# Fixed Effective Temperature¶

When the atm_option control has the value 'fixed_Teff', MESA sets $$T_{\rm surf}$$ from the Eddington-grey $$T(\tau)$$ relation

$T_{\rm surf}^{4} = \frac{3}{4} T_{\rm eff}^{4} \left( \tau_{\rm surf} + 2/3 \right).$

The effective temperature $$T_{\rm eff}$$ is specified by the atm_fixed_Teff control; see here for a discussion of how the surface optical depth $$\tau_{\rm surf}$$ is evaluated.

MESA then sets $$P_{\rm surf}$$ to equal the radiation pressure associated with $$T_{\rm surf}$$.