Colour and Temperature of Stars

Using a variable resistor and a light bulb, it is possible to demonstrate how the colour of a star is related to its temperature.

Apparatus and Materials

• Power supply
• Lamp
• Variable resistor
• Connecting wires

Health & Safety and Technical Notes

Read our standard health & safety guidance

Teaching Notes

• Most students will have noticed that when a lamp has a small current passing through it the light emitted is very red or orange. However, most students will not have thought that this could be related to why cooler stars are redder, and hotter stars are whiter or even blue.
• As the current is increased by changing the resistance in series with the lamp, more current flows through the lamp, and more energy is transferred to the lamp each second. This causes the lamp filament to get hotter and glow brighter.
• As the current increase the light emitted changes from red, through yellow and on towards white. As the current increases the temperature is also increasing. This shows a direct link between temperature and colour of light emitted.
• It is not generally possible for the light emitted from this particular set up to reach a temperature where the lamp glows blue.
• Some discharge lamps that uses gases can reach temperatures that cause the lamps to emit a very distinctive blue/white light. You may have some of these types of lamp in theatrical lanterns in your school.
• A school hall or theatre can be a good place to demonstrate this change in colour as lights are dimmed.
• This link between colour and temperature is well known amongst photographers who take the colour-temperature of light into account when taking photographs