AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER
Life cycle of a star
Space physics · Lesson 2 of 4
Teacher copy - includes the notes for whoever is teaching from it.
Warm-up
Answer each one, then check.
1. What does the Sun produce?
Light and heat
2. What force pulls matter together?
Gravity
3. What is fusion?
Nuclei joining together to form a larger nucleus
4. What is the main element in stars?
Hydrogen
5. What is a star?
A huge ball of hot gas
Learning Objectives
1. Describe the life cycle of a star like the Sun and of a star much more massive than the Sun.
2. Explain how a stable main-sequence star is formed and how it stays stable.
3. Explain how fusion in stars forms the elements.
4. Explain how elements heavier than iron are formed.
Stars
A star forms from a nebula pulled together by gravity. In the main sequence, the inward force of gravity is balanced by the outward pressure from fusion energy, so the star is stable. When the hydrogen runs out, the star changes.
The mass of a star decides how it ends: a star like the Sun becomes a white dwarf; a much more massive star explodes as a supernova.
The Life Cycle of Stars
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Mass decides the path after the main sequence. |
A flow chart of the life cycle of a Sun-sized star and a massive star.
Sun-sized or Massive?
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A STAR LIKE THE SUN |
A MUCH MORE MASSIVE STAR |
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▸ Nebula, protostar, main sequence. ▸ Expands to a red giant. ▸ Outer layers drift away; the core shrinks to a white dwarf. ▸ Cools to a black dwarf. |
▸ Nebula, protostar, main sequence. ▸ Expands to a red supergiant. ▸ Explodes as a supernova. ▸ Leaves a neutron star or a black hole. |
Why a Main Sequence Star Is Stable
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Explain why a star stays the same size for billions of years. |
1. Gravity
Pulls the star's matter inwards
2. Fusion
Hydrogen fuses to helium and releases energy, causing outward pressure
3. Balance
The forces balance so the star is stable
Answer: Inward gravitational force is balanced by outward thermal pressure.
Making the Elements
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Explain how heavier elements are formed. |
1. In stars
Nuclear fusion forms elements up to iron
2. In a supernova
The explosion has enough energy to form elements heavier than iron
3. Spread
The explosion scatters elements into space; new stars and planets form from them
Answer: Fusion makes elements up to iron; supernovae make heavier ones.
Key Points
Learn these.
▸ Main sequence. Hydrogen fuses to helium; stable for billions of years.
▸ Red giant. Hydrogen runs out; the star expands and cools.
▸ Supernova. A massive star explodes.
▸ Black hole. An object with gravity so strong that not even light escapes.
Key Terms
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Nebula A cloud of dust and gas from which stars form. |
Main sequence The long stable stage of a star's life when hydrogen fuses to helium. |
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Red giant A large, cool, red star near the end of its life. |
White dwarf A small, dense, hot star that is the core of a dead Sun-sized star. |
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Supernova The explosion of a massive star at the end of its life. |
Black hole An object so dense that not even light can escape. |
Your Task: Star Story
12 minutes
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Write the life story of the Sun and of a star ten times as massive. Where do they differ? 1. Mass controls the ending. |
A good answer shows: Sun: nebula, protostar, main sequence, red giant, white dwarf, black dwarf. Massive star: nebula, protostar, main sequence, red supergiant, supernova, neutron star or black hole.
Note: Ask which stage gives the heaviest elements.
Can I...?
☐ Describe a nebula.
☐ Describe a protostar.
☐ Explain stability.
☐ Follow the Sun's path.
☐ Follow a massive star's path.
☐ Explain fusion and elements.
☐ Define a supernova.
☐ Define a black hole.
Summary
✓ Stable when gravity and outward pressure are balanced.
✓ Sun-sized: red giant, white dwarf, black dwarf.
✓ Massive: red supergiant, supernova, neutron star or black hole.
✓ Supernovae form elements heavier than iron.
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EXAM FOCUS Describe what happens to a star like the Sun when hydrogen runs out. (3 marks) Red giant, white dwarf, black dwarf. |
Exam Practice: Life cycle of a star
Answer all questions. Use the mark allocation as a guide to how much to write. · 17 minutes
▸ Question 1 · 2 marks · State. Name the stages in the life cycle of a star like our Sun after the main sequence.
▸ Question 2 · 3 marks · Explain. Explain why a star on the main sequence is stable.
▸ Question 3 · 4 marks · Describe. Describe the life cycle of a star that is much more massive than the Sun.
▸ Question 4 · 3 marks · Explain. Explain why some elements heavier than iron are found on Earth even though they cannot be made in the main sequence.
▸ Question 5 · 4 marks · Explain. Explain how the Sun formed and why the Sun is stable.
Question 1 · 2 marks · State
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“Name the stages in the life cycle of a star like our Sun after the main sequence.” |
HOW TO ANSWER IT Command word: State. Worth 2 marks, so plan before writing.
Question 1 · mark scheme
2 marks available. Award a mark for each point made.
▸ Red giant. 1 mark
▸ White dwarf. 1 mark
▸ Model answer. Red giant, then white dwarf (then black dwarf).
Question 2 · 3 marks · Explain
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“Explain why a star on the main sequence is stable.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 2 · mark scheme
3 marks available. Award a mark for each point made.
▸ Gravity acts inwards. 1 mark
▸ Fusion energy gives outward pressure. 1 mark
▸ The forces balance. 1 mark
▸ Model answer. The inward force of gravity is balanced by the outward pressure from the energy released by nuclear fusion, so the star neither collapses nor expands.
Question 3 · 4 marks · Describe
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“Describe the life cycle of a star that is much more massive than the Sun.” |
HOW TO ANSWER IT Command word: Describe. Worth 4 marks, so plan before writing.
Question 3 · mark scheme
4 marks available. Award a mark for each point made.
▸ Nebula / protostar / main sequence. 1 mark
▸ Red supergiant. 1 mark
▸ Supernova. 1 mark
▸ Neutron star or black hole. 1 mark
▸ Model answer. A cloud of dust and gas collapses under gravity to form a protostar, then a main sequence star. When hydrogen runs out it expands into a red supergiant, then explodes as a supernova, leaving a neutron star or a black hole.
Question 4 · 3 marks · Explain
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“Explain why some elements heavier than iron are found on Earth even though they cannot be made in the main sequence.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 4 · mark scheme
3 marks available. Award a mark for each point made.
▸ Supernova forms heavier elements. 1 mark
▸ Elements thrown into space. 1 mark
▸ Incorporated into new stars and planets. 1 mark
▸ Model answer. Elements heavier than iron are made in the explosion of a supernova, which has enough energy. The explosion throws the elements into space, where they form part of new stars and planets.
Question 5 · 4 marks · Explain
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“Explain how the Sun formed and why the Sun is stable.” |
HOW TO ANSWER IT Command word: Explain. Worth 4 marks, so plan before writing.
Question 5 · mark scheme
4 marks available. Award a mark for each point made.
▸ Gravity pulls dust and gas together. 1 mark
▸ Fusion begins. 1 mark
▸ Gravity acts inwards. 1 mark
▸ Balanced by outward pressure. 1 mark
▸ Model answer. Dust and gas were pulled together by gravity. The star got hot and dense enough for nuclear fusion to start. It is stable as the inward gravitational force balances the outward pressure from fusion energy.