Red Giants, White Dwarfs and Planetary Nebulae

This lesson covers the later stages in the life of a Sun-like star — as required by the Edexcel GCSE Physics specification (1PH0), Topic 7: Astronomy. This is a Paper 2 topic. You need to understand what happens when a medium-mass star runs out of hydrogen fuel, and the stages from red giant to white dwarf.

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When the Hydrogen Runs Out

A main sequence star like the Sun fuses hydrogen into helium in its core for billions of years. Eventually, the hydrogen fuel in the core is exhausted. When this happens:

1. Nuclear fusion in the core slows and eventually stops (there is no more hydrogen to fuse in the core).
2. Radiation pressure decreases — the outward push from fusion weakens.
3. Gravity dominates — the core begins to contract (collapse inward) under its own weight.
4. As the core contracts, it heats up (gravitational potential energy is converted to thermal energy).
5. The increased temperature heats the surrounding shell of hydrogen, which begins to fuse — this is called shell burning.

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The Red Giant Stage