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
Famous quotes containing the words effective and/or temperature:
“... effective magic is transcendent nature ...”
—George Eliot [Mary Ann (or Marian)
“This pond never breaks up so soon as the others in this neighborhood, on account both of its greater depth and its having no stream passing through it to melt or wear away the ice.... It indicates better than any water hereabouts the absolute progress of the season, being least affected by transient changes of temperature. A severe cold of a few days’ duration in March may very much retard the opening of the former ponds, while the temperature of Walden increases almost uninterruptedly.”
—Henry David Thoreau (1817–1862)