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Stothers, R., and C.-W. Chin, 1981: Convective overshooting in the evolution of very massive stars. Astrophys. J., 247, 1063-1069, doi:10.1086/159115.
Two important instances of possible convective overshooting in stars of 30-120 M☉ are considered here: (1) the possible merger between the convective core and the fully convective intermediate zone in the envelope at the end of the main phase of core hydrogen burning and (2) the possible penetration by the outer convective zone into the hydrogen-shell region when the star is a red supergiant. In the first instance, convective mixing between the core and inner envelope leads to renewed hydrogen burning near the center and, consequently, to a widening of the main-sequence band in the H-R diagram. In the second instance, deep penetration by the outer convective zone forces the star out of the red-supergiant configuration and into a new configuration very near the main sequence. It would thus seem possible to account for the apparent spread of the uppermost part of the observed main sequence and for the observed concentration of very luminous supergiants toward earlier spectral types, without having to postulate the occurrence of heavy mass loss.
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