Thermal Radiation

Did you know that any object that contains any heat energy at all emits radiation? When you’re camping, if you put a large rock in your campfire for a while, then pull it out, the rock will emit the energy it has absorbed as radiation, which you can feel as heat if you hold your hand a few inches away. Physicists would call the rock a “blackbody” because it absorbs all the energy that reaches it, and then emits the energy at all frequencies (although not equally) at the same rate it absorbs energy.

All the matter in the known universe behaves this way.

Some astronomical objects emit mostly infrared radiation, others mostly visible light, others mostly ultraviolet radiation. The single most important property of objects that determines the radiation they emit is temperature.

In solids, the molecules and atoms are vibrating continuously. In a gas, the molecules are really zooming around, continuously bumping into each other. Whatever the amount of molecular motion occurring in matter, the speed is related to the temperature. The hotter the material, the faster its molecules are vibrating or moving.

Electromagnetic radiation is produced whenever electric charges accelerate that is, when they change either the speed or direction of their movement. In a hot object, the molecules are continuously vibrating (if a solid) or bumping into each other (if a liquid or gas), sending each other off in different directions and at different speeds. Each of these collisions produces electromagnetic radiation at frequencies all across the electromagnetic spectrum. However, the amount of radiation emitted at each frequency (or frequency band) depends on the temperature of the material producing the radiation.

It turns out that the shorter the wavelength (and higher the frequency), the more energy the radiation carries. When you are out in the sun on a hot day and your skin starts to feel hot, that heat is not what you need to worry about if you get sunburned easily. Most of the heat you feel is the result of infrared radiation striking the surface of your skin. However, it is the higher frequency  thus higher energy ultraviolet radiation penetrating the skin’s surface that stimulates the deeper layers to produce the melanin that gives fair complected folks the nice tan or bad sunburn. X-rays, at still higher frequencies, have enough energy to pass right through skin and other soft tissues. That is how bone and soft tissues of varying densities can be revealed by the x-ray imaging techniques used by medicine.

Any matter that is heated above absolute zero generates electromagnetic energy. The intensity of the emission and the distribution of frequencies on the electromagnetic spectrum depend upon the temperature of the emitting matter. In theory, it is possible to detect electromagnetic energy from any object in the universe. Visible stars radiate a great deal of electromagnetic energy. Much of that energy has to be in the visible part of the spectrum otherwise they would not be visible stars! Part of the energy has to be in the microwave (short wave radio) part of the spectrum, and that is the part astronomers study using radio telescopes.