Propagation of the luminosity wave during the pre-MS evolution of a 932 model. The wave originates deep in the stellar interior when the mass of the model is 12.20 M_⊙ and is shown with a blue line. The outward migration of the wave is captured in orange and the model has reached 14.60 M_⊙. The wave is about to break at the surface when the mass is 18.50 M_⊙ and is shown in purple. If the accretion rate during the breaking of the wave is higher than the critical value, the surface of the model will experience a large expansion in radius and become a red supergiant protostar. If the accretion rate is lower than the critical value, the surface of the model will expand to a much lesser extent and become a blue supergiant protostar. Once the energy is evacuated from the surface, the wave no longer exists and is shown in brown.