limb darkening

In astrophysics, the gradual decrease in observed brightness of the disk of a star from its centre to its edge (limb). This phenomenon, readily apparent in photographs of the Sun, occurs because the solar atmosphere's temperature increases with depth. Looking at the Sun's centre, an observer sees the deepest, hottest layers, which emit the most light. At the limb, only the upper, cooler layers, which produce less light, are visible. Observations of solar limb darkening are used to determine the temperature structure of the Sun's atmosphere.

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