Internal kinematics of globular clusters

With the advent of the Gaia mission, astrometry is experiencing a renaissance. Although Gaia will make important breakthroughs in many different areas, stars in the crowded central fields of globular clusters and at the faint end of the color-magnitude diagram are out of Gaia's reach. However, the stable environment of space makes the Hubble Space Telescope (HST) an excellent astrometric tool.  Its diffraction-limited resolution allows it to distinguish and measure positions and brightnesses for faint stars all the way to the center of most globular clusters.