Do Emission and Absorption Spectra Match? A Non-Physics Minded Tourist's Guide

[Paul Howard A]

Basic stuff. Do emission and absorption spectra match? If so, why wouldn't hot stellar atmospheres exhibit both, cancelling? I'm a tourist...not physics minded..

 

[DrClaude]

Core emission/absorption is close to blackbody, so emission = absorption. For the stellar atmosphere, however, you get the usual result for a gas. The light coming from the star core is going approximately in one direction, while emission from the atmosphere will be isotropic, so from an external point of view, some frequencies in the spectrum will be attenuated.

 

[Paul Howard A]

That makes a lot of sense. Thank you.

 

[Hyperfine]

There are also cases in which both an emission line and an absorption line of the same species are observed in the spectrum at slightly different wavelengths. These are known as P-Cygni line profiles. They are found in stars with an expanding atmosphere. The emission component is always to the red, the absorption component always to the blue. The line shifts result from Doppler shifts associated with the motion of the material within the atmosphere.

For an example and explanation, see: P-Cygni line profile

 

[Ken G]

Another example of emission and absorption in the same line is a consequence of hot surface layers called a chromosphere. The Sun's chromosphere is generally weak, but it gets stronger (i.e., thicker) during flare activity. In those regions where the chromosphere gets thick and hot, it can produce lines in which, far from line center you see the blackbody continuum of the star, then as you get into the line it appears darker because you are seeing the attenuation effect mentioned above, but closer still to line center you see an augmentation effect due to the hot chromosphere. (Very close to line center, you see a "central reversal", which is a scattering effect where high opacity near line center just scatters light back downward, which gets redistributed to frequencies farther from line center that escape more easily.) An example of this is the "jet bright core" profile from:
https://www.researchgate.net/profil...omospheric-anemone-jets-bright-core-taken.png
I presume the asymmetry is from blueward absorption, a mass-motion effect akin to the "P Cygni" effect above (hence the "jet" here).