Shell helium flashes are a somewhat analogous but much less violent, nonrunaway helium ignition event, taking place in the absence of degenerate matter. They occur periodically in asymptotic giant branch stars in a shell outside the core. This is late in the life of a star in its giant phase. The star has burnt most of the helium available in the core, which is now composed of carbon and oxygen. Helium fusion continues in a thin shell around this core, but then turns off as helium becomes depleted. This allows hydrogen fusion to start in a layer above the helium layer. After enough additional helium accumulates, helium fusion is reignited, leading to a thermal pulse which eventually causes the star to expand and brighten temporarily (the pulse in luminosity is delayed because it takes a number of years for the energy from restarted helium fusion to reach the surface). Such pulses may last a few hundred years, and are thought to occur periodically every 10,000 to 100,000 years. After the flash, helium fusion continues at an exponentially decaying rate for about 40% of the cycle as the helium shell is consumed. Thermal pulses may cause a star to shed circumstellar shells of gas and dust.