A star emits radiation across a range of wavelengths. As the star cools over billions of years, explain how the radiation it emits changes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Stars behave as perfect black bodies. A particular star is observed to gradually decrease in surface temperature over a very long period of time.
Model answer (4 marks)
A perfect black body absorbs all incident radiation and emits the maximum possible amount of radiation for a given temperature.
As the star cools, its temperature falls.
1. The total intensity (or power per unit area) of the radiation it emits decreases – the star becomes dimmer.
2. The wavelength at which the emission is greatest (the peak of the spectrum) shifts to longer wavelengths.
3. Consequently, the spectrum becomes dominated by radiation in the infrared part of the electromagnetic spectrum.
Thus, over billions of years the star’s light becomes weaker and moves progressively into the infrared.
As the star cools, its temperature falls.
1. The total intensity (or power per unit area) of the radiation it emits decreases – the star becomes dimmer.
2. The wavelength at which the emission is greatest (the peak of the spectrum) shifts to longer wavelengths.
3. Consequently, the spectrum becomes dominated by radiation in the infrared part of the electromagnetic spectrum.
Thus, over billions of years the star’s light becomes weaker and moves progressively into the infrared.
Examiner tips
- Use the word ‘black body’ to show you understand the definition. Explain the two effects of cooling: lower intensity and a red‑shifted peak. Mention the infrared region explicitly to hit all marks.
Common mistakes
- Confusing the star’s temperature with the temperature of the radiation. Forgetting that the peak shifts to longer wavelengths. Using vague terms like ‘dimmer’ without linking to intensity or the spectrum.
Mark scheme (4 marks)
- A perfect black body absorbs all radiation incident on it (and does not reflect or transmit any radiation)
- As temperature decreases, the total amount / intensity of radiation emitted decreases
- The peak of the emitted radiation shifts to longer wavelengths (as temperature decreases)
- The star emits radiation that is more concentrated in / towards the infrared region of the electromagnetic spectrum
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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