Reference Work Entry

Biographical Encyclopedia of Astronomers

pp 1220-1221

Date:

Kirchhoff, Gustav Robert

Born Königsberg (Kaliningrad, Russia), 12 March 1824

Died Berlin, Germany, 17 October 1887
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Kirchhoff, Gustav Robert. Courtesy of History of Science Collections, University of Oklahoma Libraries

Gustav Kirchhoff founded spectral analysis (with Robert Bunsen ) and discovered fundamental properties of the absorption and emission of electromagnetic radiation. His father, a government law councillor, was devoted to the Prussian state and encouraged his sons to similarly serve the state to the best of their abilities. Kirchhoff enrolled at the University of Königsberg, where he studied mathematical physics under Carl Gustav Jacob Jacobi (1804–1851) and Franz Ernst Neumann (1798–1895). After graduation in 1847 and a short scientific visit to Paris, he held an unsalaried lectureship in Berlin. In 1850, Kirchhoff was appointed extraordinary professor of physics at Breslau, where the arrival of Bunsen the following year inaugurated an immensely fruitful collaboration that would revolutionize astronomy. Kirchhoff moved to Heidelberg as professor of physics in 1854, following Bunsen who had gone there 2 years before.

In 1857, Kirchhoff married Clara Richelot, daughter of one of his former mathematics professors at Königsberg. This first marriage, which gave the couple four children, came to a premature end in 1869 with Clara’s untimely death. These were difficult times for Kirchhoff, as he had just the year before suffered a debilitating injury to a foot, which left him having to use crutches or a wheelchair for extended periods of time thereafter. In 1872, he married Luise Brömmel, a childless union that remained happy to the end of his life.

Increasingly unable to pursue experimental work in view of his failing health, Kirchhoff moved to Berlin as professor of mathematical physics in 1875, the same year he was elected fellow of the Royal Society. Ill health finally forced him into retirement in 1886.

Kirchhoff was a mathematical physicist by training. He made his first important scientific contributions in 1845–1846, while still a student, by using topological concepts to generalize Ohm’s law to complex networks of electrical conductors. In 1857, Kirchoff went on to demonstrate theoretically that an oscillating current would propagate in a conductor of zero resistance at the speed of light, an important step toward the electromagnetic theory of light, though he did not make that connection.

Kirchhoff’s most important contribution to astronomy was his development of spectroscopic analysis with Bunsen and their subsequent determination of the chemical composition of the Sun. Half a century before quantum mechanics would provide for it a firm physical basis, Kirchhoff and Bunsen established spectroscopy as an empirical science. Between 1859 and 1861, they demonstrated that:
  1. 1.

    incandescent solids or liquids emit continuous spectra;

     
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    the spectra of heated gases consist of a number of bright lines, characterized by different wavelength patterns for different gases; and

     
  3. 3.

    when the light from an incandescent gas or liquid traverses a heated gas, the gas absorbs light at the same wavelength is as it emits when heated to the same temperature.

     

This last principle in particular provided a natural explanation for the ubiquitous dark lines in the solar spectrum, first noted in 1802 by William Wollaston and studied in much greater detail in 1817 by Joseph von Fraunhofer .

Kirchhoff’s next step was the production of a detailed map of the solar spectrum, in the course of which he ruined his eyesight to the extent that an assistant eventually had to complete the map. In a parallel effort involving the comparison of this growing map with laboratory spectra of gases, Kirchhoff began to determine the chemical composition of the Sun’s atmosphere. He first identified the elements sodium, calcium, barium, strontium, magnesium, iron, nickel, copper, cobalt, and zinc, with the list steadily growing ever longer in the following years. Kirchhoff’s spectroscopic findings also led him to put forth a theory of the Sun’s physical constitution, whereby a hot gaseous atmosphere is assumed to overlie a hotter, incandescent liquid core. This stood in marked contrast to the still prevalent view promoted by William Herschel and John Herschel of a dark, cold solar nucleus; Kirchhoff’s efforts contributed much to the latter concept’s demise in the second half of the nineteenth century.

True to his training and inclination, Kirchhoff did not neglect theoretical aspects related to his work in spectroscopy. In 1859, as a consequence of his chemical spectral analysis, he had formulated a general principle stating that the ratio of emission to absorption of all material bodies is the same at a given temperature and wavelength. Kirchhoff’s law in turn led to his formulation in 1862 of the concept of the perfect blackbody, of vital importance in the later development of quantum theory. Although in this he had been partly anticipated by others, perhaps most notably by the British physicist Balfour Stewart , the generality and mathematical rigor of Kirchhoff’s work is such that he is now credited with the formulation of the blackbody concept.

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