  Massive stars are progenitors of supernovae, neutron stars and black holes.
During the hydrogen-core burning phase their convective cores are the prime
drivers of their evolution, but inferences of core masses are subject to
unconstrained boundary mixing processes. Moreover, uncalibrated transport
mechanisms can lead to strong envelope mixing and differential radial rotation.
Ascertaining the efficiency of the transport mechanisms is challenging because
of a lack of observational constraints. Here we deduce the convective core mass
and robustly demonstrate non-rigid radial rotation in a supernova progenitor,
the $12.0^{+1.5}_{-1.5}$ solar-mass hydrogen-burning star HD 192575, using
asteroseismology, TESS photometry, high-resolution spectroscopy, and Gaia
astrometry. We infer a convective core mass ($M_{\rm cc} = 2.9^{+0.5}_{-0.8}$
solar masses), and find the core to be rotating between 1.4 and 6.3 times
faster than the stellar envelope depending on the location of the rotational
shear layer. Our results deliver a robust inferred core mass of a massive star
using asteroseismology from space-based photometry. HD 192575 is a unique
anchor point for studying interior rotation and mixing processes, and thus also
angular momentum transport mechanisms inside massive stars.
