If you walk down the famous Hauptstraße (Main Street) in the old town of Heidelberg you may notice a memorial plate on the facade of the “Haus zum Riesen” (No. 52) which tells you that Gustav Kirchhoff and Robert Bunsen laid here the foundations of modern spectroscopy. Besides the new opportunity to detect elements in small quantities and the discovery of two new elements (caesium and rubidium) from salt of Dürkheim mineral water), a pioneering major breakthrough of this marvellous collaboration of a physicist and a chemist was the application of spectral analysis to celestial bodies – opening the field of cosmic chemistry.
Less well known is the background of their idea. In a 1912 article, Heidelberg astronomer Max Wolf mentions in a footnote an oral tradition by Henry Enfield Roscoe via Bohuslav Brauner (a student of Bunsen and Roscoe): On the occasion of a visit of the Grand Duke of Baden in Heidelberg (probably on 1 June 1860), the Heidelberg Castle was illuminated (“Schlossbeleuchtung”) at night with Bengal fire. Bunsen pointed from the roof of his laboratory a prism set against these flames and observed clearly the green lines of barium and the red lines of strontium. He said to Kirchhoff: “If we could see at that distance, which substances were glowing in these flames – why could we not also determine out of which substances the celestial bodies consist?” – Thus, the spectral analysis of the sun and the stars was born.
At a workshop at MPIK for this year’s International Summer Science School Heidelberg students, I prepared an experiment demonstrating this historic discovery. The students were looking at the flames of red and green Bengal fire through a simple hand spectroscope and could identify the spectra of strontium and barium.