Similar to Fig. 2, showing the expansion of the supernova remnant. All frames have the same size of 80 × 80 pc. At the end of the Wolf-Rayet phase (left side of left panel) the density in the hot bubble has become smooth once again. The supernova starts as a high pressure blast wave at the centre of the bubble and quickly expands. After 400 years it has swept up nearly half of the free-streaming Wolf-Rayet wind (right side of left panel). 4000 years after the explosion (left side of centre panel), the supernova remnant has swept up most of the hot bubble. After 7200 years (right side, centre panel) it collides with the outer shell. Afterwards, the supernova remnant flows back toward the centre of the bubble (12 000 years after the explosion, left side of right panel). Eventually, its motion will slow down to subsonic level and the pressure in the bubble will start to equalize (right side of right panel, 40 000 years after the explosion).