Astronomers studying a distant star detect that certain wavelengths of visible light are missing from the continuous spectrum of radiation received from the star. Explain why specific wavelengths are absent and describe what information astronomers can obtain from the pattern of missing wavelengths.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When light from a distant star is spread into a spectrum, dark lines are observed at specific positions across the continuous background of colours.
Model answer (5 marks)
The star emits a continuous spectrum from its hot, dense core or surface.
As this light travels through the cooler outer layers of the star, atoms in the gas absorb photons of specific energies.
Each element has a unique set of energy levels, so it absorbs a characteristic set of wavelengths, producing a distinct pattern of dark lines.
By comparing these dark‑line patterns with laboratory spectra, astronomers can identify the elements present in the star’s atmosphere.
If the dark lines are displaced towards longer wavelengths (red‑shifted), the star is moving away from Earth; the amount of shift indicates its radial velocity and direction of motion.
As this light travels through the cooler outer layers of the star, atoms in the gas absorb photons of specific energies.
Each element has a unique set of energy levels, so it absorbs a characteristic set of wavelengths, producing a distinct pattern of dark lines.
By comparing these dark‑line patterns with laboratory spectra, astronomers can identify the elements present in the star’s atmosphere.
If the dark lines are displaced towards longer wavelengths (red‑shifted), the star is moving away from Earth; the amount of shift indicates its radial velocity and direction of motion.
Examiner tips
- Use the word ‘absorb’ and mention the star’s core/surface for the continuous spectrum. Show the sequence: emission → absorption in outer layers → unique pattern. Include the comparison with lab spectra to identify elements. Mention red‑shift and its link to radial velocity.
- common_mistakes
- :
- Confusing emission lines with absorption lines. Forgetting to link the pattern to element identification. Not recognising that the shift gives the star’s motion.
Mark scheme (5 marks)
- The continuous spectrum of light is produced by the hot, dense core/surface of the star
- As light passes through the cooler outer atmosphere/gas of the star, certain wavelengths are absorbed
- Each element absorbs a unique/characteristic set of wavelengths, producing a unique pattern of dark lines
- By comparing the pattern of dark lines to known laboratory spectra, astronomers can identify which elements are present in the star's atmosphere
- If the dark lines are shifted towards the red end of the spectrum (red-shifted), the star is moving away from Earth / the shift in position of the lines can indicate the star's speed or direction of movement
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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