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Helium runaway is helium fusion that increases itself through the heat it generates, a type of thermal runaway. Such a runaway can occur in some phases of stellar evolution for some stars. Helium runaways necessarily end soon, due to depletion of all the available helium or through other changes in the surrounding conditions. They also fall under the term thermal pulse, a "hot" episode, and is the mechanism of helium flashes (and helium runaway is more-or-less a synonym for the term helium flash), types of which occur in AGB stars, RGB stars, and possibly power some novae.