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Brightness temperature is the temperature at which a blackbody in thermal equilibrium with its surroundings would have to be in order to duplicate the observed intensity of an object at a frequency ν. This is a useful concept only for radiation that obeys the Rayleigh-Jeans Law, and it is extensively used in radio astronomy and planetary science.
Additional recommended knowledge
For a blackbody, the Planck distribution gives:
In the Rayleigh-Jeans limit of low frequency, we find:
This can be rewritten to define the brightness temperature as:
Brightness temperature is a useful diagnostic for temperature measurement if the astronomical source is a blackbody and we are in the Rayleigh-Jeans regime. It is not useful if the source is non-thermal and/or we are in the high frequency limit.
If the Planck distribution is reintroduced into the expression for brightness temperature we find:
So for the Sun, where the temperature may be estimated to be 6000K, we can plot the brightness temperature against wavelength.
Compare with color temperature.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Brightness_temperature". A list of authors is available in Wikipedia.|