  Polstar is a proposed NASA MIDEX space telescope that will provide
high-resolution, simultaneous full-Stokes spectropolarimetry in the far
ultraviolet, together with low-resolution linear polarimetry in the near
ultraviolet. In this white paper, we describe the unprecedented capabilities
this observatory would offer in order to obtain unique information on the
magnetic and plasma properties of the magnetospheres of hot stars. This would
enable a test of the fundamental hypothesis that magnetospheres should act to
rapidly drain angular momentum, thereby spinning the star down, whilst
simultaneously reducing the net mass-loss rate. Both effects are expected to
lead to dramatic differences in the evolution of magnetic vs. non-magnetic
stars.
