oxygen, whichever is less abundant, leaving the majority species (C I or O I) to form other molecules. If oxygen dominates, it produces a “normal star” (if cool, an M star), otherwise carbon is free to form many molecules with high opacities, making carbon stars with very different spectra. At T$_{\rm eff} = 5500$K, CO formation is low, thus this molecule does not affect the chemical equilibrium contrary to what is seen in the cooler atmospheres.

The right-hand panels show changes for the same model parameters and different $\alpha$-element abundances. Large changes are visible for both models in the pressure and electron numbers. The significant differences in the pressure compared to the cooler model are due to the increased gravity. Increasing the abundance of $\alpha$ elements reduces pressure (despite the fact that electron pressure increases), because the electrons of the other main electron contributors change the continuum opacity. The $\alpha$ elements that contribute most to the total number of electrons in the ATLAS9 models are shown in Figure 6. For the T$_{\rm eff} = 4000$K, log $g = 1$ model, the main electron contributors are Ca, Mg, Na, and Al in the outer layers and Mg, Si and Fe in 