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Figure 14-13b shows the dependence of temperature for the p-p reaction and carbon cycle (CNO). The p-p reaction is the dominant process in lower temperature (in million degree K) for most of the main sequence stars. Whereas at higher temperature, the CNO process is important for massive stars with mass greater than 1.5 Msun and in the later stage of all stars. Careful examination of Figure 14-13a reveals that two protons in the p-p chain and one carbon in the CNO cycle act like a catalyze to produce a nucleus of He-4. The time scale in Figure 14-13a is the "mean reaction time per particle", which is proportional to T2/3/ (and other constants) where T is the temperature and is the density of the proton (and He-3 in one of the steps). The entities with no label in the diagrams of Figure 14-13a are the compound nuclei, which exist for a very short time of about 10-16 sec.
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