Change is physical parameters of M_⊙ models with respect to changing helium fractions. Left panel: evolution of the effective temperature as a function of the central helium abundance for different 15 M_⊙ models. Right panel: helium profile vs. the mass in different 15 M_⊙ models when the central helium mass fraction is 0.90. In both panels, the letters have the same meaning as in Fig. 2.