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definition  The darkening, or brightening, of a region on a star due to localized decrease, or increase, in the effective gravity. Gravity darkening is explained by the von Zeipel theorem, whereby on stellar surface the radiative flux is proportional to the effective gravity. This means that in rotating stars regions close to the pole are brighter (and have higher temperature) than regions close to the equator. Gravity darkening occurs also in corotating binary systems, where the tidal force leads to both gravity darkening and gravity brightening. The effects are often seen in binary star light curves. Recent theoretical work has shown that gravity darkening is not well represented by the von Zeipel theorem. This is supported by new interferometric observations of some rapidly rotating stars indicating that the von Zeipel theorem seems to overestimate the temperature difference between the poles and equator.  
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contributor  AAS_Frey.Katie original 