Explain how astronomers use the Doppler effect to determine whether a star is moving towards or away from Earth, and describe how the speed of the star's movement affects what is observed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Stars emit light across the electromagnetic spectrum. By studying this light carefully, astronomers can learn not only about a star's composition but also about how it is moving.
Model answer (5 marks)
Astronomers compare the positions of absorption lines in a star’s spectrum with the known laboratory wavelengths. If the star is moving, the observed wavelengths are displaced.
1. A star receding from Earth causes the lines to shift to longer wavelengths – a red‑shift.
2. A star approaching Earth causes the lines to shift to shorter wavelengths – a blue‑shift.
3. The amount of shift is proportional to the star’s radial speed; the faster the star, the larger the wavelength change.
4. By measuring the shift and using the Doppler formula, astronomers calculate the star’s radial velocity.
5. Thus the direction (towards or away) and speed of the star’s motion are determined from the spectral shift.
1. A star receding from Earth causes the lines to shift to longer wavelengths – a red‑shift.
2. A star approaching Earth causes the lines to shift to shorter wavelengths – a blue‑shift.
3. The amount of shift is proportional to the star’s radial speed; the faster the star, the larger the wavelength change.
4. By measuring the shift and using the Doppler formula, astronomers calculate the star’s radial velocity.
5. Thus the direction (towards or away) and speed of the star’s motion are determined from the spectral shift.
Examiner tips
- Use the terms red‑shift and blue‑shift explicitly. Show the proportionality between speed and shift. Mention comparison with laboratory lines. Keep answer concise to fit 5 marks.
Common mistakes
- Confusing red‑shift with blue‑shift. Failing to state that the shift is measured relative to known laboratory wavelengths. Not recognising that the shift magnitude indicates speed.
Mark scheme (5 marks)
- The wavelength (or frequency) of light from a moving star is shifted compared to the expected wavelength from a stationary source
- If the star is moving away from Earth, the wavelength is increased / shifted towards the red end of the spectrum (red-shift)
- If the star is moving towards Earth, the wavelength is decreased / shifted towards the blue end of the spectrum (blue-shift)
- The greater the speed of the star, the greater the shift in wavelength (or frequency)
- Astronomers identify the shift by comparing the positions of absorption lines in the star's spectrum to their expected (laboratory/known) positions
Key terms in this question
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