# Nuclear fusion

Two small nuclei, such as hydrogen, join to make a larger nucleus and release energy. It needs extremely high temperature and pressure, as inside stars.

Physics (school) · Part 08: Atoms and radioactivity · Card 8 of 8 · Last updated 2026-10-19

## Analogy

The opposite of fission: joining instead of splitting.

## In practice

The Sun makes its energy by fusing hydrogen into helium. Scientists are trying to build fusion power stations.

## Practice

### Flashcards

- **Where does the Sun get its energy?** Nuclear fusion of hydrogen into helium.

## Questions people ask

### Why is fusion so difficult on Earth?

Nuclei are positive, so they repel each other strongly. To make them collide and join, they must be heated to millions of degrees and squeezed together. No material can hold gas that hot, so experimental reactors use powerful magnetic fields to keep it away from the walls.

### Why would fusion be better than fission?

Fusion fuel is hydrogen, which is plentiful, and its waste is far less radioactive and shorter lived. There is also no runaway chain reaction. The problem is that no one yet makes more electricity from fusion than the energy needed to heat the fuel, so it is still experimental.

## Related concepts

- [Nuclear fission](https://studyvisually.com/learn/science/physics-school/physics-school-part-8/physics-school-part-8-concept-7)
- [Structure of the atom](https://studyvisually.com/learn/science/physics-school/physics-school-part-8/physics-school-part-8-concept-1)
- [Atomic number and mass number](https://studyvisually.com/learn/science/physics-school/physics-school-part-8/physics-school-part-8-concept-2)
- [Energy stores and transfers](https://studyvisually.com/learn/science/physics-school/physics-school-part-3/physics-school-part-3-concept-1)

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