Multi-epoch observations with ACS on HST provide a unique and comprehensive probe of stellar dynamics within NGC 6397 .
We are able to confront analytic models of the globular cluster with the observed stellar proper motions .
The measured proper motions probe well along the main sequence from 0.8 to below 0.1 M _ { \odot } as well as white dwarfs younger than one gigayear .
The observed field lies just beyond the half-light radius where standard models of globular cluster dynamics ( e.g .
based on a lowered Maxwellian phase-space distribution ) make very robust predictions for the stellar proper motions as a function of mass .
The observed proper motions show no evidence for anisotropy in the velocity distribution ; furthermore , the observations agree in detail with a straightforward model of the stellar distribution function .
We do not find any evidence that the young white dwarfs have received a natal kick in contradiction with earlier results .
Using the observed proper motions of the main-sequence stars , we obtain a kinematic estimate of the distance to NGC 6397 of 2.2 ^ { +0.5 } _ { -0.7 } kpc and a mass of the cluster of 1.1 \pm 0.1 \times 10 ^ { 5 } \mathrm { M } _ { \odot } at the photometric distance of 2.53 kpc .
One of the main-sequence stars appears to travel on a trajectory that will escape the cluster , yielding an estimate of the evaporation timescale , over which the number of stars in the cluster decreases by a factor of e , of about 3 Gyr .
The proper motions of the youngest white dwarfs appear to resemble those of the most massive main-sequence stars , providing the first direct constraint on the relaxation time of the stars in a globular cluster of greater than or about 0.7 Gyr .