# 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.

> Eduqas A-Level Physics — 1.6 Using radiation to investigate stars · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. The wavelength (or frequency) of light from a moving star is shifted compared to the expected wavelength from a stationary source
2. If the star is moving away from Earth, the wavelength is increased / shifted towards the red end of the spectrum (red-shift)
3. If the star is moving towards Earth, the wavelength is decreased / shifted towards the blue end of the spectrum (blue-shift)
4. The greater the speed of the star, the greater the shift in wavelength (or frequency)
5. Astronomers identify the shift by comparing the positions of absorption lines in the star's spectrum to their expected (laboratory/known) positions

## Key terms

- [Doppler effect](https://www.gradenine.co.uk/glossary/doppler-effect)

## Related

- [Revision notes for Eduqas A-Level Physics](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-astronomers-use-the-doppler-9dbe3028) · Published by Druglandscape Ltd.