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Why do red dwarf stars rely mainly on the proton-proton cycle?

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laser-activated fusion — 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.

Red dwarf stars rely primarily on the proton-proton cycle because of their relatively low core temperatures.

In stellar physics, the proton-proton chain is the dominant source of energy for cooler main-sequence stars, such as the Sun and red dwarfs, where core temperatures remain below approximately 15,000,000 K. Red dwarfs are the coolest type of main-sequence star, with surface temperatures ranging from about 2,000 to 3,500 K.123

Alternative fusion processes, such as the CNO cycle, require significantly higher temperatures to overcome the electrostatic repulsion between protons and heavier nuclei like carbon, nitrogen, and oxygen. Because red dwarfs lack the mass to generate the extreme core pressures and temperatures necessary to trigger these more complex cycles, they rely on the proton-proton chain to fuse hydrogen into helium.123