# Explain how astronomers use the Doppler effect to determine whether a distant star is moving towards or away from Earth, and why the magnitude of the observed frequency shift can be used to estimate the star's speed relative to Earth.

> IB DP Physics Higher Level (2023 syllabus) — C.5 Doppler effect · Explain · 4 marks

> Astronomers observe light from distant stars using spectrometers. The spectrum of a star contains dark absorption lines at characteristic frequencies corresponding to known elements such as hydrogen.

## Mark scheme (4 marks)

1. The observed frequency (or wavelength) of absorption lines is compared with the known rest frequency (or wavelength) of those lines measured in a laboratory.
2. If the star moves away from Earth, the observed frequency is lower (redshift); if it moves towards Earth, the observed frequency is higher (blueshift).
3. The Doppler equation relates the fractional frequency shift Δf/f (or Δλ/λ) to the ratio of the source's speed to the speed of light, so a larger shift corresponds to a greater speed.
4. Only the component of the star's velocity along the line of sight (radial velocity) contributes to the Doppler shift, so the method measures radial speed only.

## Key terms

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

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-astronomers-use-the-doppler-11246201) · Published by Druglandscape Ltd.