Phone:

A star’s color can give clues to an important property of a star: its average temperature. All objects give off “thermal radiation” – light waves emitted from the random motions of atoms inside the object. As the atoms heat up, they move around more, and thus give off more radiation. As atoms heat up and move faster, the peak wavelength of their thermal radiation changes.

Click to view animation.

If you have ever looked at a hot plate heating up on a stove, you have seen thermal radiation in action. At first, you don’t see any light coming out of the hot plate, but you feel its heat. When you heat the plate up a little, it begins to glow a dull red. As it heats up more, the plate begins to glow brighter, and its color begins to change: first orange, then yellow, then blue. If you remove the plate from the heat, it cools down, and its color sequence reverses. The animation to the right shows the color sequence you might see as the plate heats up, then cools down.

What’s going on here? The hot plate, like all other objects in the universe, emits thermal radiation. The plate emits thermal radiation at all wavelengths of the electromagnetic spectrum, but it emits most of its radiation at a certain peak wavelength, which gets shorter as the plate’s temperature goes up. At room temperature, the radiation it emits is infrared, invisible to your eyes. As the plate heats up, its peak wavelength moves to shorter wavelengths: to red, then orange, then yellow, then blue. If you continued heating the hot plate (higher than you could ever heat it on a stove), the plate would glow bright violet.

Physicists have found that every object in the universe emits thermal radiation. So of course, thermal radiation is emitted by stars – the same kind of thermal radiation emitted by hot plates on Earth. This observation answers the question from the previous page: different stars have different peak wavelengths of thermal radiation because they have different temperatures.