Rick's Cosmology Tutorial: Chapter 13 Abstract

Stellar Nuclear Processes - The pp Reaction Sequences

In this Chapter we take a detailed look at the nuclear reactions powering stars. Attention is confined in this Chapter to the hydrogen burning phase (main sequence stars). Below 18 million K the dominant reaction pathways are the so-called pp sequences. The pp sequences are the relevant reactions for solar mass stars (central temperature 13.7 miilion K). The reactions comprising the pp sequences are described.

The dominant pp sequence (ppI) in examined in some detail. The rates of each contributing reaction and the overall rate of helium-4 production is determined (for solar temperature). An exhaustive consideration of all other potential light nuclei reactions elucidates why the particular reactions of the ppI sequence are the relevant ones.

The concentrations of intermediate nuclei, such as deuterium, helium-3, tritium, neutrons and lithium-7 are determined (again, for a solar mass star). This shows that the product of the reaction sequences is essentially "just helium-4".

In this Chapter we take the basic reaction rates from the literature. However, in Chapter 14 we shall derive the rate of the first reaction in the pp sequences from quantum mechanical first principles. The same is done for the second reaction in Appendix A3. Consequently, since the first reaction is the overall rate determining step, we will be able to trace the power density of stars back to the universal constants.

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Messier 66 (NGC3627): An intermediate spiral galaxy in Leo, it is about 95 thousand light-years across.