Evolution of Single Stars. VII. Evolution of Massive Stars

Evolution of 15 and 30 M_odot stars up to central carbon ignition and of 60 M_odot star up to central helium exhaustion is followed using density criterion for convective instability. It was found that the stars spend the first half of their core helium burning stage in red and the second in blue supergiants region performing extended loops on the H-R diagram. The star starts the loop always where hydrogen burning shell approaches the jump ofchemical composition produced earlier by external convective zone. The large extension of blue supergiants region over the spectral types observed in h and hi Per and the Magellanic Clouds seems to indicate that the alternative evolutionary scheme using the temperature criterion provides better agreement with observations than our scheme. This conclusion is, however, not definitive. We suggest that the investigation of period changes of long period cepheids in Magellanic Clouds could provide a good test.