Starlight Detectives

by Alan Hirshfeld

  Visiting from England, Henry Roscoe viewed the Sun’s spectrum arrayed against that of the electrical discharge between a pair of iron electrodes energized by an induction coil:

  In the lower half of the field of the telescope were at least seventy brilliant iron lines of various colors, and of all degrees of intensity and of breadth; whilst in the upper half of the field, the solar spectrum, cut up, as it were, by hundreds of dark lines, exhibited its steady light. Situated exactly above each of the seventy bright iron lines was a dark solar line. These lines did not only coincide with a degree of sharpness and precision perfectly marvellous, but the intensity and breadth of each bright line was so accurately preserved in its dark representative, that the truth of the assertion that iron was contained in the sun, flashed upon the mind at once.

  The presence of iron lines in the Sun’s atmosphere electrified astronomers, whose prevailing theory of solar formation (later discredited) involved an accretion of iron from impacting meteors. Exercising his mathematical chops, Kirchhoff computed the odds of a chance alignment of the sixty most prominent iron lines in the Sun and the flame at less than one in a million trillion. In addition to sodium and iron, there were multiple line coincidences for barium, calcium, chromium, cobalt, copper, magnesium, nickel, and zinc. Not only did the positions of the solar lines match their laboratory counterparts, so too did the relative intensities—essentially, the widths of the lines. The lone line-coincidence for gold was judged insufficient to verify its presence in the Sun.

  In the ensuing months, Kirchhoff began to draw an eight-foot-long map of the solar spectrum, diluting the India ink in his pen to render progressively fainter lines. (While visually appealing, color is redundant in such a map; a line’s position is all astronomers need.) Suffering from severe eyestrain, Kirchhoff delegated the project’s completion to an assistant. The published map served as the benchmark reference of spectral-line patterns for the next decade.

  The scientific community quickly recognized the revolutionary dimensions of spectroscopic analysis as practiced by Bunsen and Kirchhoff. Barely two years after the Heidelberg duo’s initial report, veteran and novice researchers in Europe and the United States were applying the process to a variety of projects, spurred on by indications that solar-surface activity influenced terrestrial climate and magnetism. At a gathering of London’s Chemical Society on June 20, 1861, pioneering celestial photographer Warren De La Rue remarked that “if we were to go to the sun, and to bring away some portions of it and analyze them in our laboratories, we could not examine them more accurately.” In giving astronomers the means to determine the intrinsic nature of celestial bodies, Bunsen and Kirchhoff’s contribution was every bit as far-reaching as the introduction of the telescope itself two and a half centuries earlier. The commonality between laboratory and solar spectra extended the sense of cosmic unity already witnessed in Newtonian mechanics. Just as the orbital movements of planets, comets, and binary stars are governed by the same laws, the spectroscope showed that our Earth and Sun consist of like matter that interacts with light in identical fashion.

  The accomplishments of Bunsen and Kirchhoff were much lauded, leaving the two unprepared for the tempest over priority that blew in from England. In 1861, William Crookes, founding editor of London’s Chemical News, expressed his overtly nationalistic opinion that “our readers will feel an interest in knowing that many of the observations which are now being followed by Continental savans, have been investigated in a more or less perfect manner by English experimentalists.” Crookes’s barrage was followed by salvos from spectrum researcher William Allen Miller, who laid out a similar version of the history of spectral analysis at professional meetings in 1861 and 1862. A series of counter-lectures by Bunsen’s protégé Henry Roscoe failed to still the outcry over the originality of the Heidelberg observations.

  Bunsen and Kirchhoff affirmed that aspects of spectral analysis had been studied by others, all the way back to Joseph Fraunhofer, David Brewster, Henry Fox Talbot, and John Herschel. And that there were subsequent contributions on the subject by Crookes, Miller, George Stokes, William Swan, and John Hall Gladstone in England, Anders Ångström in Sweden, and Julius Plücker in Germany, among others. But they denied that their own research was in any way influenced by these works, much of which never came to their attention or were variously limited in scope.

  Kirchhoff countered in November 1862 with a hard-hitting rebuttal, “Contributions Toward the History of Spectral Analysis and the Analysis of the Solar Atmosphere,” which appeared in the venerable British scientific journal Philosophical Magazine. He parses the narratives of John Herschel and Henry Fox Talbot about their research on the spectra of colored flames. “In these expressions,” Kirchhoff writes, “the idea of ‘chemical analysis by spectrum-observations’ is most clearly put forward. Other statements, however, of the same observers, occurring in the same memoirs . . . flatly contradict the above conclusions and place the foundations of this mode of analysis on most uncertain ground.”

  Crookes praised William Allen Miller’s 1845 renderings of flame spectra, which he claimed were superior to those from Heidelberg, but Kirchhoff finds them to be utterly worthless for scientific use: “I have laid Prof. Miller’s diagrams before several persons conversant with special spectra, requesting them to point out the drawing intended to represent the spectrum of strontium, barium, and calcium, respectively, and that in no instance have the right ones been selected.” Kirchhoff likewise dismisses Crookes’s endorsement of the explanation of the D-line reversal phenomenon posed by Miller. He suggests brusquely that Crookes “read Miller’s words with some slight attention,” so that he might realize how the scheme produces a result that is counter to what is observed.

  Kirchhoff reminds readers that William Swan restricted his study of the D lines to hydrocarbon compounds like ether, paraffin, and turpentine; therefore, no conclusion can be drawn from that work regarding the general question of the uniqueness of elemental spectra. Anders Ångström fares no better in Kirchhoff’s history: “It is seen that the proposition which forms the basis of the chemical analysis of the solar atmosphere floated before Ångström’s mind, but only, indeed, in dim outline.”

  In a summary statement worthy of a judicial proceeding, Kirchhoff declares that no one “had clearly propounded this question [of the uniqueness of elemental spectra] before Bunsen and myself; and the chief aim of our common investigation was to decide this point. Experiments which were greatly varied, and were for the most part new, led us to the conclusion upon which the foundations of the ‘chemical analysis by spectrum-observations’ now rests.”

  The priority debate stoked by Crookes and Miller prompted an inevitable question: If English scientists were aware of the potential of spectral analysis as early as 1845, if not before, why hadn’t they developed it themselves? The issue flared at an 1861 meeting of the leading lights of English chemistry, where an exasperated Edward Frankland (yet another Bunsen trainee) pointed an accusatory finger at one of the drama’s leading players:

  I have recently read, with very great interest, the beautiful researches which Dr Miller made some sixteen years ago upon this very subject. It is really wonderful that sixteen years ago we had the real pith of the whole of this matter thrown before us, but up to the present time we have been unable to use it. I will not acquit Dr Miller for being partly to blame, for, perhaps, he himself ought to have shown us the practical way of employing the instrument at the time it was revealed to us.

  Already before Bunsen and Kirchhoff’s monumental work, the majority of scientists believed that spectral analysis could be applied—in principle, if not yet in practice—to delineate the makeup of terrestrial and solar substances. Many also trusted that every element or compound, when brought to incandescence, would prove to display a unique spectrum by which it might be identified. Yet assertions and imprecise observations are rickety bones upon which to hang a complete body of theory and practice. Until Bunsen and Kirchhoff,
there was only the feeblest momentum toward the establishment of practical spectrochemical techniques. A sustained and vigorous effort was required before all doubts about the viability of the new process were erased.

  Kirchhoff and Bunsen pursued a higher and more comprehensive level of experimental verification than any previous researcher. Their chemical samples were purer, their spectroscopic apparatus more refined, their results more compelling by virtue of an almost fanatical attention to detail. They originated an analytical technique that was independent of the underlying physics governing the interaction of matter and light. While theorists sorted out the latter, experimentalists could proceed with the spectrochemical analysis of radiating substances.

  Perhaps the major advantage the Heidelberg scientists had over their cohorts was each other. The two worked both individually and in concert, as dictated by the research needs of the moment. Their respective specializations eased their collective experimental burden, relative to that faced by a lone physicist or chemist in the laboratory. In Bunsen, Kirchhoff had a ready source of reliable emission spectra by which to identify elements in the Sun. In Kirchhoff, Bunsen had the broad theoretical outlook and instrumental expertise of the physicist. And in their collaboration, we witness the escalatory give-and-take of a musical duo, stoking their mutual creative agency through each other’s cues. Together, Bunsen and Kirchhoff comprised one of the most productive joint ventures in science.

  Perhaps Crookes’s cadre of scientists were cowed by the sheer weight and intricacy of work needed to fully develop a new spectrochemical process. In any event, such an all-out effort would have distracted them from their primary research goal, which focused on the physical more than the chemical aspects of spectra: What might these ordered arrays of spectral lines reveal about the nature of light, electricity, or thermodynamic processes?

  Equally spurious are the English claims of priority in the photochemical analysis of the Sun; their achievements are closer to a proof of concept than development of practical techniques that could be implemented by others. Indeed, as late as 1860, David Brewster and John Hall Gladstone still questioned whether Fraunhofer lines arise in Earth’s atmosphere or the Sun’s. As Bunsen and Kirchhoff’s tireless champion Henry Roscoe asserted in a letter to Gladstone on May 10, 1861, “The real importance of . . . [a] discovery is not to be measured by the first imperfect notices which have been made on the subject. . . . The discovery is really made when the true importance of these observations is shown, & when they are connected together in a scientific exact manner.”

  While the dispute over priority simmered, spectral analysis proceeded apace. The spectroscope quickly became an essential piece of equipment in virtually every physics and chemistry laboratory, and by century’s end, in many observatories.

  In 1877, Bunsen and Kirchhoff were awarded the Humphry Davy Medal of the Royal Society, an unambiguous sign that the issue of priority had been settled in most minds. Kirchhoff often related the story of a conversation with his banker, in which he mentioned how the spectroscope might, in principle, reveal the presence of gold in the Sun. The banker shot back, “What do I care for gold in the sun if I can not fetch it down here?” Shortly after he and Bunsen had received the Davy Medal—plus its weight in gold—Kirchhoff visited the banker and handed over his gleaming prize. “Look here,” he said, “I have succeeded at last in fetching some gold from the sun.”

  Chapter 16

  DECONSTRUCTING THE SUN

  It is not an uncommon thing for the physicist to tread upon the ground which a chemist thinks belongs to him, and for the chemist to tread upon the ground of the physicist. Now we have the chemist occupying the ground of the astronomer.

  —Warren De La Rue, comments to the Chemical Society, London, June 20, 1861

  DURING THE 1860S, THE SPECTROSCOPE took up a central position in efforts by astronomers to probe the chemical and physical properties of the Sun. The first step had been taken by Bunsen and Kirchhoff, who established a working inventory of elemental lines in the solar spectrum. Although heartily adopted by solar astronomers, Kirchhoff’s published spectral-line map had serious shortcomings. Each line’s position is designated, not by its physical wavelength, as Fraunhofer had done, but by its millimeters of deviation along a scale with an arbitrary zero-point. Kirchhoff had to adjust the prisms to bring successive portions of the spectrum into view, imparting the scale readings with discontinuities. Furthermore, prismatic dispersion varies with the wavelength of light: On the Heidelberg map, a pair of lines in the red part of the spectrum spans a wavelength range different from that of a pair of equally spaced lines in the violet. The precise manner of dispersion being unique to Kirchhoff’s apparatus, his map’s numerical scale is better suited to line identification—akin to Fraunhofer’s alphabetical labels—than to direct line-to-line comparisons against other’s spectral maps.

  In 1868, Swedish astronomer Anders Ångström released an extensive, wavelength-based solar-spectrum map. This was made possible by Ångström’s use of a diffraction grating instead of a prism as his spectroscope’s light-dispersing element. Introduced by Joseph Fraunhofer around 1820, a diffraction grating consists of a series of parallel, closely spaced, linear apertures through which light can pass. Fraunhofer had experimented with an array of wires stretched over the threads of a pair of bolts, then progressed to a sequence of slits peeled from gold film on a glass plate. (Gratings are also produced by cutting fine grooves into a transparent plate; a reflection grating can be made by grooving an opaque material.) Unlike its prism-based cousin, a grating spectroscope disperses colors uniformly with wavelength. Fraunhofer derived a straightforward formula that expresses the wavelength of a spectral line, depending on its deviation angle and the density of grooves in the grating.

  Ångström’s spectroscope had considerably more dispersive power than Kirchhoff’s, and the number of visible lines rose accordingly to more than twelve hundred. Once again, each line was rendered in ink as it appeared to the eye, not only in its proper place, but with its observed breadth and gradation, from pale gray to coal black. Several dark bands on Kirchhoff’s map resolved themselves into arrays of tightly packed lines. Among the solar elements added by Ångström were hydrogen (later proved to be the Sun’s primary constituent), manganese, strontium, and titanium; he rejected Kirchhoff’s claimed line coincidences for barium, copper, and zinc.

  The ascendance of Ångström’s visual map proved to be nearly as short-lived as its predecessor’s. With the same nod toward objectivity, permanence, and efficiency that would propel the camera into the depiction of faint nebulae, photographic pioneers began to record the solar spectrum onto a sensitized plate. New York astronomer Lewis M. Rutherfurd developed a succession of telescope-mounted spectroscopes during the wet-collodion era of the 1860s. Kirchhoff himself is said to have remarked that had Rutherfurd’s 1864 solar-spectrum photograph come sooner, it would have spared him a year’s crossed-eyed toil. Rutherfurd’s ultimate spectroscope, completed in 1877, featured a 1.7-inch-square diffraction grating of his own manufacture with an astonishing 17,296 rules per inch. The following year, Rutherfurd stunned attendees at a meeting of the Royal Astronomical Society when he displayed a ten-foot-wide mosaic of the solar spectrum, assembled from prints of twenty-eight negatives. Lord James Lindsay, the society’s president, remarked, “I certainly have never seen any thing before so fine as this photograph.”

  Rutherfurd distributed his machine-ruled gratings, each signed and dated, to spectroscopic professionals worldwide. Their allotment was haphazard, as American solar observer Samuel Pierpont Langley describes in an 1879 letter to his English colleague Norman Lockyer:

  On reaching New York I called at Mr Rutherfurds to see about your grating. He is away but [his assistant] Chapman told me he (Daniel C. Chapman) had sent you one, two weeks ago which I suppose you have now got. I seized upon two large ones I found there and bore them off, that being the only way of getting them, as he has apparently promised so many p
eople that he has no longer any very exact idea of the order of priority of claimants for the few he makes.

  Of the more than fifty precision diffraction gratings Lewis Rutherfurd gave away during his lifetime, three went to his uptown colleague Henry Draper. In 1872, Draper set up a heliostat to reflect sunlight into a fourteen-foot-long grating spectroscope, the projected spectrum recorded on a series of wet-collodion plates. The spectroscopic camera, or spectrograph, proved to be a keener eye than a human’s. In one section of Ångström’s hand-drawn map of the solar spectrum where 118 spectral lines had been depicted, Draper’s negative captured 293. Presenting his spectrum photograph to Britain’s Philosophical Magazine in 1873, Draper assured the editors that the “picture is absolutely unretouched. It represents, therefore, the work of the sun itself, and is not a drawing either made or corrected by hand.” So dense was the forest of spectral lines that London instrument maker John Browning wrote to Draper, “I am glad that you have stated so clearly that the plate is a perfectly untouched photograph, for I have not been able to get my friends to believe this in many instances.”

  A collective huzzah arose from the astronomical community with every release of a solar spectral-line map, the new map invariably eclipsing the old in clarity or extent. However, the iterative increase in the dispersive power of spectroscopes and spectrographs, each jump impressive in its own time, was still insufficient to resolve many questionable line coincidences. To make matters worse, systematic errors or discontinuities were discovered in the numerical scales of the maps, requiring complex corrections before they could be referenced for research. Without more definitive positional matches between solar lines and those generated in the laboratory, astronomers were unable to verify the presence of certain elements in the Sun.