A star emits electromagnetic radiation across a wide range of wavelengths. Explain how astronomers use this radiation to determine both the chemical composition and the surface temperature of the star.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Different stars produce different patterns of dark lines in their spectra, and the peak wavelength of radiation they emit varies from star to star.
Model answer (5 marks)
A star emits a continuous spectrum of electromagnetic radiation, which is the white‑light spectrum produced by its hot surface.
When this light passes through the star’s outer layers, atoms absorb photons of specific energies, creating dark absorption lines in the spectrum.
Each chemical element has a unique set of energy levels, so the pattern of absorption lines is a fingerprint that tells astronomers which elements are present in the star’s atmosphere.
The continuous spectrum also has a peak wavelength. By measuring the wavelength at which the intensity is greatest, astronomers can apply Wien’s displacement law to find the star’s surface temperature.
Because hotter stars have their peak at shorter wavelengths (blue/UV) and cooler stars at longer wavelengths (red/IR), the position of the peak directly indicates the star’s temperature.
When this light passes through the star’s outer layers, atoms absorb photons of specific energies, creating dark absorption lines in the spectrum.
Each chemical element has a unique set of energy levels, so the pattern of absorption lines is a fingerprint that tells astronomers which elements are present in the star’s atmosphere.
The continuous spectrum also has a peak wavelength. By measuring the wavelength at which the intensity is greatest, astronomers can apply Wien’s displacement law to find the star’s surface temperature.
Because hotter stars have their peak at shorter wavelengths (blue/UV) and cooler stars at longer wavelengths (red/IR), the position of the peak directly indicates the star’s temperature.
Examiner tips
- Use the word ‘absorption lines’ and ‘continuous spectrum’ early. Show the link between line pattern and element identification. Mention Wien’s law when relating peak wavelength to temperature. Keep the answer concise and to the point.
Common mistakes
- Confusing emission lines with absorption lines. Forgetting to mention the continuous spectrum. Not linking peak wavelength to temperature. Using vague terms like ‘colour’ instead of ‘wavelength’.
Mark scheme (5 marks)
- Stars produce a continuous spectrum of electromagnetic radiation / white light spectrum
- Elements in the star's atmosphere absorb specific wavelengths, producing dark lines (absorption lines) in the spectrum
- Each element produces a unique/characteristic pattern of dark lines, so the pattern of lines identifies which elements are present
- The peak wavelength of the continuous spectrum is used to determine surface temperature
- Hotter stars emit peak radiation at shorter wavelengths (e.g. blue/ultraviolet) and cooler stars at longer wavelengths (e.g. red/infrared)
Key terms in this question
surface temperature · chemical composition
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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