Re: sun's spectrum

[brian whatcott <inet@intellisys.net>]

At 21:39 1/29/98 -0800, Leigh wrote:
> > When you look at the sun's light in  a spectroscope, you see the entire
> > spectrum. Since the sun is made mainly of hydrogen, where are the hydrogen
> > lines? Or the helium lines?
....
> The Sun is a very large ball of hot gas and plasma. It is an
> "optically thick" source. Even though it radiates light with spectral
> lines characteristic of the hydrogen and helium from the apparent
> surface we call the photosphere, that is not all we see. We see also
> the continuous spectrum of light coming from the plasma, and from
> other minority elements and ions. These latter lines originate from
> deeper within the Sun, but ultimately the Sun is optically thick at
> all wavelengths.
>
> We do see spectral lines in the Sun's spectrum, but paradoxically they
> are absorption lines, not emission lines like we see in the neon sign.
> Outside the photosphere there is a layer of cooler gas called the
> "chromosphere". Gas in this layer, which is optically thin at most
> wavelengths, absorbs light of wavelengths corresponding to spectral
> lines of the elements present, and the Sun's spectrum looks like that
> of an incandescent continuous source which is interrupted by missing
> wavelengths. These lines are called "Fraunhofer lines" after the man
> who discovered them (about 1830, I think). Similar features exist in
> the spectra of stars. They are quite helpful in revealing to us
> characteristics of the stars themselves.
>
> Leigh
This is a nit-picking critique of Leigh's rather nice exposition 
(one hardly expects to score major points off this source...)

As Kirchhoff pointed out in connection with thermal radiation, good 
absorbers are in general good emitters. (In fact Prevost [1792] said 
as much.)
 So these 'absorption' lines do not in fact represent a loss of flux - 
rather a contrast effect with the continuum radiation surrounding.


brian whatcott <inet@intellisys.net>
Altus OK