Star evolution in the upper part of the Hertzsprung-Russel diagram

The observed distribution of the upper Hertzsprung-Russel diagram stars is explained under the assumption that 1) the evolution of a 64 Msun star is accompanied by the matter mixing in the semiconvective zone and by mass loss deduced from observations, 2) the 128 Msun star is subject to a mass loss with the rate 10-2 Msun/year during a short time stage. The theoretical estimate of Wolf-Rayet (WR) stars fraction agrees with the observed one if the mass loss rates become equal to the mass loss rates for WR stars when the content of hydrogen in the stellar envelope is decreased to ∼ 40% by mass