Radiation Laws and Spectra
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All objects absorb and emit electromagnetic radiation (except black holes
which do not emit anything!).
 The radiation or energy emitted by objects is termed a spectrum. This
radiation can be emitted at many wavelengths or frequencies at the same
time.
 The spectra of celestial objects can be:
 Continuous – Energy is emitted over a range of wavelengths
(frequencies) without gaps
 Pure emission – Energy is emitted only at a number of specific
wavelengths with dark gaps between the emission lines
 Continuous with absorption – Energy is emitted over a range of
wavelengths with occasional dips and gaps.
 Continuous with emission and absorption - Energy is emitted over a
range of wavelengths with occasional dips, gaps and peaks.
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A blackbody “curve” defines the spectrum of an object which can be
described by a single temperature.
In astrophysics, temperatures are usually expressed on the Kelvin scale,
where 0 K is known as “absolute zero”. Water freezes at 273 K; water boils
at 373 K.
Blackbody spectra have a specific shape with a single peak, at which the
emission reaches a maximum value.
For a blackbody, the wavelength of peak emission is proportional to
1  temperature, a relationship known as Wien’s law. As the temperature
increases, the peak wavelength decreases (moves towards bluer
wavelengths).
The total energy radiated by objects also depends on the object’s
temperature. Known as Stefan’s Law, the total energy is proportional to the
4th power of the object’s temperature:
E (total)  T 4
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The surface temperature of celestial objects can be determined by measuring
the object’s spectra, and finding the peak value of emitted light.