Kippenhahn diagrams for 120 M_⊙ models at Z = 0.0004 (left panel) and Z = 0.002 (right panel), showing the evolution of the stellar structure in terms of the distance from the center as a function of time to core collapse. The black line at the top indicates the total radius of the star. The thin black lines followed by a label indicate the radial coordinate of different mass shells inside the star. The blue-shaded areas correspond to convective regions. The solid (dashed) lines correspond to the peak (10%) of the energy generation rate for H burning (blue), He burning (green), and C burning (red).