Solar absorption lines
- From: Scott <ss@xxxxxxxxxxxxxxxx>
- Date: Sat, 08 Jul 2006 00:15:26 +1000
Hi all,
What process causes absorption lines in solar spectra (when
measured from above the Earth's atmosphere)?
I understand how absorption lines can occur in _extra_-solar
spectra due to an intervening cloud of interstellar gas/dust. The
atoms in the cloud can absorb part of the (effectively blackbody)
continuum (depending on the energy level of its electrons) & then
re-emit it in a random direction, away from the observer.
However, in the case of the Sun [1] the "cloud" (actually the upper
atmosphere of the Sun) completely surrounds the Sun so the _net_
effect of scattering is zero - for any photon heading toward an
observer that is scattered _away_, there is another photon heading
away from the observer that is scattered _toward_ them, statistically
speaking.
I thought, perhaps, that the cascade effect (photons being
absorbed & then re-emitted, potentially at different wavelengths)
could explain the absorption lines. But then I would expect to see
_emission_ lines on top of the continuum - if there's a reduction
of H-alpha, say, then there must be an abundance of some other
wavelength(s). (ie. conservation of energy)
But if I look at solar spectra, I don't see any significant
emission lines convolved with the continuum.
Any help to align my understanding with reality would be muchly
appreciated.
Scott
[1] actually this occurs in all stars, of course.
.
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