pp Chain vs CNO Cycle

What you are seeing: the two main hydrogen-burning networks in stellar cores, with their energy-generation rate compared across temperature. The pp chain (left) goes as $\varepsilon \propto T^4$ ; the CNO cycle (right) goes as $\varepsilon \propto T^{17}$ . The cross-over is near $1.8 \times 10^7$ K. The Sun (T_core = $1.55 \times 10^7$ K) is about 99% pp; an O-star is essentially 100% CNO

Figure 1. pp-chain (left) and CNO-cycle (right) reaction networks; live pp/CNO contribution as a function of core T; Q = 26.73 MeV per net synthesis. Method: epsilon_pp ~ T^4, epsilon_CNO ~ T^17, normalized so the Sun is 99% pp.

core T (10^7 K)1.55

presetSun

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WHAT TO TRY

Raise the core temperature: below about 18 million K the proton-proton chain dominates, above it the steeply temperature-sensitive CNO cycle takes over. The fraction readouts cross there.
Step through the stellar presets from M dwarf to O star: hotter, more massive cores run almost entirely on CNO, which is why massive stars burn so fiercely.
Compare the two networks: the pp chain fuses hydrogen directly, the CNO cycle uses carbon as a catalyst, but both net four protons into one helium-4 plus energy.