Re: Solar absorption lines

Scott wrote:

Now integrate this effect over the entire surface of the Sun.
There is basically 3d sphere of H atoms randomly scattering, say,
H-alpha photons over 4 pi steradians. Statistically, the observer
is going to receive a large number of _scattered_ H-alpha photons.

So I believe it's not the scattering effect that is contributing
to the absorption lines, it must be something else. You can't scatter
photons _away_ from an observer when the gas surrounds the emitter.

Scott.

Scott,

The directions of the scattered photons are distributed over 4 pi
steradians (i.e. the full sphere), but the incident photons are
distributed only over 2 pi steradians (coming only from the solar
surface), so per se the intensity should be reduced by a factor 1/2.
You can of course ask what happens to the other half (the one that's
being scattered back towards the sun) and if you assume that through
subsequent scatterings this will also eventually go towards the
observer, then you are right that the scatterings should not change the
intensity. In any case, even if the backscattered photons are lost for
some reason, the geometrical effect should result at best in a factor
1/2 reduction. So the solar absorption lines have essentially nothing
to do with a geometrical effect but are due to other mechanisms:
1) the Doppler effect (photons can essentially not penetrate the solar
atmosphere within the line as the opacity here is so high; they only
get through once they have been shifted out of the line in the course
of the scatterings due to the frequency changes by the Doppler effect).
2) Photoionization (photons within the line are trapped for so long in
the solar atmosphere that they have a high probability of ionizing
excited states of hydrogen; this means they are lost from the line)

Thomas

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