Explain how the colour of a star can be used to determine its surface temperature, and describe how the light from distant stars provides evidence that the universe is expanding.

Eduqas A-Level Physics — 6.3 Stellar evolution and cosmology (A-Level only) · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Stars emit electromagnetic radiation across a range of frequencies. Astronomers study this radiation to learn about the properties of stars and the universe.

Model answer (5 marks)

Stars can be treated as black bodies, so the spectrum of their emitted light follows Planck’s law. The wavelength at which the intensity is greatest (the peak) shifts to shorter wavelengths as the temperature rises. Consequently, hot stars have a peak in the blue/white part of the spectrum and appear blue or white, whereas cooler stars peak in the red/orange part and appear red or orange.

The light from distant galaxies and stars is observed to be shifted towards longer wavelengths – a redshift. This shift is caused by the Doppler effect as the galaxies move away from us. The amount of redshift increases with distance, giving a linear relationship (Hubble’s law). The systematic redshift of all distant objects therefore provides strong evidence that the universe is expanding, consistent with a Big Bang origin.

Examiner tips

  • Use the term ‘black body’ and mention Planck’s law to justify the colour–temperature link. Show the direction of the shift (blue to red) and explain it as Doppler redshift. Mention Hubble’s law to link redshift with distance and expansion. Keep the answer concise – 5 marks allow a short paragraph with 3–4 key points.

Common mistakes

  • Confusing the colour of a star with its intrinsic colour; ignoring the black‑body explanation. Forgetting to state that redshift is a Doppler effect. Not linking the increase in redshift with distance (missing Hubble’s law).

Mark scheme (5 marks)

  1. Stars behave as black bodies, absorbing and emitting radiation across a range of frequencies.
  2. Hotter stars emit radiation at higher peak frequencies, so they appear blue/white; cooler stars emit at lower peak frequencies, so they appear red/orange.
  3. The light from distant galaxies/stars is shifted towards the red end of the electromagnetic spectrum (redshift).
  4. This redshift indicates that stars and galaxies are moving away from us / that the universe is expanding.
  5. The further away a galaxy is, the greater its redshift, showing it is moving away faster, which supports the idea that the universe began from a single point (Big Bang).

Key terms in this question

universe

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