Variation in luminosity of the model stars of metallicity Z = 3 × 10⁻⁵ (top left) and Z = 3 × 10⁻⁷ (top right) as a funnction of the core mass. The different lines refer to stars with an intial mass 1  M_⊙ (orange), 1.5  M_⊙ (grey), 2  M_⊙ (cyan) 2.5  M_⊙ (red), 3  M_⊙ (blue), 4  M_⊙ (light green), 5  M_⊙ (magenta), 6  M_⊙ (black), and 7.5  M_⊙ (dark green). The dashed lines indicate the core mass – luminosity relation by Paczyński (1970). The bottom panels report the evolution of the temperature at the bottom of the convective envelope of the same model stars as shown in the top panels as a function of the current mass of the star.