Explain how the colour of a star can be used to determine its surface temperature, and describe how surface temperature relates to the position of a star on the Hertzsprung-Russell diagram.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Stars vary greatly in colour, from red to blue-white. The Hertzsprung-Russell (H-R) diagram plots stars according to their luminosity and surface temperature.
Model answer (5 marks)
Colour is related to surface temperature through Wien’s law – the peak wavelength of a star’s black‑body spectrum shifts to shorter wavelengths as temperature rises. Hotter stars therefore emit most of their light in the blue or blue‑white part of the spectrum, while cooler stars peak in the red or orange.
On the H‑R diagram the horizontal axis is surface temperature, decreasing from left to right. Thus a star’s colour tells you whether it lies on the left (hot, blue) or right (cool, red) side of the diagram.
The vertical axis is luminosity. Most stars lie on the main sequence, a diagonal band running from the top‑left (high temperature, high luminosity) to the bottom‑right (low temperature, low luminosity). Red giants and supergiants occupy the top‑right region (low temperature, high luminosity) and white dwarfs sit in the bottom‑left (high temperature, low luminosity).
On the H‑R diagram the horizontal axis is surface temperature, decreasing from left to right. Thus a star’s colour tells you whether it lies on the left (hot, blue) or right (cool, red) side of the diagram.
The vertical axis is luminosity. Most stars lie on the main sequence, a diagonal band running from the top‑left (high temperature, high luminosity) to the bottom‑right (low temperature, low luminosity). Red giants and supergiants occupy the top‑right region (low temperature, high luminosity) and white dwarfs sit in the bottom‑left (high temperature, low luminosity).
Examiner tips
- Mention Wien’s law and the shift of peak wavelength
- State that temperature decreases from left to right on the H‑R diagram
- Identify the main sequence and the positions of red giants and white dwarfs
Common mistakes
- Confusing the direction of the temperature axis (right‑to‑left)
- Failing to link colour to wavelength shift
- Mixing up luminosity and temperature when describing the main sequence
Mark scheme (5 marks)
- Hotter stars emit light that appears blue (or blue-white) / cooler stars appear red (or orange)
- The colour/peak wavelength of light emitted shifts with temperature — hotter stars emit at shorter wavelengths
- On the H-R diagram, surface temperature increases from right to left along the horizontal axis
- Most stars lie along the main sequence — a diagonal band from top-left (hot, high luminosity) to bottom-right (cool, low luminosity)
- Red giants / supergiants are found in the top-right (high luminosity, low temperature) and white dwarfs are found in the bottom-left (low luminosity, high temperature)
Key terms in this question
Hertzsprung-Russell diagram · surface temperature