What is the most massive star that can form? While simply posed, this question confounds

our best observations and theories. A very compact, and massive, young

star cluster near the Galactic center holds clues to answering this question. Being

so young, the cluster contains a pristine "fossil record" of the star formation

process. By analyzing Hubble Space Telescope images of stars in this cluster, I have

found observational evidence that there is an upper limit to the masses of stars of

~150 solar masses. This result is robust against errors in photometry, extinction

estimate, age estimate, non-coevality, and initial mass function slope.