In following the evolution of stars to the cooler regions of giants in the H-R diagram, core solutions are fitted with the atmospheric solutions; this parameter is the constant that joins the pressure and temperature at the end of the ionization zone. The distribution of this parameter in the H-R diagram was studied by computing atmospheric models. The value of the constant of the adiabatic relation is less for main-sequence stars for the same spectral type. For the sun, the convection zone extends to about 30% of the solar radius. (auth)