Hertzsprung-Russell diagram track of a 15 M_⊙ model. The surface and total nuclear luminosity are shown by the black solid and green dashed lines, respectively. The contributions to the nuclear luminosity include the CNO cycle during H burning (blue), the triple-α reaction (orange dashed), and additional alpha captures (red dotted) like ¹²C(α, γ)¹⁶O(α, γ)²⁰Ne and ¹⁴N(α,γ)¹⁸F(,e⁺)¹⁸O during core-He burning. After core-H exhaustion, the model rapidly expands into an RSG. Notice the increasing difference between actual and nuclear luminosities, especially below log(T_eff/K) ∼ 4.