8.1.2Space Physics
Main Sequence Stars — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions
5 min read 6 parts Lesson 3 of 10
Part 1 of 4Starting the main sequence
1
A protostar forms when gravity pulls a cloud of dust and gas together, and the gas heats up as it is squeezed.
2
Once the core becomes hot and dense enough, nuclear fusion begins there, because the core has the highest temperature and density in the star.
3
Hydrogen nuclei fuse together to form helium nuclei.
4
Each fusion reaction releases energy as heat and light, which keeps the core hot so that fusion can carry on.
5
The star has now joined the main sequence, the longest stable stage of its life. The Sun is in this stage today.
Part 2 of 4Two opposing effects
| Effect | Direction | Where it comes from |
|---|---|---|
| Gravity | Inwards, towards the centre | The attraction between all the mass in the star pulls the outer layers towards the core. |
| Radiation and gas pressure | Outwards, away from the centre | The very hot gas produced by fusion tries to expand and pushes outwards. |
Part 3 of 4Equilibrium in a stable star
Stable star
Balanced forces
- Balanced forces During the main sequence the inward force of gravity equals the outward force from the hot gases, so the forces are balanced.
Constant size
- Constant size Because the forces balance, the star neither collapses nor expands.
- Its size stays about the same for millions or billions of years.
Constant output
- Constant output Fusion in the core keeps replacing the energy that leaves the star, so the star stays stable.
Part 4 of 4Mass and the core temperature
Bigger star, hotter core
- Bigger star, hotter core The greater the mass of the star, the stronger gravity squeezes the core, so the core temperature is higher.
Heavier nuclei
- Heavier nuclei At higher temperatures, further fusion reactions can occur that make heavier nuclei than helium.
- A star of low mass such as the Sun fuses mainly hydrogen during this stage.
Key terms
| Term | Meaning |
|---|---|
| Protostar | A very young star that is still forming and has not yet started fusing hydrogen. |
| Nuclear fusion | A reaction in which light nuclei join to form a heavier nucleus, releasing energy. |
| Main sequence | The long, stable part of a star's life in which it fuses hydrogen into helium. |
| Equilibrium | A state in which opposing forces are balanced, so there is no change in size. |
| Stable star | A star in which the inward gravitational force equals the outward force from hot gas. |
Avoid these mistakes
1
Describing the Sun as burning hydrogen like a fire. Stars release energy by nuclear fusion, not by chemical combustion.
2
Saying that gravity stops once the star is stable. Gravity still acts all the time; it is balanced by the outward force.
3
Writing that the forces are equal so nothing is happening. Fusion is going on constantly in the core, and it is what supplies the outward force.
4
Mixing up the directions. Gravity acts inwards; the pressure from the hot, expanding gases acts outwards.
5
Stating that fusion makes helium into hydrogen. Hydrogen nuclei fuse to make helium.
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