Study of the spectroscopic binary [alpha] Andromedae
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Spectrum analysis may perhaps be said to date from the year 1666 when Sir Isaac Newton discovered that rays of light of different colors are of different refrangibility. Later he discovered that a beam of white light coming from a small round opening and passed through a prism would be spread out into a band of color. Fraunhoffer in 1823 substituted for the round hole a narrow slit, and found that in the case of the sun the band of color was crossed by many fine dark lines. He also turned the instrument on some of the brighter stars, and found their spectra in general different from that of the sun. For example some showed bright lines. But the true explanation of the bright and dark lines remained a mystery until the announcement in 1859 of Kirchoff's famous law.