The size of stellar reactors and rotational mixing of nuclear elements in massive stars

On the basis of observations for 25 young associations, we show evidences of overshooting in massive stars and derive a typical value of the overshooting distance dover - (0.25-0.30) Hp, where Hp is the pressure scale height at the edge of the classical core. Turbulent diffusion of nuclear elements due to rotational mixing appears to be very efficient in massive stars because of their high radiative viscosity. A bifurcation occurs in stellar evolution. Below some rotation, evolution is essentially classical; above it, mixing is very efficient and produces essentially bluewards homogeneous evolution