The average or bulk properties of electromagnetic radiation interacting with matter are systematized in a simple set of rules called radiation laws. These laws apply when the radiating body is what physicists call a blackbody radiator. Generally, blackbody conditions apply when the radiator has very weak interaction with the surrounding environment and can be considered to be in a state of equilibrium. Although stars do not satisfy perfectly the conditions to be blackbody radiators, they do to a sufficiently good approximation that it is useful to view stars as approximate blackbody radiators.
The behavior is illustrated in
the figure shown above. The Planck Law gives a distribution that peaks
at a certain wavelength, the peak shifts to shorter wavelengths for higher
temperatures, and the area under the curve grows rapidly with increasing
The Wien and Stefan-Boltzmann Laws
The behavior of blackbody radiation is described by the Planck Law, but we can
derive from the Planck Law two other radiation laws that are very useful. The
Wien Displacement Law, and the Stefan-Boltzmann Law are illustrated in the
The following figure illustrates the Wien law in action for three different stars of quite different surface temperature. The strong shift of the spectrum to shorter wavelengths with increasing temperatures is apparent in this illustration.
For convenience in plotting these distributions have been normalized to unity at the respective peaks; by the Stefan-Boltzmann Law, the area under the peak for the hot star Spica is in reality 2094 times the area under the peak for the cool star Antares.
|Some Blackbody Temperatures|
|Radio||> 10||< 10-5||< 0.03|
|Microwave||10 - 0.01||10-5 - 0.01||0.03 - 30|
|Infrared||0.01 - 7 x 10-5||0.01 - 2||30 - 4100|
|Visible||7 x 10-5 - 4 x 10-5||2 - 3||4100 - 7300|
|Ultraviolet||4 x 10-5 - 10-7||3 - 103||7300 - 3 x 106|
|X-Rays||10-7 - 10-9||103 - 105||3 x 106 - 3 x 108|
|Gamma Rays||< 10-9||> 105||> 3 x 108|
Blackbody radiation corresponds to radiation from bodies in thermal equilibrium. We will consider later the emission of non-thermal radiation, which doesn't follow a blackbody law. Such radiation is often produced by violent collisions rather than equilibrium heating. For example, in astrophysical environments radiation at the long and short wavelength ends of the above table is more likely to be produced by non-thermal processes.
Here are three Java applets illustrating some important properties of blackbody radiation.