Gravity darkening

Gravity darkening, also referred to as gravity brightening, is an astronomical phenomenon where the poles of a star are brighter than the equator, due to rapid rotation and oblate shape.

When a star is oblate, it has a larger radius at its equator than it does at its poles.

This means that equatorial regions of a star have a greater centrifugal force than the pole.

The centrifugal force pushes mass away from the axis of rotation, resulting in less overall pressure on the gas in the equatorial regions of the star.

Von Zeipel's theorem states that the radiation from a star is proportional to the local effective gravity, that is to the gravity reduced by any centrifugal force at that location on the star's surface.