Final evolution of the inner stellar structure of the model with X_C = 0.17 which experiences a shell merger between the C and Ne-burning shells (highlighted in the red box in panel a). (a) Kippenhahn diagram in which colors indicate the regions dominated by nuclear burning or neutrino losses and gray hatched regions indicate convective areas. Brown lines show contours of constant (logarithmic) density. Colored markers trace the burning front for C, Ne, and O burning. Small colored vertical lines indicate the times at which the entropy profiles are shown in the bottom panel. (b) Entropy profiles in the mass and time range highlighted in the red box in the top panel. The entropy in the neon-burning region (2.5 − 2.9 M_⊙) gradually increases until it exceeds the entropy of the C-burning layers above, triggering a merger between these regions.