Stars, cosmology and scientific evidence

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Big idea: stars, cosmology and scientific evidence

  • Key concept: Change. Stellar fusion releases energy. Galaxy redshifts and the cosmic microwave background support models of an expanding universe with a hot, dense early state.
  • Related concepts: Models and evidence. Use a scientific explanation to make predictions, then test it against observations.
  • Global context: Scientific and technical innovation. Astronomical observations test models over distances and times impossible to recreate in a classroom.

The Sun as a Star

  • The Sun is a medium-sized star at the centre of the Solar System.
  • It consists mostly of hydrogen and helium.
  • It radiates most energy in the infrared, visible and ultraviolet regions.
  • In the core, nuclear fusion converts hydrogen into helium, releasing huge energy.
  • Main-sequence stars release energy through hydrogen fusion in their cores.

The Sun as a star

The Sun as a star

Scale of the Universe

  • The Universe is a large collection of billions of galaxies.
  • A galaxy is a large collection of billions of stars.
  • The Milky Way is our galaxy; the Sun is one of its billions of stars.
  • Other stars in the Milky Way are much further from Earth than the Sun.
  • A light-year is the distance light travels in one year (≈ 9.5 × 10¹⁵ m).
  • The Milky Way's diameter is about 100 000 light-years.

Hierarchy of the Universe

Hierarchy of the Universe

Star Formation

  • Stars form from a nebula (cloud of gas and dust).
  • Gravity pulls particles together to form a protostar; density and temperature increase.
  • When the core is hot enough, nuclear fusion begins → stable star.
  • In a stable star, inward gravitational forces balance outward thermal pressure.

Balanced forces in a stable star

Balanced forces in a stable star

Life Cycle of Low-Mass Stars (≤ 8 solar masses)

  • After stable phase: red giant → planetary nebula → white dwarf.
  • Red giant: hydrogen depleted, core shrinks and heats, helium fuses to carbon, outer layers expand.
  • Planetary nebula: outer layers ejected; core collapses.
  • White dwarf: hot, dense core that cools and dims over time.

Life Cycle of High-Mass Stars (> 8 solar masses)

  • After stable phase: red supergiant → supernova → neutron star or black hole.
  • Red supergiant: fuses heavier elements up to iron; core collapses and expands repeatedly.
  • Supernova: core collapses rapidly, triggering a huge explosion; heavy elements (heavier than iron) are formed.
  • Neutron star: dense remnant at the centre; if massive enough, collapses to a black hole.
  • Nebulae from supernovae may form new stars and planets.

Galactic Redshift

  • Redshift is the increase in wavelength of light from a source moving away (Doppler effect).
  • Light from distant galaxies is redshifted compared to hydrogen on Earth → galaxies are moving away.
  • Greater redshift means the galaxy is further away and moving faster.
  • This shows the Universe is expanding.

The Big Bang Theory

  • The Universe began about 14 billion years ago from a hot, dense single point.
  • Evidence: galactic redshift (galaxies receding → expansion) and cosmic microwave background radiation (CMBR).
  • The cosmic microwave background is nearly uniform radiation observed across the sky, with small temperature variations, left over from the early Universe.
  • The radiation last scattered when the Universe became transparent had a much hotter thermal spectrum. Expansion stretched its wavelengths into the microwave region observed today.

Age of the Universe

  • Hubble's law: recession speed v is proportional to distance d: v = H₀ d.
  • Hubble constant H₀ = 2.2 × 10⁻¹⁸ per second (current accepted value).
  • Age of Universe ≈ 1 / H₀ ≈ 13.7 billion years.
  • Distance to galaxies measured using redshift and brightness of supernovae (standard candles).

Think like a scientist

  • Use teacher-provided galaxy spectra to compare measured redshift with independently estimated distance.
  • Comparison: the galaxy distance in the dataset. Outcome: the spectral redshift.
  • Control: compare shifts of the same identified spectral line. Explain why this makes the comparison fairer.
  • Evidence: Use a consistent method, repeated observations where appropriate and a table with labelled quantities and units. Keep unexpected results and investigate their cause.
  • Safety: Practical activities need teacher supervision and an appropriate risk assessment. Use the provided data or simulation where the investigation specifies it.
  • Inquiry task: State a testable question, predict the outcome using the science, then explain how your observations would support or challenge the prediction.

Evaluate the science

  • Astronomical observations test models over distances and times impossible to recreate in a classroom.
  • A redshift-distance relationship has scatter and measurement limits; individual nearby galaxies need not follow the overall trend.
  • Evaluation task: Link your conclusion to evidence, identify a limitation and suggest a specific improvement. Distinguish a measured result from an explanation of its cause.

Diapos

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  1. 1.Which of the following is the correct definition of a galaxy?

    Easy
    • AA large collection of billions of stars
    • BA large collection of billions of planets
    • CA large collection of billions of galaxies
    • DA large collection of billions of moons

  2. 2.What is the approximate diameter of the Milky Way galaxy in light-years?

    Easy
    • A100 000 light-years
    • B1 million light-years
    • C10 000 light-years
    • D1 billion light-years

  3. 3.What is the name of the force that pulls the outer layers of a star inwards during its stable phase?

    Easy
    • AGravitational force
    • BThermal pressure
    • CNuclear force
    • DElectromagnetic force

  4. 4.What is the correct order of stages in the life cycle of a star similar in mass to the Sun after the stable star phase?

    Easy
    • ARed giant, planetary nebula, white dwarf
    • BRed supergiant, supernova, neutron star
    • CPlanetary nebula, white dwarf, red giant
    • DWhite dwarf, red giant, planetary nebula

  5. 5.What does the observed redshift in the light from distant galaxies indicate?

    Easy
    • AGalaxies are moving away from Earth
    • BGalaxies are moving towards Earth
    • CGalaxies are stationary
    • DGalaxies are shrinking

  6. 6.The Hubble constant H0 is approximately 2.2 × 10⁻¹⁸ per second. A galaxy is at a distance of 1.0 × 10²³ m. What is its recessional velocity?

    Medium
    • A2.2 × 10⁵ m/s
    • B2.2 × 10⁻⁴¹ m/s
    • C4.5 × 10⁴⁰ m/s
    • D2.2 × 10⁻⁵ m/s

  7. 7.Which of the following is evidence for the Big Bang theory?

    Easy
    • AGalactic redshift and cosmic microwave background radiation
    • BThe Sun's energy from nuclear fusion
    • CThe existence of black holes
    • DThe formation of planetary nebulae

  8. 8.What is the approximate age of the Universe according to the Hubble constant?

    Easy
    • A13.7 billion years
    • B13.7 million years
    • C4.5 billion years
    • D100 billion years

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