  During most of their life, stars fuse hydrogen into helium in their cores.
The mixing of chemical elements in the radiative envelope of stars with a
convective core is able to replenish the core with extra fuel. If effective,
such deep mixing allows stars to live longer and change their evolutionary
path. Yet localized observations to constrain internal mixing are absent so
far. Gravity modes probe the deep stellar interior near the convective core and
allow us to calibrate internal mixing processes. Here we provide
core-to-surface mixing profiles inferred from observed dipole gravity modes in
26 rotating stars with masses between 3 and 10 solar masses. We find a wide
range of internal mixing levels across the sample. Stellar models with
stratified mixing profiles in the envelope reveal the best asteroseismic
performance. Our results provide observational guidance for three-dimensional
hydrodynamical simulations of transport processes in the deep interiors of
stars.
