How are we able to observe Fraunhofer lines in a star's spectrum?

Ibrahim Rosales 2022-07-23 Answered
How is it possible to observe Fraunhofer lines in the emission spectrum of any star, since the elements absorbing the radiation couldn't possibly absorb all the radiation corresponding to a particular wavelength.
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Answers (1)

kuglatid4
Answered 2022-07-24 Author has 12 answers
Absorption lines are not black. You are correct that some radiation still emerges in our direction, even in the central wavelengths of the sodium D lines.
The presence of absorption lines is because of the temperature gradient in the Sun and that the blackbody radiation field is T 4 .
As we move outwards in the solar photosphere, at some point, the integrated material above that point is insufficient to stop most photons escaping. This is a definition of where the photosphere is.
However, the depth of this point is wavelength dependent. If we are looking at a "continuum" wavelength, then the opacity is relatively low (dominated by H- recombination) and the radiation escaping comes from a relatively deep layer. If we are looking at the centre of a Na D-line then the opacity is high and the emission comes from a region higher up in the photosphere. Because of the temperature gradient, the former layer is hotter than the latter. And because of the T 4 dependence, the former is bright while the latter is comparatively dark.
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