8.1.3Space Physics
The Life Cycle of Solar Mass Stars
5 min read 4 parts Lesson 3 of 10
Section 1 of 4Birth to the Main Sequence
Stars form from clouds of gas and dust, and they settle into a stable phase once fusion begins in the core.
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A cloud of dust and gas, called a nebula, begins to collapse under its own gravity.
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As the cloud contracts, its core heats up and the object becomes a protostar.
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When the core becomes hot enough, hydrogen nuclei fuse to form helium, and the star joins the main sequence.
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Fusion releases energy, and the outward pressure from radiation balances the inward pull of gravity for billions of years.
Section 2 of 4Death of a Solar Mass Star
When the hydrogen in the core runs out, the star changes dramatically in a series of stages.
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When the hydrogen in the core runs out, fusion stops in the core and the core starts to contract.
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The outer layers expand and cool, and the star becomes a red giant.
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The red giant sheds its outer layers, which form a shell of gas called a planetary nebula, leaving a hot core behind.
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The remaining core cools into a white dwarf, which gradually fades over a very long period of time.
Section 3 of 4Stages and Their Features
Each stage of the life cycle has a distinctive size, temperature and source of energy.
Stages and Their Features
Main sequence
- Main sequence A stable star that is balanced between gravity and pressure from hydrogen fusion in its core.
Red giant
- Red giant A very large, cool star that fuses hydrogen in a shell around its core once the core hydrogen has been used up.
White dwarf
- White dwarf A small, very dense and hot remnant that no longer undergoes fusion, so it slowly cools down.
| Stage | Key feature | Energy source |
|---|---|---|
| Main sequence | Stable size and balanced forces | Hydrogen fusion in the core |
| Red giant | Very large, cool surface | Fusion in a shell around the core |
| White dwarf | Small, very dense and hot | No fusion, so it cools over time |
Section 4 of 4Exam Checklist & Tips
8.1.3 – Key Points for the Exam
- Nebula A cloud of dust and gas in space from which stars form.
- Protostar A young star that is still forming and has not yet started fusion in its core.
- Main sequence The stable stage in which a star fuses hydrogen into helium in its core.
- Red giant A large, cool star formed when a star runs out of hydrogen in its core.
- White dwarf The small, dense remnant left behind when a solar mass star has lost its outer layers.
Common Mistakes
- Saying the Sun will become a black hole, when a star of solar mass ends as a white dwarf.
- Describing a red giant as a young star that has just formed, when it is a late stage in the life cycle.
- Confusing a white dwarf with a supernova, which happens only to much more massive stars.
- Stating that fusion stops immediately after the star joins the main sequence, when it continues for billions of years.