These grains have a series of points protruding inward, as shown in the sketch, such that as the points burn off, they keep the area roughly constant. In a first approximation one can see that the periphery of the "star" should be equal to the outer(circular) periphery of the grain, so
that the burning area is equal at beginning and end. Detailed geometric constructions have been developed that keep the area very nearly constant throughout the burn.