Logarithm of the density of the circumstellar medium in g/cm³ after 1.17 and 2.35 Myr (left panel), after 4.42 and 4.58 Myr (center panel), and after 4.774 and 4.783 Myr (right panel). On the left, the shells of the two bubbles encounter each other (left panel, left side) and are destroyed in the process. The remnants are pushed toward the lightest of the stars (left panel, right side). On the right, the 40 M_⊙ star (at Y =  −2.5 × 20¹⁹ cm) goes through the RSG phase, which forms a thin shell at the wind termination shock (center panel, left side) and then the WR phase. The WR wind sweeps up the RSG wind and the RSG shell is destroyed in the collision. The debris of the collision moves outward into the hot shocked gas (central panel, right side). Finally, the 40 M_⊙ star explodes as a supernova, which expands into the circumstellar bubble (right panel, left). Once it has hit the outer shell, the supernova bounces back and starts flowing inward (right panel, right).