Absorption lines and emission lines in stellar spectra

  • #1
Haynes Kwon
Gold Member
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Summary:

Why do we see more absorption lines in stellar spectra than emission lines?

Main Question or Discussion Point

Why do we see more absorption lines in stellar spectra than emission lines?
 

Answers and Replies

  • #2
mathman
Science Advisor
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406
The surface of the sun is cooler than the interior.
 
  • #3
283
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Emission lines are present in spectrum of only few types of stars, see
https://en.m.wikipedia.org/wiki/Emission-line_star

Most of the stars exhibits only absorption lines (superimposed on the continuous blackbody radiation). As the light radiated in photosphere propagates through upper layers of stellar atmosphere, some of the wavelengths are absorbed by colder gas.
 
  • #4
sophiecentaur
Science Advisor
Gold Member
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Why do we see more absorption lines in stellar spectra than emission lines?
The light from the very hot interior or near the surface will have a continuous spectrum. The material is very dense and the atoms / ions will be moving very fast. Those factors cause the emitted lines to become bands, which are not easy to identify.
Otoh, the cooler, lower pressure gas which surrounds a star will absorb only narrow band ranges (lines) of the spectrum.
I think you would only be likely to find emission lines from low density gases (say, on nebulae) that are subjected to high enough levels of high energy particles and photons from 'nearby' stars. They can produce line emission spectra as they are under more like 'lab' conditions.
The power of emission lines will be low, too, because of the circumstances (very distant from their energy source) - much lower than the absorbed power by gases which surround a star. They will be visible against the Dark background of deep space.
I think that makes some logical sense.
PS Those conditions can happen where the star is of the kind described in the above link where there is a high level of UV energy from a very hot star and that causes excitation of the nearby gases to emit optical energy photons against a relatively low optical background from the star itself.
 

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