Life Cycle of a Star Notes

Oxford AQA IGCSE Physics: Revision notes

Key facts

  • Stars form when gravitational attraction pulls together clouds of hydrogen and helium.
  • On the main sequence, fusion of hydrogen to helium balances gravity and the star is stable.
  • A Sun-like star becomes a red giant, then a white dwarf, then a black dwarf.
  • A much bigger star becomes a red supergiant, then a supernova, then a neutron star or black hole.
  • Elements heavier than iron form only in a supernova.

How stars form

Gravity pulls clouds of hydrogen and helium together until the core is hot and dense enough for fusion.

Stars form when dust and gas, mainly hydrogen and helium, are pulled together by gravitational attraction. The cloud contracts and heats up. With enough mass, the core becomes hot and dense enough for nuclear fusion to begin. Leftover material orbiting the new star may form planets.

  1. 1

    Cloud of dust and gas

    Mainly hydrogen and helium

  2. 2

    Gravity pulls it together

    The cloud contracts and heats up

  3. 3

    Core becomes hot and dense

    If there is enough mass

  4. 4

    Nuclear fusion begins

    A star is born

How a star forms

What pulls the dust and gas together to form a star?

The main sequence

Fusion pushes outwards while gravity pulls inwards, so the star stays a stable size for a long time.

The main sequence is the stable, long period of a star's life. Hydrogen nuclei fuse into helium, mostly in the core where temperature and density are greatest. The outward force from fusion energy (radiation pressure) balances the inward pull of gravity.

The more massive the star, the hotter its core and the heavier the nuclei it can make.

StarFusion pushes outGravity pulls in
On the main sequence the outward push from fusion balances the inward pull of gravity

Gravity

  • Pulls inwards

Fusion energy

  • Pushes outwards

Balanced

  • Star stays a stable size

Why is a main sequence star a stable size?

Sun-like stars

A Sun-like star swells to a red giant, sheds its outer layers and cools as a white dwarf, then a black dwarf.

The star's mass decides its whole life cycle. For a Sun-like star: hydrogen fuses to helium on the main sequence for billions of years. It then expands and cools as a red giant. The outer layers are shed, leaving a hot, dense white dwarf, which cools over a very long time to a black dwarf that no longer emits light.

  1. 1

    Main sequence

    Hydrogen fuses to helium

  2. 2

    Red giant

    Expands and cools

  3. 3

    White dwarf

    Hot, dense core

  4. 4

    Black dwarf

    Cools and stops emitting light

Life cycle of a Sun-like star

What does a Sun-like star become after the red giant stage?

Massive stars

A much bigger star becomes a red supergiant, explodes as a supernova and leaves a neutron star or black hole.

On the main sequence, hydrogen fuses faster than in a Sun-like star. The star becomes a red supergiant, fusing helium to carbon and then heavier elements. It ends as a huge explosion, a supernova. The core left behind is an extremely dense neutron star or, depending on the remaining mass, a black hole.

  1. 1

    Nebula

    a cloud of gas and dust collapses

  2. 2

    Main sequence star

    much more massive than the Sun

  3. 3

    Red supergiant

    fuses helium, then heavier elements

  4. 4

    Supernova

    a huge explosion

  5. 5

    Neutron star or black hole

    depends on the mass of the core left behind

Life cycle of a massive star

Sun-like star

Stage 1:
Main sequence
Stage 2:
Red giant
Stage 3:
White dwarf
Stage 4:
Black dwarf

Much bigger star

Stage 1:
Main sequence
Stage 2:
Red supergiant
Stage 3:
Supernova
Stage 4:
Neutron star or black hole

Which stage does a much bigger star have that a Sun-like star does not?

Origin of elements

Stars make the elements: up to iron by fusion in normal stars, and heavier elements only in a supernova.

Fusion in stars produces all naturally occurring elements. Elements up to iron form during a star's life cycle. Elements heavier than iron form only in a supernova, where conditions are extreme. The explosion spreads these elements through the universe, seeding future stars and planets.

  1. 1

    Stars fuse nuclei

    elements up to iron form during the star's life cycle

  2. 2

    Supernova

    elements heavier than iron form only here

  3. 3

    Explosion scatters elements

    into space

  4. 4

    New stars and planets form

    from the enriched gas

Then back to step 1

Where the elements come from

Up to iron

  • Fusion during a star's normal life

Heavier than iron

  • Only in a supernova

Where are elements heavier than iron formed?

Try an exam question

Describe what happens to a star much bigger than the Sun after it leaves the main sequence, and explain how this produces elements heavier than iron.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.