A helium runaway shell is the fast fusion of helium (triple alpha process) in a shell within the core of an AGB star. It is a type of thermal pulse, a relatively brief period when the star's temperature is significantly higher. AGB stars periodically burn hydrogen in a shell surrounding the core, and then burn helium when enough is present to ignite. The resulting helium flash can last as long as 400 years, and are spaced at 10,000 to 100,000 year intervals. These flashes can cause the star to emit gas and dust in a superwind.