Density (top) and fraction of ejected gas as a function of distance from the most massive star (radius) for three different times (solid black: 2.84 Myr, dotted red: 3.49 Myr, dashed blue: 4.58 Myr), averaged at constant radius. In the main-sequence phase of the 60 M_⊙ star (2.84 Myr), there is a sharp contact surface at 20 pc, where the ejected gas fraction suddenly drops to 10^-3. During the Wolf-Rayet phase, the wind power increases. Rayleigh-Taylor instabilities at the accelerating shell create an extended mixing layer. 20 000 yr before the supernova of the 60 M_⊙ star (dashed blue curves), the mixing layer extends between about 27 and 40 pc, if defined by ejected gas fractions between 1 and 10 per cent.