Kippenhahn diagrams following the energy generation and loss rates and the structure of convective regions as a function of time remaining until core collapse, from core helium burning until a few days before core collapse. Represented are the evolutionary calculations with initial helium star masses of 8 (top) and 20 (bottom) M_⊙, with metallicities of Z = 0.01 (left) and Z = 0.04 (right). The colour denotes the intensity of the energy generation (red) and loss (blue) rate. The hatched regions denote convective regions. The helium and carbon burning regions are indicated with arrows. The point where the products of helium burning are exposed at the surface, and the model transitions from WN to WC mass loss rates are indicated with horizontal arrows.