A star appears white when viewed from Earth. Over millions of years, the star cools and its colour changes to red. Explain, in terms of the electromagnetic spectrum, why the colour of the star changes as it cools.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Stars emit light across a range of wavelengths. The peak wavelength of light emitted by a star depends on its surface temperature.
Model answer (4 marks)
A hotter star emits radiation with a shorter peak wavelength, while a cooler star emits with a longer peak wavelength. White light contains a mixture of short‑wavelength (blue/white) and longer‑wavelength light. As the star cools, its peak wavelength shifts toward the red part of the visible spectrum, which has the longest wavelengths. Consequently the light that reaches Earth is dominated by red wavelengths, so the star appears red.
Examiner tips
- Use the word ‘peak wavelength’ and link temperature to wavelength shift.
- Mention that red light has the longest wavelength in the visible range.
- Explain that the shift moves the peak into the red region.
- Show the causal chain: cooling → longer peak wavelength → red appearance.
Common mistakes
- Confusing colour with temperature without mentioning wavelength shift.
- Saying the star simply ‘gets red’ without explaining the spectrum.
- Using ‘blue’ instead of ‘red’ as the longer wavelength colour.
Mark scheme (4 marks)
- Hotter objects emit radiation at shorter (peak) wavelengths / cooler objects emit radiation at longer (peak) wavelengths
- Red light has a longer wavelength than white/blue-white light in the visible spectrum
- As the star cools, the peak wavelength of emitted radiation shifts towards / into the red region of the visible spectrum
- Therefore the star appears red because the dominant/peak wavelength detected corresponds to red light
Key terms in this question
Related
- All Eduqas GCSE Physics revision notes →
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- Decode the mark scheme abbreviations →
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