stars with proper motions large enough to measure. The largest proper motion corresponded to a tangential velocity of 1400 km/s. They found that this motion is organized about the position of SgrA* to within 0.1 arc sec. The projected positions and velocities are consistent with Keplerian motion. This means that essentially all of the mass is closer to the center than any of the stars measured. The best estimate of the mass in the central object is (2.6 ± 0.2) X 106 M0. The observations don't

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