Re: Solar absorption lines

Hi George,

If a patch of gas is illuminated by a source at
some brightness covering 2 pi steradians and then
emits the same energy into 4 pi sterardians it
must appear less bright.

Yes, but if the gas covers 4 pi steradians (surrounds the emitter)
cancellation occurs.

Why can't there be a balance when the gas surrounds the emitter,
as in the case of the Sun?

The Sun doesn't surround a patch of gas in
the chromosphere, that's the key. (see below)

I'm talking about a situation where the _gas_ surrounds the
_emitter_.

Suppose a photon is scattered away from us:

(Sun) -> * Earth
|
v


You suggest another is scattered to replace it:

(Sun) * -> Earth
|
^
|

The situation I am referring to is different.

My ASCII-art is not so good, so I drew a diagram & scanned it in.
Please look at:

http://members.optusnet.com.au/scottsmedley/tmp/scatter.png

I now refer to this diagram.

Consider photon A emitted from the photosphere & heading directly
toward the observer. If the photon is of the right wavelength, say
656nm (H-alpha), then it can be absorbed by a H atom in the
chromosphere and re-emitted in a random direction, identified by A'.
Obviously the observer would never see this photon. You, & several
others, have said as much already.

Now consider photon B which (were it not for the chromosphere) would
not normally be seen by the observer. There is a small chance that this
photon can be scattered directly toward the observer, identified by B'.

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.
.

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