Gustav Kirchhoff read the Sun's ingredients from its light
In 1859 Gustav Kirchhoff showed that the Sun contains sodium, without leaving his laboratory in Heidelberg. By passing light through a spectroscope, he and the chemist Robert Bunsen learned to identify elements by their colour fingerprints. Two years later the same method revealed two metals nobody had seen before, caesium and rubidium.
Kirchhoff was born in 1824 in Königsberg, Prussia, the son of a lawyer. While still a student there, in 1845, he worked out as a seminar exercise the two rules for electrical circuits that now bear his name: current flowing into any junction equals current flowing out, and voltage drops around any closed loop sum to zero. The exercise became his doctoral thesis. He went on to teach at Berlin and Breslau before moving to Heidelberg in 1854, where the partnership with Bunsen began.
Together they improved the spectroscope Joseph von Fraunhofer had built in 1814. Kirchhoff summarised what they saw in three rules. A hot solid, liquid or dense gas glows at every wavelength, giving a continuous rainbow. A thin glowing gas shines only at particular wavelengths. And a thin, cooler gas placed in front of a continuous source removes those same wavelengths, leaving dark lines. He had no idea why; atomic energy levels would not explain the lines until Niels Bohr's model of 1913.
He also coined the term black body in 1860 and proposed a universal law of thermal radiation, proved in 1861. The unknown function hiding in that law is what later led Max Planck to the quantum of action, and from there to quantum mechanics. In 1857 he calculated that an electrical signal in a wire with no resistance would move at the speed of light.
His range was unusual. He fixed a long-standing flaw in the theory of vibrating plates by showing how to get the correct conditions at a free edge, which is why thin-plate theory is still named after him and Augustus Love. He put Huygens' principle of wave propagation on firmer mathematical footing, proved a theorem in graph theory about counting spanning trees, and related heats of reaction to heat capacities. He married his mathematics professor's daughter, Clara Richelot, returned to Berlin in 1875, and died there in 1887; his grave lies a few metres from the Brothers Grimm.
Source: Gustav Kirchhoff
