Evolutionary Models of Stars of 15 and 30 M_{sun}

Evolutionary models are computed for stars of 15 and 30 I with a composition given by the mass abundances X = 0.75, Y = 0.23, Z = 0.02. Parallel calculations are carried out with and without allowance for semiconvection or full convection in shells of intermediate mass. In the case of semiconvection, the stability criterion that has been adopted is dT/dP = (dT/dP) d. After the main-sequence phases the models with intermediate mixing develop fully convective shells that greatly alter the behavior of the models during core helium burning. The models with the convective shells spend much of their helium-burning lifetimes as blue supergiants, whereas the models without convective shells rapidly become red supergiants. From an examination of the observations of h and x Persei it is concluded that the convective-shell models are more likely to be physically correct.