Lesson plan
Source function
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 −1 cm −2 sr −1 Hz −1 and in SI are W m −2 sr −1 Hz −1 . The source function can be written : S_{\lambda} \ \stackrel{\mathrm{def}}{=}\ \frac{j_{\lambda}}{\kappa_{\lambda}} where j_{\lambda} is the emission coefficient, \kappa_{\lambda} is the absorption coefficient (also known as the opacity). Putting this into the equation for radiative transfer we get : -\frac{1}{\kappa_{\lambda} \rho} \frac{dI_{\lambda}}{ds} = I_{\lambda} - S_{\lambda} 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.
Article summary
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 −1 cm −2 sr −1 Hz −1 and in SI are W m −2 sr −1 Hz −1 . The source function can be written : S_{\lambda} \ \stackrel{\mathrm{def}}{=}\ \frac{j_{\lambda}}{\kappa_{\lambda}} where j_{\lambda} is the emission coefficient, \kappa_{\lambda} is the absorption coefficient (also known as the opacity). Putting this into the equation for radiative transfer we get : -\frac{1}{\kappa_{\lambda} \rho} \frac{dI_{\lambda}}{ds} = I_{\lambda} - S_{\lambda} 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.
Key claims
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Discussion questions
- Summarise the article in your own words. Which single claim would you verify first, and why?
- Which claim in this article is most likely to be revised as the sources update? How would you check its current status?
- If two sources disagreed about one of these facts, what would you need in order to trust one over the other?
Bibliography
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