Evolutionary track of a 15 M_⊙ He star with solar metallicity (Z = 0.02) and overshooting parameter f_ov = 0.01. Panel (a): temporal evolution of the compactness parameter ξ_2.5 until seconds before core collapse. Key evolutionary phases are marked as follows: pentagram (central He depletion), square (central C ignition), triangle (central C depletion), cross (central Ne depletion), open circle (central O depletion), and left-pointing triangle (central Si depletion). Panel (b): Kippenhahn diagram from core He-burning to the onset of core collapse. The color bar shows the nuclear energy generation rate (ϵ_nuc, red) and neutrino loss rate (ϵ_ν, blue). The green, purple, and red-hatched regions represent the convective, overshooting, and semiconvective mixing regions, respectively. The gray solid indicate the classical Chandrasekhar mass, M_Ch, 0, as defined in Eq. (6). The dashed black, dotted red, and dash-dotted magenta lines denote the iron, He-free (M_He-free), and C-free (M_C-free) cores, respectively. Vertical dotted lines mark specific depletion and ignition events. Panel (c): same as panel (b), but the color represents the specific gravothermal energy, ϵ_grav ≡ −Tds/dt (red: contraction , blue: expansion). Panel (d): same as panel (b), but the color represents the electron degeneracy parameter, η ≡ μ/k_BT, where μ is the chemical potential, k_B is the Boltzmann constant, and T is temperature. η ≪ −1: non-degenerate; η ≈ 0: partial degeneracy; η ≫ 1: strong degeneracy.