Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2017 / S2 / 2 / 1 / Solution

The effective temperature is defined by $\boxed{L=4\pi R_{\ast }^2\sigma T_{\mathrm{eff}}^4}$, with bolometric luminosity $L$, photospheric radius $R_{\ast }$ and Stefan-Boltzmann constant $\sigma$. It is the blackbody temperature giving the same total surface flux, not a claim that each spectral line forms at that temperature.