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Stars and the Life Cycle of StarsAQA GCSE Physics: Revision notes

Section 1

How does a star form and reach a stable stage?

A star begins life as a nebula — a cloud of dust and gas pulled together by gravitational attraction. As the cloud contracts, it heats up and forms a protostar.

Once the core becomes hot and dense enough, fusion reactions begin, releasing huge amounts of energy. This marks the start of the main sequence stage, where the star spends most of its life in equilibrium: gravitational collapse balanced by the outward push of fusion energy.

Key termsprotostarmain sequence
Exam tip

Our Sun is currently a main sequence star — this stage lasts billions of years.

Section 2

What is the life cycle of a star like the Sun?

A star of similar mass to the Sun follows this sequence:

  1. Nebula — cloud of dust and gas
  2. Protostar — contracting, heating cloud
  3. Main sequence star — stable, fusion balances gravity (billions of years)
  4. Red giant — outer layers expand and cool as the core fuel runs low
  5. White dwarf — the outer layers are shed, leaving a hot, dense core
  6. Black dwarf — the white dwarf cools until it no longer emits light
Key termsred giantwhite dwarfblack dwarf
Example

A useful way to remember this sequence: Nebula → Protostar → Main sequence → Red giant → White dwarf → Black dwarf.

Section 3

What is the life cycle of a much more massive star?

A star with much greater mass than the Sun follows a different, more dramatic sequence:

  1. Nebula
  2. Protostar
  3. Main sequence star
  4. Red super giant — a much larger, more extreme version of a red giant
  5. Supernova — a huge, violent explosion
  6. Neutron star or black hole — depending on the remaining mass of the core
Key termsred super giantsupernovaneutron starblack hole
Common mistake

A common error is thinking all stars end as black holes. Only the most massive stars can form a black hole — smaller massive stars leave a neutron star instead.

Section 4

How are new elements formed by stars?

Fusion processes inside stars produce almost all the naturally occurring elements found in the universe.

  • During a star's normal life, fusion builds up progressively heavier elements from hydrogen, up to and including iron
  • Elements heavier than iron cannot be made by fusion inside an ordinary star — they are only produced during the extreme conditions of a supernova
  • The supernova explosion then distributes these elements throughout the universe, scattering them into space where they can eventually become part of new stars, planets — and even living things
Key termsfusion (in stars)
Think of it like this

Every atom of iron or gold on Earth heavier than iron was forged either inside a star or in the explosive death of a star — we are, quite literally, made of stardust.

Must Know

  • Sun-like star life cycle: nebula → protostar → main sequence star → red giant → white dwarf → black dwarf
  • Much more massive star life cycle: nebula → protostar → main sequence star → red super giant → supernova → neutron star or black hole
  • The main sequence stage is a stable equilibrium between gravitational collapse and outward fusion energy
  • Fusion in stars produces naturally occurring elements up to iron
  • Elements heavier than iron are produced only in a supernova explosion
  • A supernova distributes elements throughout the universe

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