Life Cycle of a StarOxford AQA IGCSE Physics: Revision notes
Section 1
How do stars form?
Stars form when enough dust and gas — mainly hydrogen and helium — is pulled together by gravitational attraction.
- As the cloud of dust and gas contracts, it heats up
- If there is enough mass, the core becomes hot and dense enough for nuclear fusion to begin
- Smaller amounts of leftover material orbiting the new star may form planets
Section 2
What happens during the main sequence?
The main sequence is the stable, long-lasting period of a star's life.
- Energy is released by the fusion of hydrogen nuclei into helium
- The core is where temperature and density are greatest, so it is where most fusion takes place
- The outward force from fusion energy (radiation pressure) balances the inward pull of gravity, keeping the star a stable size
The more massive a star is, the hotter its core, and the heavier the nuclei it is able to create by fusion.
When explaining stability during the main sequence, always mention that gravitational force (inward) is balanced by the outward force from fusion — this pairing is what examiners look for.
Section 3
How does a star's mass determine its life cycle?
A star's entire life cycle — how it lives and how it dies — is determined by its mass. There are two contrasting pathways:
| Stage | Sun-like star | Star much bigger than the Sun |
|---|---|---|
| 1 | Main sequence | Main sequence |
| 2 | Red giant | Red supergiant (fuses helium to carbon, then heavier nuclei) |
| 3 | White dwarf | Supernova |
| 4 | Black dwarf | Neutron star or black hole |
Section 4
What is the life cycle of a star like the Sun?
- Main sequence — hydrogen fuses to helium in the core, stable for billions of years
- Red giant — the star expands and cools as it runs out of hydrogen in the core
- White dwarf — the outer layers are shed, leaving a hot, dense core
- Black dwarf — the white dwarf cools down over a very long time and stops emitting light
Section 5
What is the life cycle of a star much bigger than the Sun?
- Main sequence — hydrogen fuses to helium, but much faster than in a Sun-like star
- Red supergiant — the star expands massively; helium fuses to carbon, and then progressively heavier elements are fused
- Supernova — a huge explosion as the star collapses and then violently expels most of its material
- Neutron star or black hole — depending on the remaining mass, the collapsed core becomes either an extremely dense neutron star or a black hole
A common error is saying all stars end as black holes. Only the most massive stellar remnants become black holes; less massive remnants become neutron stars, and Sun-like stars end as white/black dwarfs.
Section 6
Where do the elements in the universe come from?
Fusion processes in stars produce all naturally occurring elements.
- Elements up to iron are formed by fusion during a star's normal life cycle (main sequence and giant/supergiant phases)
- Elements heavier than iron are only formed during a supernova explosion, where the extreme conditions allow further fusion
- Supernova explosions then distribute these elements throughout the universe, seeding future stars and planets (including the elements that make up the Earth and living things)
The iron in your blood and the gold in jewellery were both forged inside stars — iron by ordinary fusion, gold by a supernova.
Must Know
- Stars form from gravitational attraction pulling together clouds of hydrogen and helium gas
- On the main sequence, fusion of hydrogen to helium balances gravity, keeping the star stable
- The more massive a star, the hotter its core and the heavier the elements it can fuse
- A Sun-like star: main sequence to red giant to white dwarf to black dwarf
- A star much bigger than the Sun: main sequence to red supergiant to supernova to neutron star or black hole
- Elements heavier than iron are only formed in a supernova; supernovae distribute elements throughout the universe
That's the notes covered.
Carry on to the next subtopic.