Radiation Laws

Stars, like other hot bodies, radiate electromagnetic energy of all different wavelengths. Energy due to temperature is called thermal radiation. The temperature of a star determines which wavelength is brightest.

Stars radiate energy practically as a blackbody, or theoretical perfect radiator. The intensity of radiation emitted over a range of wavelengths depends only on the blackbody's temperature. Wien's law of radiation states that the wavelength, ^max, at which a blackbody emits the greatest amount of radiation is inversely proportional to its temperature (T). The formula is

where ^max is in centimeters and T is in kelvin (K). Thus the hotter a star, the shorter the wavelength at which it emits its maximum radiation.

Some stars are thousands of degrees hotter than others. You can judge how hot a star is by its color (wavelength). The hottest stars look blue-white (short wavelength), and the coolest stars look red (long wavelength). Look in the sky for the examples cited in Table 2.1.

Short Wavelength (nanometers) Long

Figure 2.5. The Sun's thermal radiation spectrum. All blackbody radiation spectrums have the same shape. Hotter stars emit more energy at all wavelengths, and the peak shifts to shorter wavelengths.

Short Wavelength (nanometers) Long

Figure 2.5. The Sun's thermal radiation spectrum. All blackbody radiation spectrums have the same shape. Hotter stars emit more energy at all wavelengths, and the peak shifts to shorter wavelengths.

TABLE 2.1 Four Hot and Cool Stars

Surface Temperature

Season Star Constellation Color (K)

Summer Vega Lyra Blue-white 10,000

Summer Antares Scorpius Red 3,000

Winter Sirius Canis Major Blue-white 10,000

Winter Betelgeuse Orion Red 3,400

The Stefan-Boltzmann radiation law states that the total energy (E), emitted by a blackbody is proportional to the fourth power of its absolute temperature (T). Thus a star that is twice as hot as our Sun radiates 24, or 16, times more energy than the Sun.

A radiation spectrum shows how much energy a body radiates at different wavelengths, which wavelengths it radiates most intensely, and the total amount of energy it radiates at all wavelengths (indicated by the area under the curve).

Examine Figure 2.5. (a) The Sun radiates most intensely in the_

wavelengths. (b) The total amount of energy that the Sun radiates as visible light is (more, less)_than the amount radiated outside the visible region.

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