Evolution of Very Massive Stars.

Models for the initial state and for the early evolution phases of stars with masses between 30 and 200 solar masses have been constructed It was found that, as evolution proceeds in these very massive stars, the convective instability moves outward. This effect is caused by the low values of produced by the radiation pressure. In consequence, an intermediate semiconvective zone develops in which the convection is so slow that it does not contribute to the energy transport but is fast enough to modify the composition, so that convective neutrality is maintained in every layer of this zone throughout the early evolution phases. The resulting models show that these very massive stars will use up as much as two-thirds of their total initial hydrogen content before exhaustion in the core occurs. Accordingly, the evolution proceeds somewhat more slowly for these stars than had previously been estimated; even a star of 200 solar masses spends over two million years in the evolutionary phases before its core gets exhausted