  Current burning issues in stellar physics, for both hot and cool stars,
concern their magnetism. In hot stars, stable magnetic fields of fossil origin
impact their stellar structure and circumstellar environment, with a likely
major role in stellar evolution. However, this role is complex and thus poorly
understood as of today. It needs to be quantified with high-resolution UV
spectropolarimetric measurements. In cool stars, UV spectropolarimetry would
provide access to the structure and magnetic field of the very dynamic upper
stellar atmosphere, providing key data for new progress to be made on the role
of magnetic fields in heating the upper atmospheres, launching stellar winds,
and more generally in the interaction of cool stars with their environment
(circumstellar disk, planets) along their whole evolution. UV
spectropolarimetry is proposed on missions of various sizes and scopes, from
POLLUX on the 15-m telescope LUVOIR to the Arago M-size mission dedicated to UV
spectropolarimetry.
