The German physicist Gustav Robert Kirchhoff (1824-1887) is best remembered for his pioneering work in spectroscopy that permitted investigation of the chemical composition of stars.
Gustav Kirchhoff was born on March 12, 1824, in Königsberg, East Prussia, the son of a lawyer. He attended the local gymnasium and entered the University of Königsberg at the age of 18. Among his teachers were Franz Neumann, the noted theoretical physicist, and Friedrich Richelot, the mathematician. Shortly after he received his doctorate in 1847, he married Richelot's daughter, Clara; they had two sons and two daughters. Also in 1847, he received a rarely awarded travel grant from the university for a study trip to Paris, but the political situation forced him to cancel the plans. In 1848 Kirchhoff became privatdozent in Berlin, and 2 years later he obtained the post of extraordinary (associate) professor at Breslau. It was there that he first met Robert Bunsen. By 1854 both Kirchhoff and Bunsen were working together in Heidelberg.
The investigation of spectra with prisms had been going on for decades. There had also been several guesses made as to the identity between some lines in the solar spectrum and in spectra produced in laboratories. But it was Kirchhoff who, one afternoon in the summer of 1859, looked at the interaction of sunlight and the light of table salt burning in the flame of the Bunsen burner and said, "There must be a fundamental story here." When he returned to the laboratory the next day, he had the solution to his observation. It is known as Kirchhoff's law of radiation: the relation between the powers of emission and the powers of absorption for rays of the same wavelength is constant for all bodies at the same temperature. This law also implies that the bodies absorb more readily the radiation of such wavelengths as they tend to emit. Furthermore, the law implies that the greater the opacity of a body, the more complete its spectrum, and that the true emission spectrum of a substance is obtained in its gaseous state. Kirchhoff's now famous paper, written with Bunsen and published in 1859, also stated that "the dark lines [Fraunhofer lines] of the solar spectrum which are not caused by the terrestrial atmosphere, arise from the presence in the glowing solar atmosphere of those substances which in a flame produce bright lines in the same position."
Kirchhoff and Bunsen became celebrities overnight. Subsequent scientific developments did full justice to the elation of the moment. Spectroscopy turned out to be the magic key to a great number of practical discoveries, and half a century later it ushered in the era of modern atomic physics. In a sense, Kirchhoff's great success in spectroscopy drew attention away from his varied contributions to every branch of physics. He occupied the chair of theoretical physics at the University of Berlin from 1875 until his death on Oct. 17, 1887.
A short but informative account of Kirchhoff's life and work was written by Robert von Helmholtz and is available in English in the Smithsonian Institution Report (1889). The first two chapters of Edward Charles Cyril Baly, Spectroscopy (1905; 3d ed. 1927), contain much useful information on the origins and further developments of classical spectroscopy.