Strömgren gave a brief analysis of three points dealt with in his article on ‘ Evolution of Stars’ (A.J 57, 65, 1952):

1. For main-sequence stars the extent of the convective regions changes with the mass. The more massive stars (A stars brighter than 2m-3m) have a convective core roughly corresponding to the Cowling model. The outer convection zone, first considered in connexion with problems of the solar interior by Biermann in 1938, is quite narrow. Passing from bright to faint main-sequence stars one finds that the extent of the convective core decreases, vanishing for faint stars. This is due to the fact that the energy-production mechanism changes from the carbon cycle to the proton-proton process while the relation between opacity and distance from the centre does not change very appreciably. At the same time the outer hydrogen convection zone increases in thickness and reaches appreciable depths.