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The core helium burning (CHeB) phase

At this stage the helium core contracts and therefore heats up. Eventually the temperature needed to ignite the helium is reached ( 108K) and the triple-alpha process begins. This causes the helium flash which occurs because the helium core is degenerate and the triple-alpha process is very sensitive to temperature. Instead of expanding as the temperature increases (as we expect for an ordinary gas), a degenerate gas conductsa the energy produced to other parts of the core and thus the onset of helium-burning in a small part of the core quickly spreads to throughout the whole core. This is the helium flash. Eventually the core temperature reaches ~350 X 106K causing the gas to become non-degenerate, expand and cool. The helium flash marks the end of the RGB phase. This is represented on the H-R diagram (Fig. 2.2) as point E.

During the core helium burning phase, helium is fused to produce carbon and oxygen in a convective core within a thin hydrogen burning shell. Helium burning includes the triple-alpha process, which is the mechanism by which 12C is produced from three 4He nuclei by the reaction:

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where the `*' indicates an excited state of the 12C nucleus. The third alpha must be added faster than 8Be can decay. Since the half-life of 8Be is ~7 X 10-16 seconds, it is clear that the equilibrium abundance of 8Be is very small. Helium burning can also lead to the alpha-rich nuclides 16O, 20Ne and 24Mg by tex2html_wrap_inline710 addition reactions starting from 12C. Furthermore, alpha-addition to the 14N seed (abundant following the CN cycle) can follow this chain:

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The end member, 22Ne, may be of importance in interpreting meteoritic data (see chapter 3). It may also be important in the s-process.

a In a degenerate gas, the electrons are distributed more or less uniformly throughout the medium, surrounding the nuclei. The nuclei themselves are regularly spaced and become more tightly constrained as the pressures increase, until they are so fixed with respect to each other that they resemble a crystal lattice. Under such conditions, the material more closely resembles a solid than a gas. The degenerate gas is, therefore, a very good energy conductor


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