Te Reo Physics Resources

Multimedia Physics and Science Teaching Resources


Hot Star Cool Star | Tīrama, Tīrama Ngā Whetū


Blackbody radiation is explored using a halogen light bulb with a dimmer. The results are used to discuss hot and cool stars.

Principles Illustrated
As an object gets hotter it glows more brightly, and the highest intensity radiation is at a shorter wavelength.



English version

Te Reo Māori Version


The diffraction grating glasses are used to see the spectrum of light produced by the hot filament, or at least the visible part that gets through the glass of the bulb. As the filament warms, the brightness of all the visible colours increases but the intensity of the blue increases relative to the red. So while an object glowing with a reddish colour is hot, an object glowing with a bluish colour is even hotter. Bluish stars are hotter than reddish stars.

Inexpensive diffraction grating glasses are often available from us or from our source: Rainbow Symphony.

As our “sun” we used a 60 W Osram Halolux 84472 Halogen bulb in some of our experiments, but we have also used an ordinary incandescent bulb with clear glass. One advantage of the Osram bulb is that the filament is oriented vertically, as are the diffraction gratings.

Hot and cool stars can be found in Orion
Blackbody radiaton curve
The Blackbody radiation curve
Blackbody radiaton curve
The Blackbody radiation curve

Other Information


There are the usual electrical safety issues, and also the bulb can get very hot.
Individual teachers are responsible for safety in their own classes. Even familiar demonstrations should be practised and safety-checked by individual teachers before they are used in a classroom.

See also Star Scene Investigation
This demonstration can be expanded by viewing infrared light from a remote control using a webcam, or a heater using a modified webcam. Click here for more information.


This teaching resource was developed by the Te Reo Māori Physics Project with support from