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

Ibrahim Rosales

Ibrahim Rosales

Answered question


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.

Answer & Explanation



Beginner2022-07-24Added 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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