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):

1. The green curve is the proton-proton (pp) chain reaction. For more information on the proton-proton (pp) chain reaction, see Star file: nuclear_burning_pp.html and Star file: nuclear_burning_ppi_chain.html.
2. The blue curve is the CNO cycle.
3. The red curve is the triple-alpha process.
4. 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.
5. 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.

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. For a plot of the solar composition, see file solar_composition.html.

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. Note, CNO cycle in stars requires beta decay processes (which are effected by the weak nuclear interaction) and these slow down the overall CNO cycle and allow stable hydrogen burning which is needed for main-sequence stars and the stable phases of post-main-sequence stars (wikipedia: CNO cycle: Cold CNO cycles).

   Note, the proton-proton chain is also slowed down by a beta decay process, the nuclear reaction

   p + p → D + ν_e + e⁺ + 0.42 MeV

   in which a beta+ decay (β+ decay) converts a proton (p) into a neutron (n) to make the deuteron (D,H-2). This β+ decay is 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 product 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.

   Note, carbon is all-important for life as we know it since that requires complex molecules and carbon is the only atom that permits complex molecules.