Mass fraction of ¹H (blue line) and ⁴He (orange line) as a function of mass coordinate for an example single-star model with an initial mass of 20 M_⊙ at core helium depletion. Vertical lines mark different choices of the helium core mass definition. In red, we indicate the specific nuclear energy generation rate throughout the model. The first peak in the nuclear energy generation rate around 5.8 M_⊙ corresponds to helium-shell burning and the second peak around 7.6 M_⊙ to hydrogen-shell burning.