Limb darkening

Limb darkening is an effect where the middle part of a star (as well as the Sun) looks brighter than the edge, or limb.

Limb Darkening Media

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  A filtered image of the Sun in visible light, showing the limb-darkening effect as a dimmer luminosity towards the edge or limb of the solar disk. The image was taken during the 2012 transit of Venus (seen here as the dark spot at the upper right).

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  An idealized case of limb darkening. The outer boundary is the radius at which photons emitted from the star are no longer absorbed. L is a distance for which the optical depth is unity. High-temperature photons emitted at A will just barely escape from the star, as will the low-temperature photons emitted at B.

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  Limb darkening geometry. The star is centered at O  and has radius R . The observer is at point P  a distance r  from the center of the star, and is looking at point S  on the surface of the star. From the point of view of the observer, S  is at an angle θ from a line through the center of the star, and the edge or limb of the star is at angle Ω.