They are the fine stripes that are observed when the light of a celestial object is dispersed in the different wavelengths that compose it by means of a spectroscope.
Each line is representative of a certain chemical element present in the star and has a well defined wavelength.
When an electron passes from one energy level to another, it emits a photon with a certain energy. These photons give rise to emission lines in a spectroscope.
The spectral lines were first seen in the spectrum of the sun by William Wollaston in 1802. However, they were not systematically studied until 1814, when an optometrist named Joseph von Fraunhofer observed and cataloged them. Fraunhofer carefully documented the positions of the lines, but did not attempt to explain why they were there. In late 1850, physicist Gustav Kirchhoff decided to investigate this matter further with the help of chemist Robert Bunsen.
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