Hertzsprung-Russell diagram track of the 15 M_⊙ model in Fig. 1, but over-plotted with coloured symbols based on the value of W_m in the envelope. Blue squares if positive W_m, reflecting the envelope’s ability to regain thermal balance, while red circles if negative W_m, indicating an unstable scenario. Symbols are highlighted with a thick black border if the deviation from thermal balance exceeds 1% between actual and nuclear luminosity.