On the left, we present a Kippenhahn diagram of two SERIES I stellar models as a function of remaining evolution time. On the right, we present a section of the same diagrams but as a function of model number in which the location of some important stages can be seen more accurately. Green hatched areas denote regions of convection. The intensity shown in the colorbar, indicates the net energy rate. Turquoise line shows the CO core. Top panel: structure of a 2.5 M_⊙ helium star with Z = 0.02 and no overshooting. During e-captures on ²⁴Mg and subsequent carbon burning, the core becomes convective. Bottom panel: structure of a hybrid 1.8 M_⊙ helium star with Z = 0.02 and f_OV = 0.014. Convective overshooting (purple hatched area) leads to larger core during the helium main sequence phase. Compressional heating ignites the carbon resulting in a thermal runaway.