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Name: Justin C.
Status: educator
Grade: other
Location: N/A
Country: N/A
Date: 11/29/2005


Question:
My query is: why is it assumed that the light coming from inner regions of the Sun contains a continuous spectrum? (This assumption must be made if we are to assume all dark lines are due to absorption at some point 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) that all wavelengths of photons are emitted?


Replies:
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.

Vince Calder



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