Stellar mass in M_⊙ vs age in years (left panel). The asterisks along each curve are the star masses in the mass interval 0.6 to 120 M_⊙. The color code of each curve indicates the chemical composition. Curves are also labeled in accordance with the mass abundance of metals Z according to the law ΔY/ΔZ = 2.5, where Y is the mass abundance of helium and the initial values are Z₀ = 0.0001 and Y₀ = 0.23. The values of the metalliciies are Z = 0.0001 (black), Z = 0.0004 (blue), Z = 0.004 (green), Z = 0.008 (magenta), Z = 0.020 (red), and Z = 0.050 (dark red). The mass abundance of hydrogen is given by X = 1 − Y − Z. Same as in the left panel but for luminosity vs age (right panel). The luminosity dips at ages above 1 to 1.5 Gyr correspond to the transition masses M_HeF. Along each curve, stars more massive than M_HeF undergo a quite He-ignition of the core, while those equal or lighter than M_HeF undergo core He-flash.