The diagram to the right shows how Hayashi tracks change with changes in chemical composition. Z is the star's metallicity, the mass fraction not accounted for by hydrogen or helium. For any given hydrogen mass fraction, increasing Z leads to increasing molecular weight. The dependence of temperature on molecular weight is extremely steep—it is approximately ∂ ∂ ln ⁡ ⁡ T eff ∂ ∂ ln ⁡ ⁡ μ μ ≈ ≈ − − 26. Decreasing Z by a factor of 10 shifts the track right, changing ln ⁡ ⁡ T eff by about 0.05.