Effective Temperature
The surface of a star is defined to have a temperature given by the effective temperature in the Stefan–Boltzmann law. By using a simple model for stellar atmospheres, assuming local thermal equilibrium in a plane parallel geometry and the Eddington approximation, the effective temperature of the sun can be shown to occur at an optical depth of 2/3. This indicates the surface of a star is not at the top of the atmosphere where the optical depth is defined as zero: Stars are observed at a depth inside the atmosphere. Stars, excepting neutron stars, have no solid surface. Therefore, the photosphere is typically used to describe the Sun's or another star's visual surface.
Read more about this topic: Photosphere
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