  A vital condition for life on Earth is the steady supply of radiative heat by
the Sun. Like all other stars, the Sun generates its emitted energy in its
central regions where densities and temperatures are high enough for nuclear
fusion processes to take place. Because stellar cores are usually covered by an
opaque envelope, most of our knowledge about them and their life-giving nuclear
processes comes from theoretical modelling or from indirect observations such
as the detection of solar neutrinos and the study of stellar pulsations,
respectively. Only in very rare cases, stars may expose their cores, e.g., when
a tiny fraction of them evolves into Wolf-Rayet or helium hot subdwarf stars.
However, for the vast majority of stars, namely unevolved stars that burn
hydrogen to helium in their centres, direct observational clues on the cores
are still missing. Based on a comprehensive spectroscopic and asteroseismic
analysis, we show here that the bright B-type star $\gamma$ Columbae is the
stripped pulsating core (with a mass of $4$-$5\,M_\odot$, where $M_\odot$ is
the mass of the Sun) of a previously much more massive star of roughly
$12\,M_\odot$ that just finished central hydrogen fusion. The star's inferred
parameters indicate that it is still in a short-lived post-stripping structural
readjustment phase, making it an extremely rare object. The discovery of this
unique star paves the way to obtain invaluable insights into the physics of
both single and binary stars with respect to nuclear astrophysics and
common-envelope evolution. In particular, it provides first observational
constraints on the structure and evolution of stripped envelope stars.
