Limb darkening is an effect observed in the spectra and brightness of celestial bodies, particularly stars and planets. It refers to the phenomenon where the edges (or "limbs") of a star or planet appear darker than the center when viewed in terms of brightness or intensity. This effect arises due to the temperature gradient in the outer layers of a star. The center of the star is generally hotter and emits more light than the outer edges or "limbs.
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Limb darkening is the decrease of emergent specific intensity toward the apparent edge of a stellar disc. A transit depth then depends on the intensity sampled by the planet as well as its projected area; the uniform-disc area ratio is only an approximation.