Nuclear fusion is the process where two light atomic nuclei combine (fuse) to form a single, heavier nucleus. Because the total mass of the resulting nucleus is slightly less than the combined mass of the original nuclei, the missing mass (known as the mass defect, Δm) is converted directly into energy via Einstein's mass-energy equivalence equation:
E = Δm c2
How Fusion Generates Energy in Stars
Stars, including our Sun, are powered by nuclear fusion, converting massive amounts of hydrogen into helium in their dense, high-temperature cores.
For fusion to occur, positively charged hydrogen nuclei (protons) must overcome their mutual electrostatic repulsion (the Coulomb barrier). This requires extreme environmental conditions:
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High Temperatures: Provides nuclei with sufficient thermal kinetic energy to approach each other at high velocities and breach the repulsive barrier.
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High Pressures & Densities: Increases the frequency of nuclear collisions per unit volume.
When two protons approach within approximately 1 femtometer (10-15 m) of each other, the attractive strong nuclear force overcomes electrostatic repulsion, binding the particles together and releasing substantial energy.
The video below details how main-sequence stars power themselves by fusing hydrogen into helium through nuclear reaction pathways such as the proton-proton (p-p) chain.