Lignes Fraunhofer

In 1802, William Hyde Wollaston first noted the existence of dark lines in the solar spectrum. However, it was Joseph von Fraunhofer who, in 1814, independently rediscovered and systematically studied these lines using a high-quality prism and telescope, mapping over 570 of them. He labeled the most prominent lines with letters from A to K, a notation still used today. The profound significance of these lines was not understood until 1859, when Gustav Kirchhoff and Robert Bunsen established that the pattern of lines was unique to each chemical element. They realized that the dark lines in the solar spectrum matched the bright emission lines of known elements when heated. This led to the groundbreaking conclusion that the Fraunhofer lines are absorption spectra. Light from the Sun’s continuous spectrum, produced by its hot, dense core (photosphere), passes through the cooler, less dense outer layers (chromosphere). Atoms in the chromosphere absorb photons at the precise frequencies corresponding to their allowed electronic transitions, re-emitting them in random directions. This process effectively removes those specific frequencies from the light reaching Earth, creating the dark lines. This discovery transformed astronomy from a science of position and motion to one of astrophysique, enabling the study of the physical nature and chemical makeup of distant stars.