Study GuideAstronomy–The Structure of Stars1. Energy GeneraƟon The CNO Cycle1.1How Stars Produce EnergyStars shine because they produce energy in their cores. This energy comes fromnuclear fusion, aprocess in which light elements combine to form heavier ones and release energy.The exact fusion process depends on themass of the star.1.2Energy ProducƟon in Low-Mass StarsInlow-mass stars like the Sun, energy is mainly produced through theproton–proton (pp) cycle.In this process, hydrogen nuclei fuse together to eventually form helium, releasing energy thatsupports the star against gravity.1.3Why High-Mass Stars Need a Different ProcessHigh-mass stars have amuch stronger gravitational pull.To balance this force, their cores reachmuch higher pressures and temperatures.At these higher temperatures, nuclear reactions other than the proton–proton cycle become moreefficient. As a result, a different energy-producing process takes over.1.4The Carbon–Nitrogen–Oxygen (CNO) CycleFor stars that aremore than about twice the mass of the Sun, the main source of energy is thecarbon–nitrogen–oxygen cycle, commonly called theCNO cycle.In this cycle:•Carbon, nitrogen, and oxygen nuclei act as catalysts•Hydrogen nuclei are fused into helium•Energy is released at each step•The carbon nucleus is regenerated at the end, allowing the cycle to repeatEven though carbon, nitrogen, and oxygen are involved,hydrogen is still the fuel, andhelium isstill the final product.Preview Mode
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