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.