Describe how astronomers use the electromagnetic radiation received from a star to determine its surface temperature and what they can conclude about a star's chemical composition.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Stars emit electromagnetic radiation across a range of wavelengths. By studying this radiation carefully, astronomers can discover a great deal about the physical properties of stars that are too far away to visit.
Model answer (5 marks)
The colour of the light, or the wavelength at which the star’s spectrum peaks, is used to find its surface temperature – a hotter star emits a peak at shorter (blue/white) wavelengths, a cooler star at longer (red/orange) wavelengths. By measuring the peak wavelength with a spectrograph, astronomers calculate the temperature using Wien’s law. The spectrum also shows dark absorption lines. These lines are produced when elements in the star’s outer atmosphere absorb photons of particular wavelengths. Each element has a distinctive set of line positions, so by matching the observed pattern to known line lists, astronomers identify the elements present and thus the star’s chemical composition.
Examiner tips
- Use the colour/peak wavelength to explain temperature; mention Wien’s law. State that absorption lines reveal composition and that each element gives a unique pattern. Keep answer concise and use exam terminology.
Common mistakes
- Confusing emission lines with absorption lines; not linking colour to temperature; failing to mention that each element has a characteristic line pattern.
Mark scheme (5 marks)
- The colour / peak wavelength of the radiation emitted by a star indicates its surface temperature
- Hotter stars appear blue/white and cooler stars appear red/orange
- The spectrum of a star contains dark absorption lines
- These absorption lines are caused by elements in the star's outer atmosphere absorbing specific wavelengths
- Each element produces a unique/characteristic pattern of lines, so the chemical composition of the star can be identified
Key terms in this question
electromagnetic radiation · surface temperature · chemical composition
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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