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How does the CNO cycle produce energy in hotter stars?
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nuclear fusion
Interior of the U.S. Department of Energy's National Ignition Facility (NIF), located at Lawrence Livermore National Laboratory, Livermore, California. The NIF target chamber uses a high-energy laser to heat fusion fuel to temperatures sufficient for thermonuclear ignition. The facility is used for basic science, fusion energy research, and nuclear weapons testing.
The CNO cycle (carbon-nitrogen-oxygen cycle) is a sequence of thermonuclear reactions that serves as the primary energy source for hotter stars on the main sequence. In this process, carbon acts as a catalyst to facilitate the conversion of four hydrogen nuclei into one helium nucleus.123
The cycle proceeds through a repeating loop of proton captures and beta decays:
- A carbon-12 (C) nucleus captures a proton to form nitrogen-13 (N), which then decays into carbon-13 (C).12
- The C nucleus captures another proton to become nitrogen-14 (N).12
- The N nucleus captures a proton to form oxygen-15 (O), which decays into nitrogen-15 (N).12
- Finally, the N nucleus captures a proton and breaks down, releasing a helium nucleus (alpha particle) and regenerating the original C nucleus.12
A fraction of the mass is released as energy according to the mass-energy equivalence formula, E = mc2. Because this process requires higher temperatures to overcome the electrostatic repulsion of the involved nuclei, it is the dominant energy source in stars significantly hotter or more massive than the Sun.123