Section 1 of 5Colour and Temperature
The colour a star appears is set mainly by the temperature of its surface.
Surface temperature
- The temperature of the outer layer of a star, measured in kelvin (K).
Hot stars
- Hot stars emit most of their light at shorter wavelengths, so they appear blue.
Cool stars
- Cooler stars emit most of their light at longer wavelengths, so they appear red.
Infrared emission
- Warm objects emit mainly infrared radiation, and very hot objects also emit visible light.
Colour order
- From hottest to coolest, stars appear blue, white, yellow, orange and then red.
Section 2 of 5Typical Stars
Approximate surface temperatures for each colour of star.
Blue
- The hottest group, with surfaces of around 30 000 K.
White
- Hot stars with surfaces of around 10 000 K, such as Sirius A.
Yellow
- Stars with surfaces of around 6000 K, a group that includes our Sun.
Red
- The coolest group, with surfaces of around 3000 K, including red dwarfs.
| Colour | Approximate surface temperature | Example |
|---|---|---|
| Blue | About 30 000 K | Hot, massive main sequence stars |
| White | About 10 000 K | Sirius A |
| Yellow | About 6000 K | The Sun (about 5800 K) |
| Red | About 3000 K | Red dwarfs and red giants |
Section 3 of 5Changes During Expansion and Contraction
Changes During Expansion and Contraction
Expansion
- When a star expands its surface cools, so it becomes redder, as happens in a red giant.
Contraction
- When a star contracts its surface heats up, so it becomes whiter, as happens in a white dwarf.
Exam tip
- For stars, red means cool and blue means hot, which is the reverse of the everyday association of red with heat.
Surface versus core
- Classification uses the surface temperature, which is much lower than the temperature in the core.
Section 4 of 5Worked Example
Question
- Star A appears blue-white and Star B appears orange.
- Which star has the higher surface temperature, and what would happen to the colour of Star B if it expanded?
- Step 1: Bluer stars are hotter, and blue-white is bluer than orange, so Star A is hotter.
- Step 2: Orange indicates a cooler surface than blue-white, so Star B has the lower surface temperature.
- Step 3: An expanding star cools at its surface, so Star B would become redder if it expanded.
Answer
- Star A has the higher surface temperature;
- if Star B expanded, its surface would cool and it would look redder.
Section 5 of 5Exam Checklist & Tips
8.2.1 – Key Points for the Exam
- Surface temperature The temperature of the outer layer of a star.
- Kelvin (K) The SI unit of temperature, used for the surface temperature of stars.
- Infrared Electromagnetic radiation with a wavelength longer than visible red light.
- Red giant A large, cool, luminous star formed when a main sequence star expands.
- White dwarf A small, hot star formed when a solar-mass star collapses after shedding its outer layers.
Common Mistakes
- Saying that red stars are the hottest; blue stars have the highest surface temperatures.
- Giving star temperatures in degrees Celsius; stellar temperatures are given in kelvin.
- Assuming that a star's colour depends on its size; colour is set mainly by its surface temperature.
- Describing a red giant as hot because it is large; its surface is cooler than that of the Sun.