A student looks at a star through a converging lens and notices that the star appears as a point of light. Explain how the colour of a star can be used to determine its surface temperature, and describe how the colour of a star changes as its surface temperature increases.

Eduqas GCSE Physics — 2.1 Light, lenses and the universe · Explain · 4 marks · View as Markdown

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

Stars emit light across a range of colours. The colour of a star depends on its surface temperature.

Model answer (4 marks)

The colour of a star is directly related to its surface temperature because the star emits a spectrum of light that follows Planck’s law. A cooler surface emits most of its energy at longer wavelengths, giving a red or orange appearance. As the surface temperature rises, the peak of the spectrum shifts to shorter wavelengths, so the star’s colour moves from red towards white or yellow and eventually to blue. The hottest stars therefore appear blue or blue‑white.

Examiner tips

  • Use the phrase ‘directly related to’ to link colour and temperature.
  • Mention the shift from red to blue as temperature increases.
  • Include the hottest stars appear blue‑white.
  • Show understanding of spectral shift (red → white → blue).

Common mistakes

  • Confusing colour with brightness; colour is about wavelength, not intensity.
  • Saying all hot stars are white; they are actually blue or blue‑white.
  • Omitting the shift from red to blue as temperature rises.

Mark scheme (4 marks)

  1. The colour of a star is related to / indicates its surface temperature
  2. Cooler stars appear red (or orange)
  3. As surface temperature increases, stars appear to change colour from red towards white / yellow / blue
  4. The hottest stars appear blue (or blue-white)

Key terms in this question

surface temperature · colour · converging lens · star

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