Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.

Blackbody radiation and the colors of stars

Temperature

When you look at me, you see light which is emitted by the flourescent bulbs overhead and bounces off me or my clothing. If I turn off the lights, and close all the shades and drapes so that no outside light can enter the room, will you see me? No.

But a rattlesnake could. How? The rattlesnake has organs which can detect the heat my body gives off. Heat is, in fact, one kind of electromagnetic radiation: the kind we call infrared. Even if no lights shine upon me, my skin still emits this infrared radiation.

In fact, to a rough approximation, all objects emit radiation in a manner which depends largely on their temperature. Human eyes can't see this radiation when an object is at room temperature ... but they can begin to sense it when the temperature rises to many hundreds of degrees. For example, you can see the radiation emitted by both of the objects below.



  Q:  Which of the two is hotter?  How do you know?



  Q:  Which of the two would do more damage to 
      your hand if you were to reach out and 
      grab it?  Why?



As the temperature of an object rises, the radiation it emits changes in two ways:

You don't need sophisticated equipment to determine temperatures very roughly; just look at the sky with your own eyes.


Credit & Copyright: Juan Carlos Casado


Credit and Copyright: Akira Fuji

Red stars are cool (temperatures below 4500 K), and blue or blueish white stars are hot (temperatures above 9000 K).

Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.