Nuclear Fusion and Stars — AQA GCSE Physics
Stars produce their energy by nuclear fusion, joining light nuclei into heavier ones.
Nuclear fusion in stars
Nuclear fusion is the joining of two light nuclei to form a heavier nucleus, releasing energy. Stars, including the Sun, release energy by fusing hydrogen nuclei into helium.
Fusion requires extremely high temperatures and pressures to overcome the repulsion between the positively charged nuclei. These conditions exist in the cores of stars.
How a star forms
- A star forms from a cloud of dust and gas (a nebula), pulled together by gravity.
- As the cloud collapses, it becomes hotter and denser, forming a protostar.
- When it is hot and dense enough, hydrogen fusion begins, and a main sequence star is formed.
A stable star
During its long main sequence phase, a star is stable because two forces are balanced:
- the inward force of gravity, and
- the outward pressure caused by the energy released by fusion.
Fusion in stars also creates heavier elements (up to iron); the heaviest elements form in supernova explosions.
Exam tips
- Stars release energy by fusing hydrogen into helium.
- Fusion needs very high temperature and pressure.
- A star forms from a nebula pulled together by gravity → protostar → main sequence.
- A stable star balances gravity (inward) with fusion pressure (outward).