Continuous Solar Spectra
Name: Justin C.
My query is: why is it assumed that the light coming
regions of the Sun contains a continuous spectrum? (This assumption must
made if we are to assume all dark lines are due to absorption at some
in the journey of the light; If light has been absorbed it must have been
there in the first place!). Is it just that there are so many different
possible jumps (because there are so many elements present in the Sun)
all wavelengths of photons are emitted?
You raise a valid question. Indeed the temperature of the Sun (or other
"hot" star) is treated as a blackbody radiator because the temperature of
the "inside" is a very hot plasma (a collection of charged particles). On
the surface, there are sunspots (storms) that are cooler and appear
"black" compared to the surroundings but are still bright and hot compared
to "normal" terrestrial temperatures. It is known from lab experiments
that hot plasmas produce essentially a continuum of radiation, so that is
some of the data supporting continuous radiation from "inside" the Sun.
Also the theoretical model of how stars produce radiation from the
reactions occurring within the Sun/stars is reasonably well understood and
the model predicts a continuum of radiation. Even if there were some
variations in the internal solar spectra it would be quite difficult to
distinguish it because the Planck blackbody radiation is just so intense
at those temperatures, small variations would be swamped by that
radiation. And we are talking about "stable" stars, during a supernova or
other instable stars, or neutron stars and black holes etc. things are
much more complicated.
It is not just speculation that the absorption lines observed in high
resolution spectra of the Sun (and other hot stable stars) are due to cooler
gases surrounding the bright source. It is possible to measure the relative
intensity of those absorptions (from any number of elements and/or
molecules) and from that data obtained in terrestrial labs to infer the
temperature of the absorbers from the relative intensities of the
absorptions. This applies not only to visible light but also applies to
spectra extending from x-rays, ultraviolet, visible, infrared, and even
microwave frequencies. By piecing all the data together to make an
internally consistent match to the observed solar (or other star) spectra it
is possible to determine whether the absorbers/emitters that are on the
stellar surface, the various levels of the solar atmosphere, and even those
absorbers/emitters that are in space between earth and the light source.
Nonetheless, I think you raise a perceptive question.
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Update: June 2012