The life cycle of a star

    OCR
    GCSE

    The life cycle of a star is determined fundamentally by its initial mass, initiating with the gravitational collapse of a nebula into a protostar. During the stable main sequence phase, candidates must explain the equilibrium between inward gravitational attraction and outward radiation pressure arising from the nuclear fusion of hydrogen. The curriculum requires differentiation between the evolution of solar-mass stars—progressing to red giants and white dwarfs—and massive stars, which evolve into red supergiants and culminate in supernovae, leaving neutron stars or black holes. Mastery involves linking these processes to nucleosynthesis and the cosmic dissemination of elements heavier than iron.

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
    5
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that a nebula collapses under its own gravity to form a protostar
    • Credit responses that identify nuclear fusion of hydrogen into helium as the energy source of a main sequence star
    • Award 1 mark for explaining stability in terms of balanced forces: gravitational attraction inwards equals radiation pressure outwards
    • Candidates must state that elements heavier than iron are only produced during a supernova explosion
    • Award 1 mark for correctly identifying the end stage of a solar-mass star as a white dwarf cooling to a black dwarf

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You have correctly listed the stages, but you must explain *why* the star expands into a Red Giant (fusion of helium/loss of equilibrium)"
    • "Avoid using the word 'burning'; replace it with 'nuclear fusion' to secure the mark"
    • "You identified the supernova, but failed to mention that this is the specific site for creating elements heavier than iron"
    • "Good comparison of the two pathways; ensure you explicitly state that mass is the determining factor for which path a star takes"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that a nebula collapses under its own gravity to form a protostar
    • Credit responses that identify nuclear fusion of hydrogen into helium as the energy source of a main sequence star
    • Award 1 mark for explaining stability in terms of balanced forces: gravitational attraction inwards equals radiation pressure outwards
    • Candidates must state that elements heavier than iron are only produced during a supernova explosion
    • Award 1 mark for correctly identifying the end stage of a solar-mass star as a white dwarf cooling to a black dwarf

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When describing the main sequence, explicitly name the forces: 'gravity' acting inwards and 'radiation pressure' acting outwards
    • 💡Memorise the two distinct pathways as a flowchart: Sun-like (Red Giant → White Dwarf) vs Massive (Red Supergiant → Supernova → Neutron Star/Black Hole)
    • 💡Avoid generic terms like 'fuel'; specify 'hydrogen nuclei' or 'helium nuclei' to access higher marks

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Stating that stars 'burn' hydrogen rather than undergoing 'nuclear fusion'
    • Incorrectly suggesting that the Sun will eventually become a black hole (it lacks sufficient mass)
    • Confusing the forces involved; citing 'magnetic' or 'atmospheric' pressure instead of radiation/gas pressure
    • Believing that fusion stops immediately when a star becomes a Red Giant, rather than shifting to heavier elements

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Gravitational collapse and protostar formation
    Main sequence equilibrium (Radiation pressure vs Gravity)
    Evolutionary path of solar-mass stars (Red Giant to White Dwarf)
    Evolutionary path of massive stars (Supernova to Neutron Star/Black Hole)
    Nucleosynthesis and the origin of heavy elements

    Likely Command Words

    How questions on this topic are typically asked

    Describe
    Explain
    Compare
    State
    Predict

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