  In this paper we present new models of massive stars based on recent
advancements in the theory of diffusive mixing and a new empirical formulation
of the mass-loss rates of red supergiant stars. We compute two sets of stellar
models of massive stars with initial chemical composition [Z=0.008, Y=0.25] and
[Z=0.020, Y=0.28]. Mass loss by stellar wind is also taken into account
according to the empirical relationship by de Jager et al. (1988). Despite the
new mixing prescription, these models share the same problems of older models
in literature as far as the interpretation of the observational distribution of
stars across the HRD is concerned. Examining possible causes of the failure, we
find that the adopted rate of mass loss for the red supergiant stages
under-estimates the observational values by a large factor. Revising the whole
problem, first we adopt the recent formulation by Feast (1991), and secondly we
take also into account the possibility that the dust to gas ratio varies with
the stellar luminosity. Stellar models are then calculated with the new
prescription for the mass-loss rates. The models now possess very extended
loops in the HRD and are able to match the distribution of stars across the HRD
from the earliest to the latest spectral types both in the Milky Way, LMC and
SMC. Finally, because the surface H-abundance is close to the limit adopted to
start the Wolf-Rayet phase (WNL type), we suggest that a new channel is
possible for the formation of low luminosity WNL stars, i.e. by progenitors
whose mass can be as low as 20 \msol, that have evolved horizontally across the
HRD following the blue-red-blue scheme and suffering large mass loss during the
red supergiant stages.
