Globular clusters (GCs) are old, collisional systems so the stars inside them interact on short timescales and have had a very long time in which to do so. These many and frequent interactions significantly affect both the structural and kinematic properties of GCs, so by studying their structure and kinematics we can learn about the evolution history of GCs.
Some effects manifest as energy equipartition in the velocity distributions, that is a change in the velocity properties as a function of stellar mass. This phenomenon has been studied in theory but observations have been challenging owing to the need for very accurate measurements for faint stars as well as bright ones. The Hubble Space Telescope (HST) is the only observatory with the depth and precision to provide kinematics for a wide range of stellar masses, and even so we are pushing the limits of the observatory and our analysis techniques.
In my talk, I will focus on what we are learning about energy equipartition in Milky Way GCs from analysis of their kinematics using proper motions from HST. I will also discuss the advances that current and future observatories will enable.