Nuclear fusion is the fundamental process that drives the entire life cycle of a star, from its formation to its eventual death.

Role of Nuclear Fusion in the Life Cycle of a Star

• Star Formation and Main Sequence Phase : Stars form from collapsing clouds of gas and dust, called nebulae. When the core temperature of a protostar reaches around 10 million kelvin, nuclear fusion ignites, primarily fusing hydrogen atoms into helium through processes like the proton-proton chain or the carbon-nitrogen-oxygen (CNO) cycle. This fusion releases enormous energy, creating radiation pressure that balances gravitational collapse, stabilizing the star in a phase called the main sequence. The star remains in this stable state for most of its life, with the duration depending on its mass-massive stars burn fuel quickly and have shorter lives, while smaller stars burn fuel slowly and live much longer

• Post-Main Sequence Evolution : As hydrogen in the core is depleted, fusion slows, causing the core to contract and heat up. This triggers fusion in shells around the core and eventually leads to helium fusion in the core for stars with sufficient mass. The star expands into a red giant or supergiant phase during this process. Heavier stars can fuse progressively heavier elements in concentric shells around the core

• End of Fusion and Star Death : Once nuclear fusion can no longer sustain the star-because iron fusion requires energy instead of releasing it-the core collapses. For low to medium-mass stars like the Sun, this results in the ejection of outer layers as a planetary nebula and the formation of a dense white dwarf core. For high-mass stars, the core collapse triggers a supernova explosion, leaving behind a neutron star or black hole. The material expelled enriches the interstellar medium, contributing to the formation of new stars

Summary

Nuclear fusion:

• Powers the star by converting hydrogen to helium, providing the energy that balances gravity.
• Determines the star’s stability and lifespan during the main sequence.
• Drives the star’s expansion and evolution into giant phases as fusion shifts to heavier elements.
• Ultimately controls the star’s death, whether as a white dwarf or through a supernova, depending on the star’s mass.

Thus, nuclear fusion is the engine of a star’s life cycle, governing its birth, evolution, and death