Proper-motion studies of several open and globular clusters have yielded accurate, two-component (radial and tangential) velocity dispersion profiles for the brighest stars in these systems. It is shown that velocity dispersions derived from proper motions, unlike those derived from radial velocities, contain enough information to determine the shape and amplitude of the velocity ellipsoid at all radii in a spherical cluster. This information, in turn, is sufficient to yield the cluster potential without any additional assumptions about the kinematics or the mass-to-light ratio. Proper-motion data for the open star cluster M35 imply a dynamical mass within the central 3.75 pc between 1600 and 3200 solar masses (95 percent confidence), consistent with the mass of a realistic stellar population within the same radius.
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Leonard et al. (1989) studied this question.