helium shell flash

The helium shell flash is one of a series of helium burning episodes in the thin helium shell that surrounds the dormant carbon core of an asymptotic giant branch star; the helium burning shell does not generate energy at a constant rate but instead produces energy primarily in short flashes. During a flash, the region just outside the helium-burning shell becomes unstable to convection; the resultant mixing probably leads to the s-process as well as to the upward movement of carbon produced by helium burning. The overheating from a flash also causes an expansion of the star's upper layers, followed by a collapse, leading to large-scale pulsations.