Stars

    OCR
    GCSE

    Stars form from the gravitational collapse of nebulae, initiating nuclear fusion where hydrogen nuclei combine to form helium, releasing vast amounts of energy. A star's stability during the main sequence results from the equilibrium between inward gravitational attraction and outward radiation pressure. The ultimate fate of a star—evolving into a white dwarf, neutron star, or black hole—is strictly determined by its initial mass. This stellar evolution drives nucleosynthesis, creating elements heavier than lithium and dispersing them via supernovae to form future planetary systems.

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
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    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that stability in a main sequence star is caused by the balance between inward gravitational force and outward radiation pressure
    • Credit responses that identify the fusion of hydrogen nuclei into helium nuclei as the primary energy source in the main sequence
    • Award 1 mark for describing the transition to a Red Giant: hydrogen fuel depletes, the core collapses, and outer layers expand and cool
    • Candidates must explicitly link the formation of elements heavier than iron to the extreme energy conditions of a supernova explosion
    • Award 1 mark for correctly differentiating the end states: White Dwarf/Black Dwarf for low mass, Neutron Star/Black Hole for high mass

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly listed the stages, but you need to explain *why* the star expands into a Red Giant (unbalanced forces)"
    • "Be careful with force directions: remember that gravity always acts inwards towards the centre of mass"
    • "You mentioned fusion, but didn't specify that hydrogen nuclei fuse into helium nuclei during the main sequence"
    • "Excellent distinction between the two pathways; to reach the top band, mention that elements heavier than iron are only formed in supernovae"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that stability in a main sequence star is caused by the balance between inward gravitational force and outward radiation pressure
    • Credit responses that identify the fusion of hydrogen nuclei into helium nuclei as the primary energy source in the main sequence
    • Award 1 mark for describing the transition to a Red Giant: hydrogen fuel depletes, the core collapses, and outer layers expand and cool
    • Candidates must explicitly link the formation of elements heavier than iron to the extreme energy conditions of a supernova explosion
    • Award 1 mark for correctly differentiating the end states: White Dwarf/Black Dwarf for low mass, Neutron Star/Black Hole for high mass

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When describing the main sequence, do not just say 'forces are balanced'; you must name them: 'gravitational attraction' and 'radiation pressure' (or thermal expansion)
    • 💡For 6-mark extended response questions, structure your answer by clearly separating the two distinct evolutionary paths based on the initial mass of the protostar
    • 💡Ensure you use the term 'nuclei' rather than 'atoms' when describing fusion (e.g., 'hydrogen nuclei fuse'), as electrons are stripped at these temperatures

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing the life cycle pathways by suggesting the Sun will eventually become a black hole or neutron star
    • Stating that 'gravity pushes out' or 'pressure pulls in', demonstrating a fundamental misunderstanding of the direction of acting forces
    • Failing to distinguish between the fusion of light elements (occurring in stars) and the synthesis of heavy elements (occurring only in supernovae) when asked about the origin of matter
    • Describing the 'death' of a star as the fire going out, rather than a cessation of fusion leading to gravitational collapse

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Stellar Life Cycles and Mass Dependence
    Nuclear Fusion and Nucleosynthesis
    Hydrostatic Equilibrium (Balance of Forces)
    Hertzsprung-Russell (HR) Diagrams and Classification

    Likely Command Words

    How questions on this topic are typically asked

    Describe
    Explain
    Compare
    State
    Predict

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