Logarithm of the density of the circumstellar medium in g/cm³ after 4.86 and 5.60 Myr (left panel), after 6.58 and 7.0 Myr (center panel), and after 7.01 and 7.07 Myr (right panel). On the left, the mass, ejected by the first supernova has spread throughout the bubble, which, due to the injection of energy, expands rapidly. The density inside the bubble decreases and the Rayleigh-Taylor instabilities in the outer shell become more pronounced. In the central panel, the 25 M_⊙ star has reached the RSG phase and forms a shell at the wind termination shock that moves away from the star as the thermal pressure in the bubble decreases over time. Finally, on the right, the 25 M_⊙ star explodes as a supernova, which sweeps up the RSG shell and fills the bubble.