AQA Physics 8463 4.8.1.2
The life cycle of a star
Gravity pulls in; fusion pushes out. Set a star’s mass, then step through its life, predicting each stage before you see it.
Triple Science · Foundation tier
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Two bonfires.
A huge bonfire and a small campfire are lit at the same time, each with a pile of fuel to match its size.
If you had to guess, does a bigger star burn its fuel faster and die sooner, or slower and live longer?
Birth and balance
Gravity in, fusion energy out.
A star forms from a cloud of dust and gas called a nebula. Gravity pulls the dust and gas together to form a protostar, which gets hotter. When it is hot enough, hydrogen nuclei fuse to form helium, releasing energy: the star is born. On the main sequence the star is stable, because the inward force of gravity is balanced by the outward pressure from fusion energy. Try each force on its own.
Both together, balanced: the star stays the same size. This is the main sequence, and it lasts a very long time.
The life cycle
The star’s mass decides its path.
Every star starts the same way. After the main sequence the path splits, depending on whether the star is about the size of the Sun or much more massive. Choose a mass, then predict each next stage.
Main sequence: Hot enough for hydrogen to fuse into helium. Gravity and the outward pressure from fusion energy balance: the star is stable for a very long time.
What comes next?
Where the elements come from
Fusion makes new elements.
Fusion processes in stars produce all of the naturally occurring elements. Elements up to iron are formed by fusion in stars. Elements heavier than iron are produced in a supernova, and the explosion of a massive star spreads the elements throughout the universe. Tap an element to see where it was made.
Think again · the Sun’s ending
“One day the Sun will explode as a supernova.”
Your call
ChooseWhat will the Sun become after its red giant stage?
Sort
Sun-sized, massive, or both?
Place each stage on the path it belongs to.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Describe
- The stages, in order, for the mass given.
- Explain
- Why a star is stable: the two forces, balanced.
- Compare
- The two paths after the main sequence.
Key fact
A main-sequence star is stable because gravity balances the outward pressure from fusion energy. Its mass decides what it becomes: a white dwarf, or a supernova then a neutron star or black hole.
Key note · AQA 8463 4.8.1.2
The life cycle of a star
- A star forms when gravity pulls together dust and gas in a nebula, making a protostar. When it is hot enough, hydrogen fuses into helium.
- Main sequence: stable, because gravity is balanced by the outward pressure from fusion energy.
- About the Sun’s mass: red giant, white dwarf, black dwarf.
- Much more massive: red supergiant, supernova, then a neutron star or black hole.
- Fusion in stars makes elements up to iron; elements heavier than iron are produced in a supernova, which spreads the elements through the universe.
Photograph this card. Tomorrow, cover it and say each line first.
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Not sure why the Sun won’t go supernova?
Stars in the figures are not to scale.