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physical phenomena > energy source > exothermic fusion process > helium burning

helium burning
subject			fact

helium burning	has duration 5 × 105 years
	has optimum density 700 g/cm3
	requires minimum mass at star birth 0.5 solar masses
	has optimum temperature 2 × 108 K
	has part product carbon
	has next higher temperature reaction carbon burning
	has product carbon, oxygen
	has reactant helium
	has definition The stage when a star fuses helium into carbon and oxygen. All stars born with more than half a solar mass eventually burn helium.
	is a kind of exothermic fusion process
exothermic fusion process	liberates energy
fusion	has antonym fission
	produces energy
	has reaction probability
	has catalyst
	has minimum temperature
physical process	has domain physics

Kinds of helium burning :

• helium flash (3 facts) - The onset of runaway helium burning under degenerate conditions. The helium flash occurs in the hydrogen-exhausted core of a star in the red-giant phase of evolution. When gravitational pressure has brought the degenerate core to a temperature of about 10⁸ K, the helium nuclei can start to undergo thermonuclear reactions. Once the helium burning has started, the temperature builds up 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 where the thermal pressure expands the core and thus removes the degeneracy and limits the flash. The helium flash can only occur when the helium core is less than the 1.4 M_sun Chandrasekhar mass limit and thus it is restricted to low-mass stars.
• helium shell flash (2 facts) - It has been shown that helium shell burning outside a degenerate core is unstable; the helium-burning shell does not generate energy at a constant rate but instead produces energy primarily during 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.

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