Caption: "This log-log plot shows the rate of nuclear energy generation (ε i.e., probably energy per unit mass per unit time Cl-378) for main-sequence stars as a function of temperature (T). The green curve is the proton-proton (pp) chain reaction, the blue curve is the CNO cycle, and the red curve is the triple-alpha process. The yellow vertical line represents the core temperature of the Sun, and it shows the pp chain reaction is the primary source of energy generation in the Sun. The dashed line joining the pp chain reaction curve to the CNO cycle is the net energy generation rate of the combined hydrogen nuclear burning rates. The overall slope of the curves shows the greater temperature sensitivity of the CNO cycle and triple-alpha process relative to the pp chain reaction." (Somewhat edited.)

Features:

1. The lack of numbers on the axes suggest the image author may be being somewhat qualitative.

2. Density and composition for the log-log plot? Probably, just main-sequence star central density and cosmic composition for which the solar composition is a fiducial composition. See also file solar_composition.html for a plot of the of the solar composition.

3. In the CNO cycle, carbon (C, Z=6), nitrogen (N, Z=7), and oxygen (O, Z=8) isotopes are used as catalysts in the nuclear burning process. These isotopes are consumed in various nuclear reactions, but recreated in others. There is NO net change in their abundances.

   The CNO cycle is believed to be the dominant hydrogen burning in main-sequence stars of ⪆ 1.3 M_☉ for which the stellar core temperature ⪆ 17*10**6 K which is a bit higher than the solar core temperature of the 15.7*10**6 K (Wikipedia: CNO cycle).

4. The rapid increase in hydrogen burning rate with stellar core temperature for the CNO cycle is an important factor in the rapid decrease in stellar lifetime with stellar mass.

5. The CNO cycle actually does rely on the weak nuclear interaction via beta decays (Wikipedia: CNO cycle: Cold CNO cycles), but the reliance is evidently much less important than that of the proton-proton chain via the

   p + p → D + ν_e + e⁺ + 0.42 MeV nuclear reaction,

   where a weak nuclear interaction must convert a proton (p) into a neutron (n) to make the deuteron (D,H-2) which the rate determining step of the proton-proton chain (Wikipedia: The proton-proton chain).

6. The triple-alpha process is NOT important for producing energy in main-sequence stars. However, it is in post-main-sequence stars and its ash is carbon (C, Z=6) and, by a side effect process, another product is oxygen (O, Z=8). Collectively the triple-alpha process and the side effect process can be called helium burning. Some of the cosmic composition of carbon and oxygen is produced in helium burning in post-main-sequence stars and some comes from supernova explosions.