Research Associate Tobias Fritz, along with graduate students Sean Linden and Paul Zivick in Nitya Kallivayalil’s Near-field Cosmology group, combined images from Gemini South’s wide-field adaptive optics system with data from the Hubble Space Telescope to determine the proper motion of a distant cluster of stars. The observations, the first to use ground-based adaptive optics to precisely measure the motion of a cluster at such a large distance, allowed astronomers to set a lower limit for the mass of our Milky Way while providing clues about the cluster’s origin.