Neon burning

In the heavier stars, a stage in which 20Ne is destroyed occurs subsequently to the carbon burning, but before the absorption of oxygen. The a-particles formed are used up by the nuclei already present (also from neon itself) in so-called neon burning. 

These reactions take place in the inner zone of stars heavier than 15 solar masses. Hydrostatic carbon burning is followed by explosive neon burning at temperatures of around 2.5 x 109K. Under these conditions, phosphorus (31P) can be formed, although complex side reactions also occur. In comparison with the formation of... 

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

Because phosphorus has buta single stable isotope it sheds no light on isotopic anomalies within presolar material. P nucleosynthesis derives almost entirely from a single main-line source, neon burning, which occurs at the end of carbon burning in massive stars that will become Type II supernovae. 

Neon burning is induced by photodisintegration of e into an a-particle and The a-particle is then captured by another e nucleus to make Mg or by Mg to make Si. Secondary products include Al, and S. 

Neutron capture reactions are common in massive stars because of the abundance of neutrons available there. Lookback at the equations for carbon-, oxygen-, and neon-burning (page 70). Notice that alpha particles are common products of such reactions. Astrochemists have determined that the three most common sources of neutrons in such reactions are the following reactions ... 

At the end of the neon burning the core is left with a mixture of alpha particle nuclei ieO and 24Mg. After this another core contraction phase ensues and the core heats up, until at Tg 2, ieO begins to react with itself ... 

As the shock wave passes through the outer layers of the star, it heats and compresses them. This causes further nuclear reactions. For example, when the shock passes through the silicon and sulfur-rich zones of the star, it heats them and causes explosive silicon burning. Oxygen-rich layers experience explosive oxygen burning, neon-rich layers experience explosive neon burning, etc. The ejecta from a massive star then enrich the interstellar medium with new elements. 

This process occurs in heavy stars after neon-burning in the core. But it does not give the answer to the abundance of phosphorus on Earth or the other planets. Although oxygen burning does produce phosphorus in heavy stellar cores, further nucleosynthesis uses up almost all of it to produce, in the end, a star composed of 90 percent silicon and sulfur. So another mechanism must be responsible for phosphorus production and distribution throughout the universe. 

Supernova explosions seem to be the answer. The shock of the explosion allows even outer shells of the star s atmosphere to heat up to temperatures in the range of 2 billion K—hot enough to allow neon to fuse. This explosive neon burning and its subsequent reactions result in the production of P in the star s atmosphere. The matter created in this way is ejected with enough force to overcome the star s diminished gravitational attraction, and the elements are free to sail into the universe, to be collected and incorporated in newly forming stars and planets. 

In stars of mass greater than about eight times the mass of the Sun, neon-burning temperatures can be reached, and the neon produced can interact with photons (energy) to form oxygen via the reverse reaction of that shown above... 

The next stage is neon burning starting at 10 K, in which photons first disintegrate Ne and liberate He, which in turn reacts with the undissociated Ne to build up Mg and further nuclei ... 

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