Stellar cores, that is, the central regions where densities and temperatures are high enough for nuclear fusion processes to take place, are usually covered by an opaque envelope. Only in very rare cases, stars may expose their cores, for example, when a tiny fraction of them evolve into Wolf–Rayet or helium hot subdwarf stars. However, for the vast majority of stars, namely unevolved stars that burn hydrogen into helium in their centres, direct observational clues on the cores are still missing. On the basis of a spectroscopic and photometric analyses, here we show that the bright B-type-star γ Columbae is the stripped pulsating core (with a mass of 4–5 M_⊙, where M_⊙ is the mass of the Sun) of a previously much more massive star of roughly 12 M_⊙ that just finished central hydrogen fusion. The discovery of this star, which is still in a short-lived post-stripping structural re-adjustment phase, 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 observational constraints on the structure and evolution of stripped-envelope stars.