Density profiles vs. Lagrangian mass at the time of core collapse for a set of stellar models with an initial mass on the main sequence between 11 and 30 M_⊙ (Woosley et al. 2002). The origin is at star center (left column) or at the He core edge (right column). The H-rich envelope is drawn with a thick black line and corresponds in all models to (extended) regions with a density below about 10⁻⁷ g cm⁻³. The He core is drawn with a thick red line and corresponds in all models to (compact) regions with a density above about 1 g cm⁻³. The vertical scale is squeezed but is suitable to reveal the evolution of the He core mass and the H-rich envelope mass for different ZAMS masses. Left panel: smooth progression in the He core mass with initial progenitor mass, and the varying progenitor mass at the onset of core collapse. Right panel: nonlinear evolution of the H-rich envelope mass with initial progenitor mass is emphasized. In these models, stars with ZAMS masses of 11–21 M_⊙ contain similar H-rich envelope masses (8–9 M_⊙) at the onset of core collapse.