Which process dominates hydrogen fusion in cores exceeding 1.3 times the Sun's mass?

Answer

CNO cycle

For stellar cores that are significantly more massive than the Sun, specifically those exceeding approximately 1.3 solar masses, the extreme temperatures and pressures favor a different pathway for converting hydrogen into helium. This process is formally known as the CNO cycle, short for Carbon-Nitrogen-Oxygen cycle. Unlike the direct proton-proton chain, the CNO cycle utilizes trace amounts of carbon, nitrogen, and oxygen nuclei already present in the star's material to act as catalysts. These catalyst nuclei are consumed and regenerated throughout the fusion sequence, facilitating the merger of hydrogen into helium. A key characteristic of the CNO cycle is its extreme sensitivity to temperature; even small rises in core heat result in disproportionately massive increases in the rate of energy production, leading to much faster fuel consumption compared to lower-mass stars.

Which process dominates hydrogen fusion in cores exceeding 1.3 times the Sun's mass?

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