Helium-burning

Eventually, the hydrogen fuel of the star will be exhausted and further gravitational collapse will occur. This will give rise to a temperature increase up to 1—2 x 108 K (with a density of 108 kg/m3). In this red giant, helium burning will commence. 

Three-body reactions are rare, but the reaction proceeds through a resonance in 12C at 7.65 MeV corresponding to the second excited state of 12C (Jtt = 0+). This excited state has a more favorable configuration than the 12C ground state for allowing the collision to occur. (In a triumph for nuclear astrophysics, the existence of this state was postulated by astrophysicists to explain nucleosynthetic rates before it was found in the laboratory.) 

After a significant amount of 12C is formed, one gets the a-capture reactions  

A brief interlude of neon burning then occurs with reactions such as  

Applying the homology version of the hydrostatic equation (5.8) to the envelope, one obtains another condition for the pressure at the edge of the core  

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When hydrogen is burned up in the nuclear furnace of a star, helium burning takes over, forming carbon, which in turn leads to oxygen, etc. Subsequent emission processes releasing a-particles, equilibrium processes, neutron absorption, proton capture, etc. lead to heavier elements. 

When 10% of the hydrogen in the core has been consumed gravitational contraction again occurs until at a temperature of 2 X 10 K helium burning (fusion) can occur. This is followed by a similar depletion, contraction and temperature rise until nuclear reactions involving... 

S2-4 Helium burning as additional process for nucleogenesis 19S4 Slow neutron absorption added to stellar reactions 195S-7 Comprehensive theory of stellar synthesis of all elements in observed cosmic abundances 196S 2.7 K radiation detected... 

The main nuclear reactions occurring in helium burning are ... 

Further helium-burning reactions can now follow during which even heavier nuclei are synthesized ... 

In a sense the o-process resembles helium burning but is distinguished from it by the quite... 

The origin of chemical elements has been explained by various nuclear synthesis routes, such as hydrogen or helium burning, and a-, e-, s-, r-, p- and x-processes. "Tc is believed to be synthesized by the s (slow)-process in stars. This process involves successive neutron capture and / decay at relatively low neutron densities neutron capture rates in this process are slow as compared to /1-decay rates. The nuclides near the -stability line are formed from the iron group to bismuth. 

In each case, the pressure is mainly fixed by the density and quite insensitive to temperature. A consequence of this is that, when a new nuclear reaction (e.g. helium-burning) sets in in degenerate material, the additional heat cannot be taken up (as it would be in an ideal gas) by expansion and there is a thermal runaway (Mestel 1952). The temperature rises very strongly until it becomes high enough for the degeneracy to be removed, and the star adjusts to a new structure with lower central density. 

Helium-burning on CNO material previously converted into 14N by H-burning leads to the series of reactions... 

Hoyle successfully predicts existence of a 7.6 MeV resonance state of the carbon-12 nucleus on grounds that otherwise little carbon would survive further processing into oxygen during stellar nucleosynthesis by helium burning, whereas in fact the C/O ratio is about 0.5. Discovery of strange particles. 

The neutron bursts take place in helium burning shells surrounding the inert carbon-oxygen core. The neutrons released here are grafted onto iron and its kin. 

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