The hydrogen-burning stage will continue until the hydrogen in the centre of the star is nearly depleted. When this happens, the stability of the star is lost and it continues on a new evolutionary path. The stage of hydrogen burning in the core is projected to last about 1010 years for the Sun, implying that it is about halfway through this stage.
The diminishing number of protons in the centre of the star causes this region to contract slowly, and consequently, the temperature rises. The hydrogen burning is still taking place in a thick shell around the centre, and as a result of the rising temperature, the reaction rate increases and more energy is produced. In order to accommodate the increased flow of energy, the outer shell of the star has to expand. The radius may become some 50 times larger, and the surface temperature drops by about 1500 K, which causes the light emitted to change its spectrum in the direction of longer wavelengths. For this reason these stars are recognised as "red giants". Their luminosity is typically 1000 times that of the Sun (Iben, 1970).
As the temperature of the hydrogen-burning shell increases, the chain of hydrogen to helium sub-processes that dominates in stars like the Sun (the pp-chain described below) receives competition from another chain of reactions (the CNO-chain), which also results in a net hydrogen to helium conversion. Eventually, this process takes over. At this time, the conditions in the centre will allow the production of neutrino-antineutrino pairs, which escape from the star, carrying away large amounts of energy. The energy loss from the central region is compensated by a flow of matter inwards, mostly in the form of high-energy electrons. The central temperature rise is slowed down by the neutrino radiation, and it takes some 3 x 109 years (from the departure from the "main sequence" stage of hydrogen only burning in the core) until the temperature is sufficiently large to allow the burning of helium.
The burning of helium is achieved by the reaction 3 42He ^ 126C + y + 7.28 MeV.
The gamma-emission is associated with a definite decay process from an excited level of 12C to its ground state, releasing 7.66 MeV of energy. Thus, the kinetic energy of the helium nuclei should furnish 0.38 MeV, which implies that the transition rate becomes significant only at temperatures above 108 K. Some of the carbon nuclei react with another helium nucleus to form oxygen,
Since the temperature dependence of the helium-burning process is very strong just above 108 K, the star experiences a "helium flash" with a maximum temperature around 3 x 108 K. However, a new equilibrium is reached in which the helium burns stably in the centre at a temperature of 108 K, while hydrogen burns in a shell around the core, each contributing about half the energy production. This period may last of the order 108 years, and it is followed by alternating periods of instability and stability, as helium is depleted in the core and a helium-burning shell is formed.
Repeated contraction and temperature increase in the centre can ignite carbon- and oxygen-burning processes (109 K), resulting predominantly in the creation of 2412Mg and 2814Si. At a temperature around 3 x 109 K, gamma-rays cause the magnesium and silicon nuclei to evaporate a significant number of helium nuclei, which may be captured by other Mg, Si or heavier nuclei to successively create a number of nuclei in the mass range A = 30-60.
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