Your eyes are stellar thermometers. Every time you look into the night sky and notice the color of a star you have taken the temperature of that distant, massive ball of cosmic gas! This is only possible through the power of Blackbody Radiation.

Giant star Betelgeuse. This star is larger than the orbit of the Earth.

Its cool (a few thousand degrees), extended atmosphere radiates as a blackbody
giving it the characteristic red color.
(credit StScI)

You can't take an astronomy course without learning about blackbody radiation. It's one of those ideas that form the cornerstone for what astronomers know and how they know it. But what, really, is a blackbody? How did astronomers (actually physicists) come up with that slightly misleading name? How do blackbodies end up being so important? The point of this applet is to let you learn about blackbody radiation for yourself but first, here is a summary of the fundamentals.

A short definition of a blackbody might go like this: An object is a "blackbody" if the radiation it emits into space originates completely from its temperature. This means the radiation produced by the object comes from light waves mixing it up with the jiggling motions of all the zillions of atoms that make up the object. Inside a blackbody, radiation can not travel very far before it is absorbed by a jiggling atom. It is then quickly re-emitted, travels a short distance and then gets absorbed again by another atom. This happens zillions of times so there is a constant interplay between the matter and the radiation bouncing around in a blackbody. The one-to-one relation between the amount of jiggling heat motion of atoms and the spectral signature they produce make blackbodies unique, distinctive and of primary importance. Blackbody spectra do not depend on an object's chemical composition, its size or its age.

Don't let the name confuse you, blackbodies do not have to be black. They can be blue or red or yellow. So where does the name come from? Anything colored black absorbs all the wavelengths (colors) of light that fall onto it. Blackbodies do this as well and that is why physicists came up with the name "blackbody". How can blackbody appear blue or red? You'll get a better sense of how this works when you play with the applet. For now the important idea is that a blackbody pump radiation into space in a very special way. Anything which has heat and is dense enough will emit as a blackbody. That means you, the chair you’re sitting on and the Earth on which the chair rests are all blackbodies.

Every blackbody emits light with an easily identified pattern, its "spectral" signature (also called a spectral energy distribution or more specifically the blackbody curve). The blackbody curve is the particular way the total light emitted by a blackbody varies with its frequency. The number of red photons, the number of green photons, the number of infrared and ultraviolet photons are all exactly specified by the blackbody curve. Now here is the killer point - the exact form of the curve depends only on the object's temperature. Every blackbody at 2000 degrees emits light with exactly the same curve. The spectral signature of a 2000 degree iron bar in a blast furnace is identical to a 2000 degree star a trillion, trillion miles away in deep space. That is what makes blackbodies so useful and that is why the color of a star is also a measure of its temperature. If you go to the section called Blackbody radiation in astrophysics you will learn a little more about how blackbodies figure in astronomy. Then you'll be ready to play with the Applet.