Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2015 / S2 / 2 / 1 / Solution

The effective temperature is defined by the Stefan–Boltzmann law applied to a star's total luminosity:Here $R$ is the adopted photospheric radius and ${\sigma }_{\mathrm{SB}}$ is the Stefan-Boltzmann constant. This defines the temperature of a black body radiating the same total flux per unit surface area; it does not assert that every wavelength of the stellar spectrum follows a perfect Planck spectrum.