Fusion is the energy source in stars because light nuclei combine to form more tightly bound nuclei, converting a small amount of mass into a large amount of energy. This applies to both SL and HL in IB Physics topic E.5, Fusion and stars.
How Fusion Releases Energy
In nuclear fusion, light nuclei combine to form a heavier nucleus. The products have a greater binding energy per nucleon, meaning their nucleons are more tightly bound and the final nucleus has less mass than the original nuclei.
This difference is called the mass defect. It is released as energy according to Einstein's mass-energy relation:
Because , even a small mass defect produces enormous energy. For example, if approximately 0.7% of 1 kg of hydrogen is converted into energy during fusion, then