What phenomenon causes the edge (limb) of a star to appear dimmer than its center, affecting the shape of the transit light curve?

Answer

Limb darkening.

Limb darkening is the phenomenon where the edge of a star appears dimmer than its center because observers are looking through less of the star's atmosphere when viewing the edge tangentially.

What phenomenon causes the edge (limb) of a star to appear dimmer than its center, affecting the shape of the transit light curve?

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