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All bodies (objects) emit electromagnetic radiation, no matter what their temperature is. The type of radiation emitted depends on the temperature of the body.

Hot bodies emit a continuous range of electromagnetic radiation at different energy values – this means that the radiation that is emitted is spread out over a range of different frequencies and wavelengths.

As a body gets hotter, it emits more high frequency electromagnetic radiation than low frequency. The higher the frequency, the shorter the wavelength. This means that the intensity of the radiation emitted is greater for a hotter body.

As the surface temperature of a hot body, such as a star, increases:

• the intensity of radiation it emits increases
• the wavelength that corresponds to the peak intensity gets shorter

The graph for a hot star (such as a blue supergiant) peaks over a shorter wavelength than a cooler star (such as a red giant). The intensity of radiation for each frequency is higher for a very hot star than for a cooler one.

Black bodies - Higher

A perfect black body is a theoretical object. It would have these properties:

• it would absorb all the radiation that falls on it
• it would not reflect or transmit any radiation

An object that is good at absorbing radiation is also a good emitter, so a perfect black body would be the best possible emitter of radiation.

There are no known objects that are perfect at absorbing or emitting all the radiation, of every possible frequency, that may be directed at them. Some objects do, however, come close to this and these are referred to as black bodies.

Stars are considered to be black bodies because they are very good emitters of most wavelengths in the electromagnetic spectrum. This suggests that stars also absorb most wavelengths. Whilst there are a few wavelengths that stars do not absorb or emit, this figure is very low, so they can be treated as black bodies. Planets and black holes are also treated as nearly perfect black bodies.