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| The '''source function''' is a characteristic of a stellar atmosphere, and in the case of no [[scattering]] of photons, describes the ratio of the emission coefficient to the absorption coefficient. It is a measure of how photons in a light beam are removed and replaced by new photons by the material it passes through. Its units in the cgs-system are erg s<sup>-1</sup> cm<sup>-2</sup> sr<sup>-1</sup> Hz<sup>-1</sup> and in SI are W m<sup>-2</sup> sr<sup>-1</sup> Hz<sup>-1</sup> . The ''source function'' can be written | |
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| :<math>S_{\lambda} \ \stackrel{\mathrm{def}}{=}\ \frac{j_{\lambda}}{\kappa_{\lambda}}</math>
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| where <math>j_{\lambda}</math> is the emission coefficient, <math>\kappa_{\lambda}</math> is the absorption coefficient (also known as the [[opacity (optics)|opacity]]). Putting this into the equation for [[radiative transfer]] we get
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| :<math>-\frac{1}{\kappa_{\lambda} \rho} \frac{dI_{\lambda}}{ds} = I_{\lambda} - S_{\lambda} </math>
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| where ''s'' is the distance measured along the path traveled by the beam. The minus sign on the left hand side shows that the intensity decreases as the beam travels, due to the absorption of photons.
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| ==See also==
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| * [[Radiative transfer]]
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| * [[Opacity (optics)]]
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| ==References==
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| * {{cite book
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| | author=B.W. Carrol, D.A. Ostlie
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| | title=An Introduction to Modern Astrophysics
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| | publisher=Addison-Wesley Publishing Company, Inc.
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| | year=1996
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| | isbn =0-321-21030-1 }}
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| [[Category:Astrophysics]]
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| [[Category:Stellar astronomy]]
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| {{star-stub}}
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Revision as of 03:49, 20 February 2014
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