S- and r-process isotopes

The following stage is core collapse caused by electron capture or photodisintegration of iron. According to the traditional view, collapse leads to formation of a neutron star which cools by neutrino emission and decompression of matter when it reaches nuclear density (10 g cm ). The rebound that follows generates a shock wave which is capable of reigniting a good few nuclear reactions as it moves back out across the stellar envelope. 

Strengthened by absorption of high-energy neutrinos, the shock wave expels the outer layers. Seen from outside, the star appears to have exploded. On Earth, this fabulous event is greeted with the joyous shout of Supernova  

Globally, then, oxygen, neon and magnesium originate in hydrostatic shell combustion and the quantity synthesised and ejected increases with the mass of the progenitor star, whilst sulphur, argon, calcium and iron are essentially due to explosive nucleosynthesis and the ejected mass is much less variable from one star to another. 

Neutron capture may occur over a time-scale that is long enough to allow all possible P decays to take place, in which case we refer to the s process, or quite the opposite may happen, and we have the r process, as discussed earlier. The two processes lead to quite distinct abundance distributions (Fig. 5.5). 

In a continuous flow of neutrons, the abundance of each element is inversely proportional to the probability (cross-section) of neutron capture. Nuclei 

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