Re: Solar absorption lines
- From: willner@xxxxxxxxxxxxxxx (Steve Willner)
- Date: 10 Jul 2006 17:34:37 -0400
In article <44ae6c8c$0$22360$afc38c87@xxxxxxxxxxxxxxxxxxxx>,
Scott <ss@xxxxxxxxxxxxxxxx> writes:
What process causes absorption lines in solar spectra (when
measured from above the Earth's atmosphere)?
From your subsequent posts, I suspect you are thinking of low-densityplasmas. (So, I suspect, are several people who have replied.)
Stellar atmospheres are different because they are a lot denser. The
buzzword would be that (to a first approximation) you have "local
thermodynamic equilibrium," not "detailed balance." In other words,
a common physical process is that a photon is absorbed and excites an
atom. The atom then decays _not_ by radiation but by a collision,
imparting the extra energy to an atom or ion nearby. Of course the
inverse process also occurs, but it's less common because there is a
net transfer of energy outward. This gives absorption lines without
creating corresponding emission lines in other directions.
There's also a geometric issue. Most spectra of the Sun are of a
small area of the disk, so you wouldn't see the "balancing" emission
lines in these spectra even if the lines existed. However, this is
not the major effect. There are some "whole-disk" spectra of the
Sun, and of course there are plenty of whole-disk spectra of sunlike
stars that show absorption lines. (Postings about the chromosphere
"flash spectrum" are correct, but the net emission doesn't come close
to balancing the absorption as can be seen in the whole-disk
spectra.)
The best way to start thinking about stellar atmospheres is a
"plane-parallel" model. Imagine a single square centimeter on the
surface of the Sun and a very long column below it. At any depth,
the gas has a fixed temperature and density, the same in all
horizontal directions, but density and temperature both increase as
you go down. (We ignore the temperature inversion; absorption lines
are formed below it, so for this purpose just take the temperature
minimum as the "surface.") Do you see why this model has to produce
absorption lines as viewed from above? If so, your question is
answered. If not, do you understand the concept of "optical depth?"
That's the key to answering your question in more detail than in the
first paragraph.
If you want to understand this at a serious level, you will need to
find a textbook. Dimitri Mihalas and John Jefferies are two authors
who have written good texts, but there may be even better ones around
these days. (No prizes for guessing how long it has been since I
actually studied this stuff!)
--
Steve Willner Phone 617-495-7123 swillner@xxxxxxxxxxxxxxx
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
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