Starlight Detectives

by Alan Hirshfeld

  Wollaston found that the Sun’s apparently seamless spectrum is, in fact, interrupted at intervals by dark lines, that is, partial or complete absences of color. He (mistakenly) supposed that these lacunae are natural divisions among what he regarded as the solar rainbow’s four primary colors, reduced from Newton’s proposed seven. Wollaston next revisited terrestrial sources of illumination, clarifying Thomas Melvill’s earlier observations. “By candlelight,” he reported to London’s Royal Society in 1802, “a different set of appearances may be distinguished. When a very narrow line of the blue light at the lower part of the flame is examined alone, in the same manner, through a prism, the spectrum, instead of appearing a series of lights of different hues contiguous, may be seen divided into 5 images, at a distance from each other. The 1st is broad red, terminated by a bright line of yellow; the 2nd and 3rd are both green; the 4th and 5th are blue, the last of which appears to correspond with the division of blue and violet in the solar spectrum.”

  In other words, the solar spectrum is a multihued field, riven by a series of dark lines, whereas the spectrum of a flame or an electric arc is a series of bright lines against an otherwise dark field. This apparent complementarity does not presume that the two types of spectra are mirror images of one another—that each dark line in the Sun’s spectrum aligns with a bright line in the spectrum of a flame. Yet in the closing phrase of his description, Wollaston does cite one example of an evident coincidence in the spectral positions of a solar line and a terrestrial line. The relationship of dark lines in the Sun to their bright, laboratory counterparts would perplex astronomers, physicists, and chemists for decades to come.

  Solar spectral lines, as depicted by William Wollaston in 1802.

  From an instrumental perspective, the birth of solar and celestial spectroscopy dates to 1814, when master optician Joseph Fraunhofer magnified the Sun’s spectrum with a small telescope. Adversity had dogged Fraunhofer for much of his youth. Orphaned at age eleven, he found himself bundled by his guardian into the back of a Munich-bound mail wagon, on his way to an apprenticeship with a glass cutter. The household proved to be a virtual prison, and the supposed apprenticeship a six-year term of domestic servitude. Books were forbidden, as well as a candle for his windowless room.

  On July 21, 1801, when Joseph Fraunhofer was fourteen, the four-story building in which he worked collapsed. By chance, he had been standing beside a pile of crates, which arrested the fall of a beam that would have crushed him. Four hours later, he was extricated under the watchful eyes of Maximilian Joseph, Prince-Elector and eventual King of Bavaria. Fraunhofer found himself a sudden celebrity, his rise from the ruins a phoenix-like symbol of rebirth amid the nation’s economic and political turmoil. That he was an orphan—indeed a fragile-looking one—only heightened the tale’s flash. So disquieted was Maximilian by Fraunhofer’s plight that he invited him for a private visit at his estate.

  The royal palace at Nymphenburg, in the west of Munich, is a sprawling, red-roofed edifice surrounded by gardens, fountains, and expansive lawns. The unworldly Fraunhofer was surely rapt in wonder as he approached the central château, past the reflecting pool and the sculpted forms of Greek deities. His entrance into the building’s Great Hall would have been equally jaw-dropping. Exuberant frescoes adorned the walls around him, while scantily clad gods, goddesses, and nymphs cavorted across the vaulted ceiling above his head.

  Any unease Fraunhofer might have felt in the face of this Rococo-era excess was dispelled by the prince himself, a reform-minded leader who habitually engaged with commoners while strolling the streets of the city. The two sat for an amiable chat, during which Maximilian evidently perceived in this humble apprentice an aspirational energy to be nurtured. When their conversation was over, Maximilian presented his visitor with eight golden Karolinen—half a year’s salary for a typical worker. Indeed, in a gesture that must have moved the orphaned Fraunhofer, the prince offered to help him as a father would a son.

  Maximilian assigned one of Bavaria’s most prominent citizens, the statesman and entrepreneur Joseph von Utzschneider, to see to Fraunhofer’s education. Utzschneider would profit as much as his young charge from their arrangement, for Fraunhofer proved remarkably adept at science and mathematics. He joined Utzschneider’s optical instruments firm in 1806 as an assistant, and by 1814 was running the entire production side of the company.

  Primed by his readings on the theoretical foundations of optics, Fraunhofer found traditional glass- and lens-making methods to be positively medieval compared to the mathematical rigor in his books. Like generations of predecessors, opticians carved lenses intuitively, as would a sculptor, out of glass whose refractive properties were only approximately known. This haphazard process might produce just one acceptable lens out of every four or five completed. And the wider and thicker the lens, the greater the odds of a misshapen form or material flaw. Middling opticians maximized their chances of success by limiting production to small lenses. Yet in early 1800s Europe, the press for scientific and technological supremacy ran up against the limits of seat-of-the-pants optical fabrication. Finely crafted lenses were required for high-precision surveying instruments and astronomical telescopes.

  To effectively compete against the entrenched English optics firms, Fraunhofer sought to replace—or at least supplement—the trial-and-error manufacturing model by mathematics-based design and rigorous testing. “In such a way,” one biographer notes, “he fought the personal art and ability of artists like [English optician Jesse] Ramsden by means of the arms of applied science.”

  The performance of a multielement achromatic lens depends critically on the chemical compositions and shapes of its nested glass components. To optimize these qualities, the refractivity and dispersive power of various formulations of glass had to be determined by experiment. Refractivity—the degree to which a light ray is deflected within a piece of glass—is wavelength-dependent; thus, a prism casts a rainbow, while a lens generates ill-defined, color-fringed images. To enhance the former and suppress the latter, Fraunhofer stressed the need to quantify the optical parameters of glass over a wide range of wavelengths.

  Joseph Fraunhofer.

  Fraunhofer’s testing protocols hinged on the development of a reliable source of monochromatic light rays: “It would be of great importance to determine for every species of glass the dispersion of each separately coloured ray. But, since the different colours of the spectrum do not present any precise limits, the spectrum cannot be used for this purpose. . . . It was, however, absolutely necessary for me to have homogeneous light of each colour.”

  Having failed to generate monochromatic rays by passing lamplight through tinted glass or chemical filters, Fraunhofer instead constructed a prism-based apparatus to isolate particular colored lines in the spectral emission of a sodium lamp. Measuring the deviation of these lines with a surveyor’s theodolite, he successfully deduced the refractive properties of the glass in the prism. He noted in particular a brilliant yellow line, unaware that Wollaston had reported its existence more than a decade earlier.

  Out of curiosity, Fraunhofer applied his spectroscopic device to sunlight. Seeking the vivid yellow line he had previously spotted in the sodium-lamp spectrum, he was amazed to find what he termed a virtual infinitude of dark lines strewn among the colors. (Fraunhofer was unaware of Wollaston’s prior observations of dark lines.) In the end, he tallied the spectral positions of 574 such features, some grayish and hair-fine, others black and comparatively broad. Fraunhofer labeled the most prominent lines A, B, C, D, and so on, from red to violet, with occasional lowercase letters interspersed among the list—designations that are still used today.

  The bright yellow line cast by lamplight was missing here, its position taken up by a close-together pair of dark lines Fraunhofer would come to designate D. (Upon further magnification, D’s bright, sodium-lamp analog proved to be a doublet as well.) The significance of this cosmic-terrestrial alignment was unclear to Fraunhofer
, nor could he explain why the solar D lines were dark, whereas their laboratory counterparts were bright. However, on one critical issue, he did weigh in: the Fraunhofer lines, as they were soon to be called, originate in the Sun itself, and are neither optical artifacts of the spectroscope nor the result of selective absorption of sunlight within Earth’s atmosphere.

  By 1820, English craftsmen, who had been masters of the optics trade throughout the eighteenth century, found themselves lagging Fraunhofer in every facet of production: glassmaking, optical design and testing, and instrumental sophistication. To their chagrin, most English opticians were unable to make spectroscopes of sufficient grade to display all but the most prominent Fraunhofer lines, much less the nearly six hundred he had counted. (English makers were subject to a confiscatory excise tax on crown and flint glass, which crippled optical research and manufacture. A government-sponsored project to improve the quality of optical glass foundered, as did outright attempts to obtain relevant information through bribery. In 1824, John Herschel visited Fraunhofer in the hope of gleaning insight into his techniques, but was denied entry to the glass foundry and workshop.)

  That Fraunhofer saw the solar dark lines at all, while previously unaware of their existence, is a testament to the resolving power of his spectroscope and his attentiveness as an observer. The lines’ inconspicuousness is captured by a recollection from 1830 by English mathematician and computing pioneer Charles Babbage. Some years earlier, John Herschel had invited Babbage to his home to view the Fraunhofer lines. While setting up the spectroscope, Herschel remarked to Babbage:

  I will prepare the apparatus, and put you in such a position that [the lines] shall be visible, and yet you shall look for them and not find them: after which, while you remain in the same position, I will instruct you how to see them, and you shall see them, and not merely wonder you did not see them before, but you shall find it impossible to look at the spectrum without seeing them.

  Babbage continues, “On looking as I was directed, notwithstanding the previous warning, I did not see them; and after some time I inquired how they might be seen, when the prediction of Mr. Herschel was completely fulfilled.”

  Fraunhofer adopted his eponymous lines as reference markers to assess the refractivity of glass samples. In principle, two identical samples should refract a particular line to the same position along a graduated scale. That is, the projected images of the lines should coincide; any relative displacement of the images indicates a difference in composition. Fraunhofer could thus alter the chemical recipes of his various glass mixtures—add, say, more or less lead oxide—to produce lenses of unprecedented homogeneity, clarity, and optical specification. In precisely matched pairs, these component lenses formed the world’s best achromatic telescope objectives.

  In the first halting steps toward celestial spectroscopy, Fraunhofer equipped a four-inch refractor telescope with a prism and viewed spectra of the Moon, planets, and several bright stars. He found that lunar and planetary spectra largely mimic the Sun’s, suggesting that these bodies shine by reflected sunlight. Spectra of stars, on the other hand, are diverse. Although the overall line patterns are largely preserved from star to star, the comparative prominence of individual lines in the spectra of red stars like Betelgeuse differs from that of white stars like Sirius; and both, in turn, differ from the Sun’s characteristic spectrum. Fraunhofer reports in his pioneering paper from 1817: “I have seen with certainty in the spectrum of Sirius three broad bands which appear to have no connection with those of sunlight. . . . In the spectra of other fixed stars of the first magnitude one can recognize bands, yet these stars, with respect to these bands, seem to differ among themselves.”

  Unlike his predecessors, Fraunhofer had taken a distinctly scientific approach to optical design and fabrication. He developed apt experiments, constructed sophisticated apparatus for measurement, and reported his findings in accepted scientific forums. Yet, at heart, Fraunhofer was an artisan. His observations were conducted in service to manufacturing imperatives, not to the explication of nature. He targeted stars, for instance, to ascertain whether spectral lines are an instrumental artifact of light passing through a slit; the spectra of stars could be obtained without a light-restricting slit (yet still showed spectral lines). Fraunhofer’s working methods were never published, nor were outsiders permitted entry to his glassmaking or lens-grinding workshops. Having cast new light on the once-inaccessible realms of atoms and stars, Fraunhofer resumed his true calling: the making of precision optical instruments. He was equally innovative in this arena, presenting astronomers with an advanced generation of refractor telescopes. In his absence, the nascent field of stellar spectroscopy slumbered for almost fifty years.

  After Fraunhofer, laboratory scientists in England and France studied the spectra of light emitted by substances incinerated in flames and electric arcs. In 1826, English photographic pioneer William Henry Fox Talbot evoked the freewheeling character of this frontier research:

  A cotton wick is soaked in a solution of salt, and when dried, placed in a spirit lamp. It gives an abundance of yellow light for a long time. A lamp with ten of these wicks gave a light little inferior to a wax candle; its effect upon all surrounding objects was very remarkable, especially upon such as were red, which became of different shades of brown and dull yellow. A scarlet poppy was changed to yellow, and the beautiful red flower of the Lobelia fulgens appeared entirely black.

  Talbot was drawn to the flame’s mesmeric yellow cast, which he ascribed to the combustion of the element sodium in the salt solution that infused the wick of his burner (although elsewhere he proposes both sulfur and “crystallized water” as the activating agent). A prism-view revealed the source of the distinctive color to be a brilliant yellow spectral line. To Talbot’s bewilderment, the same yellow line showed up, like an uninvited guest, in the spectrum of every incandescent chemical—even those supposedly devoid of sodium. Why would elemental spectra, each one arising from a supposedly unique arrangement of matter, all share this particular spectral line?

  Despite this irreconcilable state of affairs, Talbot painted a future where “a glance at the prismatic spectrum of a flame may show it to contain substances which it would otherwise require a laborious chemical analysis to detect.” The ubiquitous yellow spectral line muddled the universal assumption that each chemical element possesses its own unique spectral pattern, and that spectroscopic classification of matter—perhaps even identification of new elements—might be feasible. It would be decades before these lofty aspirations were achieved, in large part due to impure test samples and lack of rigor in the laboratory. (Talbot himself frankly admitted as much when he confessed, “I am not much of a chemist, but sometimes amuse myself with experiments.”)

  Between 1820 and 1860, there was scant scholarly momentum behind the development of spectrochemical analysis; spectroscopic researchers were chiefly concerned with the production of monochromatic light for use in optical testing or photochemical experiments—or as ammunition in the hot debate over whether light is a particle or a wave. Those who did venture into the spectrochemical arena narrowed the scope of their investigations: they sought a spectroscopic means to differentiate between known, but chemically similar, substances. By the 1850s, one could distinguish the spectrum of, say, lithium from that of strontium or nitric-acid gas from bromine, but no trail was blazed toward Talbot’s more expansive notion of spectral analysis.

  Yet even as the vast inventory of chemical substances remained beyond reach of the spectroscope, the conundrum of the intruding yellow line was solved. A trio of British scientists—John Hall Gladstone, William Crookes, and William Swan—independently isolated the culprit: it was, as Talbot had suggested, the element sodium, in the form of sodium chloride—common salt. Such was spectroscopy’s power to amplify the salience of matter that chemists had been flummoxed by nature’s most mundane crumb. As nineteenth-century historian Agnes Clerke described it:

  [Salt] floats in the a
ir; it flows with water; every grain of dust has its attendant particle; its absolute exclusion approaches the impossible. And withal, the light that it gives in burning is so intense and concentrated, that if a single grain be divided into 180 million parts, and one alone of such inconceivably minute fragments be present in a source of light, the spectroscope will show unmistakably its characteristic beam.

  Given the omnipresence of sodium and its high reactivity in a flame, chemists realized that they had to painstakingly leach every trace of dissolved salt out of their samples before applying any spectrochemical process. John Hall Gladstone spoke for the chemical community when he concluded in 1857 that the analysis of flame spectra, “though doubtless very accurate in certain cases, is of limited and difficult application.” Barely two years later, Robert Bunsen and Gustav Kirchhoff wrestled this limited and difficult application into submission and opened the door to cosmochemical analysis through light.

  Chapter 15

  LABORATORIES OF LIGHT

  [I]n order to examine the composition of luminous gas, we require, according to this method, only to see it; and it is evident that the same mode of analysis must be applicable to the atmospheres of the sun and the brighter fixed stars.

  —Gustav Kirchhoff and Robert Bunsen, “Chemical Analysis by Spectrum-observations,” 1860

  ON NOVEMBER 15, 1859, ROBERT BUNSEN wrote with evident excitement to his longtime colleague Henry Roscoe, in England: “At present, Kirchhoff and I are engaged in a common work that does not let us sleep. Kirchhoff has made a wonderful, entirely unexpected discovery in finding the cause of the dark lines in the solar spectrum, and he can increase them artificially in the sun’s spectrum or produce them in a continuous spectrum and in exactly the same position as the corresponding Fraunhofer lines. Thus a means has been found to determine the composition of the sun and fixed stars with the same accuracy as we determine strontium chloride, etc., with our chemical reagents.” As Bunsen laid out the particulars, Roscoe realized that he was privy to the unfolding of a scientific revolution.