Re: sun's spectrum

[Leigh Palmer <palmer@sfu.ca>]

> 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?

A lit neon sign tube is filled with a tenuous gas and hot plasma. It
is called an "optically thin" source because it is nearly transparent
at most wavelengths. When one looks at a neon sign tube with a
spectroscope one sees bright lines characteristic of the element neon.
There is also a much dimmer continuum of light coming from the free
electrons in the plasma.

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