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George Ellery Hale: Solar Physics and Related Inventions, 1927


When a mixture is heated up until it glows brightly and that excited "glow" is passed through a narrow slit and then through a prism, only certain bright color lines in the resulting spectrum are observed. The spectral lines emitted are uniquely characteristic of the elements in the mixture and each element has a unique, unchanging line pattern, an analytical "fingerprint."

When sunlight is examined in the same way, the fingerprint isolates the characteristics of elements in the Sun's atmosphere. Gases in the Sun's atmosphere are slightly cooler than its surface and the chemical elements in the gases absorb certain frequencies of the light energy. The result is a spectrum with a number of dark lines corresponding to the energy absorbed by those elements, an absorption spectrum fingerprint.