Galaxies

    OCR
    GCSE

    Galaxies are massive systems of stars, interstellar gas, dust, and dark matter, held together by gravity. The topic encompasses the structure of the Milky Way, the classification of galaxies, and the analysis of light from distant galaxies to determine their motion. Crucially, the observation of red-shift in galactic spectra provides the primary evidence for the expansion of the universe and the Big Bang theory, linking the recession velocity of galaxies to their distance from Earth.

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    Objectives
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    Exam Tips
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    Pitfalls
    5
    Key Terms
    5
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for defining a galaxy as a large collection of billions of stars held together by gravity
    • Credit responses that identify the Milky Way as the specific galaxy containing our Solar System
    • Award 1 mark for describing red-shift as the observed increase in wavelength of light from distant galaxies
    • Candidates must link the magnitude of red-shift to recession velocity: distant galaxies move away faster
    • Award marks for explaining that light from the most distant galaxies shows the greatest red-shift, providing evidence for an expanding universe

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You have correctly identified the Milky Way, but ensure you distinguish it clearly from the Solar System in terms of scale"
    • "Good description of red-shift. To improve, explicitly link the 'shift' to an increase in wavelength"
    • "You mentioned galaxies moving away, but for higher marks, you must explain that the *further* away they are, the *faster* they are moving"
    • "Excellent use of evidence. Now consider adding CMBR to your explanation to provide a complete argument for the Big Bang theory"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for defining a galaxy as a large collection of billions of stars held together by gravity
    • Credit responses that identify the Milky Way as the specific galaxy containing our Solar System
    • Award 1 mark for describing red-shift as the observed increase in wavelength of light from distant galaxies
    • Candidates must link the magnitude of red-shift to recession velocity: distant galaxies move away faster
    • Award marks for explaining that light from the most distant galaxies shows the greatest red-shift, providing evidence for an expanding universe

    Examiner Tips

    Expert advice for maximising your marks

    • 💡Memorize the scale hierarchy: Planet < Star < Solar System < Galaxy < Universe. OCR frequently asks for this ordering in low-tariff questions
    • 💡When explaining red-shift, explicitly state that 'wavelength increases' and 'frequency decreases'. Do not use vague terms like 'light stretches'
    • 💡For 6-mark Level of Response questions on the Big Bang, ensure you discuss both Red-shift (expansion) and CMBR (origin energy) to access the top band

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing the hierarchy of space objects, often incorrectly stating the Solar System contains the Galaxy or treating 'Universe' and 'Galaxy' as interchangeable
    • Stating that 'stars are moving away' or 'planets are moving away' instead of 'galaxies are moving away' when explaining red-shift
    • Misinterpreting red-shift as the galaxy changing colour (becoming red) rather than the spectral lines shifting towards the red end of the spectrum
    • Failing to mention that the expansion of the universe implies it originated from a single point (The Big Bang)

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Gravitational binding of stellar systems
    The Milky Way and Solar System location
    Red-shift and the Doppler Effect
    The Big Bang Theory and Cosmic Expansion
    Dark Mass and Dark Energy

    Likely Command Words

    How questions on this topic are typically asked

    State
    Describe
    Explain
    Calculate
    Suggest

    Practical Links

    Related required practicals

    • •{"code":"Modeling Expansion","title":"Balloon Model of the Universe","relevance":"Demonstrates how galaxies move further apart as space expands, not by moving through space"}

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