Star Colour & Temperature

IGCSE Edexcel Physics
8.7–8.8 Star colour and temperature
Key Concepts: A star's colour depends on its surface temperature: blue/white stars are hottest; red stars are coolest. Order (hottest → coolest): blue-white → white → yellow → orange → red. The Sun is a yellow star (~5500 °C). Hotter stars emit shorter-wavelength (more blue) light; cooler stars emit longer-wavelength (more red) light.

Section A — Star Colour and Temperature

1. State which colour of star is hottest and which is coolest. [2]
2. List the star colours in order from hottest to coolest. [3]
3. Explain why hotter stars appear blue while cooler stars appear red, in terms of the wavelength of light emitted. [3]
4. State the approximate surface temperature and colour of the Sun. [2]

Section B — Luminosity and Brightness

5. State the difference between the luminosity of a star and its apparent brightness as seen from Earth. [2]
6. Two stars have the same luminosity. Star A is twice as far away as Star B. Compare their apparent brightnesses. [2]

Total marks: 14

Mark Scheme

1. Hottest: blue [1]; coolest: red [1] [2]
2. Blue-white → white → yellow → orange → red [3 — 1 mark if 3–4 correct in order; 2 marks for all 5 in correct order; 3 marks for all 5 plus an explanation]
3. Hotter objects emit radiation of shorter wavelengths [1]; blue light has a shorter wavelength than red [1]; so hotter stars emit more blue light; cooler stars emit more red light [1] [3]
4. Approximately 5500 °C (accept 5000–6000 °C) [1]; yellow [1] [2]
5. Luminosity is the total energy output of the star per second [1]; apparent brightness is how bright the star appears from Earth — it depends on both luminosity and distance [1] [2]
6. Star A appears less bright [1]; apparent brightness follows an inverse-square law — double the distance gives one quarter the brightness, so Star A appears four times dimmer than Star B [1] [2]