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Secrets of Sunlight and Stellar Spectra

In 1666, Isaac Newton demonstrated that the seemingly white light from the sun contained all of the colors of the rainbow by passing a narrow beam of sunlight through a series of glass prisms. Newton used the Latin word spectrum (pl. spectra) to describe the resulting rainbow of colors [2]. Over the next 200 years, a chain of scientists continued along the path that Newton had forged, each building upon the work of those before him.

When the spectrum created by sunlight was highly magnified, a series of dark vertical lines superimposed on the continuous colored spectrum were discovered. (Fig. 3). Similar lines were found in the spectra of starlight, but the lines were of varying width, sharpness and intensity, and appeared in different positions along the spectrum. Every star studied had its own unique spectral "fingerprint".

 
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Figure 3
Example of solar spectrum showing the dark lines. This type of spectrum, an absorption line spectrum, is also produced by stars. Image obtained from the Observatoire de Paris website.

Laboratory experiments revealed that the gasses created by burning different chemical elements in a flame produced spectra of a different nature; instead of dark lines on a bright background, bright lines on a dark background were produced (Fig. 4). An effort was well underway to catalog the bright line spectra for all of the then known elements, when it became apparent that the series of bright lines in the laboratory spectra could often be matched to the dark lines found in the solar and stellar spectra. This was the first evidence that the sun and the stars contained many of the same elements found on Earth.

 
orifig4.gif (11633 bytes) Figure 4

Diagram illustrating the three types of spectra. An emission line spectrum is produced by a hot, thin gas. Such were the spectra produced in the laboratory experiments on the chemical elements. A continuous spectrum is produced by a hot, dense object, while an absorption line spectrum is the result of the light from a hot, dense object passing through a cloud of cooler, less dense gas. This is the type of spectrum produced by the sun and stars, as the extremely hot, dense inner material of the star radiates its energy in the form of light through its cooler outer atmosphere. Diagram adapted from one contained in online course notes for Physics class at Brown University.  

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Copyright ©2001 C. Gino