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As you saw the in the previous Explore exercise, the temperature of an object determines the peak wavelength of its spectrum. If you have ever looked at a hot plate heating up on a stove, you have an idea of why this might be true. 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, is made of atoms. The atoms in the hot plate are moving around constantly, emitting light. As the hot plate heats up, its atoms move faster, and as they move faster, they emit higher-energy light.

The light emitted by the plate because of its moving atoms is called “thermal radiation.” The plate emits thermal radiation at all wavelengths of light, 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 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.

Because all objects in the universe are made of atoms, 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 explains the discovery you made in the last Explore exercise: different stars have different peak wavelengths of thermal radiation because they have different temperatures.