= Solution
For an ensemble of nearly circular stellar orbits with randomly oriented planes, radial motion is small while tangential motion is substantial: $\boxed{\sigma_r^2\ll\sigma_t^2}$. The two tangential variances remain equal by <spherical symmetry>. There need not be bulk rotation even though individual stars move on nearly circular paths. The anisotropy parameter $\beta=1-\sigma_t^2/\sigma_r^2$ is strongly negative; its exactly circular limit is singular as $\sigma_r\to0$.
Back to article page
