Time evolution of the luminosity associated with the hydrogen-burning shell during the first 3 × 10⁵ years after the end of the MS. Solid lines are for models computed with the Ledoux criterion, and dashed lines are for models computed with the Schwarzschild criterion. Black lines are for 20 M_⊙ models at solar metallicity, blue lines are for 25 M_⊙ models at solar metallicity, and red lines are for 25 M_⊙ models at a metallicity of Z = 0.006.