Logarithm of the density [cgs.] for the circumstellar medium in the absence of an interstellar magnetic field. The left panel shows the circumstellar bubble on a 40 × 40 pc frame after 1 (left side) and 2 (right side) Myr have past since the start of the main sequence. The bubble is spherically symmetric, except for local thin-shell and Rayleigh-Taylor instabilities. In the centre (frame size 60 × 60 pc), the star leaves the main sequence after 4.3 Myr (left side) and becomes a red supergiant, which lasts until 4.5 Myr after the start of the simulation (right side), forming a new shell at the wind termination shock. The right panel (frame size 70 × 70 pc) shows the transition to the Wolf-Rayet stage, which forms a new shell that sweeps up the red supergiant wind (left panel, 10 000 yrs into the Wolf-Rayet stage) and collides with the red supergiant shell. The fragments of this collision move out into the old main sequence bubble (right side, 100 000 years into the Wolf-Rayet fase).