The helium flash is the onset of runaway helium burning in the core of a low-mass star (such as the Sun). The helium flash happens in the hydrogen-exhausted core of a star that has become a red giant. When gravitational pressure has raised the temperature of the dormant helium core to a temperature of about 100 million K, the helium nuclei start to undergo thermonuclear reactions. Once the helium burning has started, the temperature climbs rapidly (without a cooling, stabilizing expansion), and the extreme sensitivity of the nuclear reaction rate to temperature causes the helium-burning process to accelerate. This in turn raises the temperature, which further accelerates the helium burning, until a point is reached at which the thermal pressure expands the core and thereby limits the flash. The helium flash can only occur when the helium core is less than the 1.4-solar-mass Chandrasekhar limit and thus it is restricted to fairly low-mass stars.