Starlight Detectives

by Alan Hirshfeld

  They also needed to implement an effective framework of measurement, one that would fix the positions of stellar lines relative to an ironclad fiducial standard that all astronomers could use. Analogous to Bunsen and Kirchhoff’s solar spectroscope, the new apparatus had to permit simultaneous viewing of a star’s spectrum alongside a calibrated spectrum generated in real time within the observatory. And these parallel spectra must be identically scaled in wavelength, so as to “enable the observer to determine with certainty the coincidence or noncoincidence of the bright lines of the elements with the dark lines in the light from the star.”

  The completed spectroscope employed two flint-glass prisms and was bolted securely to the eye-end of the Clark refractor. The stellar spectra were observed through an auxiliary telescope fitted with crosshairs. For comparison, an external mirror-and-prism arrangement deflected light into the spectroscope from an incandescent source in the observatory. The English climate being what it is, Huggins spent considerable “downtime” in his laboratory, applying a higher-dispersion, table-mounted spectroscope to the emissions of twenty-nine chemical elements provided by Miller. These spectra he laid out in charts for quick visual reference against the stellar line-arrays.

  To maximize their precision of measurement, Huggins and Miller sought a real-time reference spectrum against which to gauge the positions of stellar spectral lines. The Fraunhofer lines could not be used, as the Sun was below the horizon during their telescopic observations. Instead, they settled on a readily available and virtually inexhaustible source of light: the incandescent air between a pair of electrodes. The discharge produced about a hundred widely spaced emission lines—a sufficient number of fiducial marks against which stellar lines could be measured. (By analogy, one might index the positions of Manhattan skyscrapers from a distance by holding up a ruler against the cityscape.) However, a downside to the process was that electricity occasionally jumped from the induction coil to the metal housing of the spectroscope, giving Huggins a rude shock.

  Huggins and Miller understood the technical exigencies imposed by the nature of their work. Virtually every component of their spectroscopic apparatus was finely crafted and adjustable to minimize optical distortions; brass covers shielded sensitive components from dust, stray light, and air currents; even the microscopic flexure of the spectroscope in its various orientations was factored into every observation. Standing before their colleagues, many of whom were leery of new technology, the two men would have to justify every procedural step, every decimal point of precision, every scientific assertion. Huggins’s fanatical attention to detail reflected equally the scientific imperative and the desire to avoid professional embarrassment. Technology was not his end game, as it appears to have been for Joseph Fraunhofer; it was the vehicle by which he might explicate nature, and simultaneously secure his credibility as a scientist.

  Once again, the issue of priority attended the arc of scientific advancement in practical spectroscopy. The case against the originality of the solar and laboratory research published by Bunsen and Kirchhoff percolated in England. Meanwhile, Huggins and Miller found themselves competing with a host of researchers similarly inspired to apply a spectroscope to the stars: Giovanni Battista Donati in Florence, Pietro Angelo Secchi at the Collegio Romano, Lewis M. Rutherfurd in the United States, and even classical-astronomy stalwart George Biddell Airy at the Royal Greenwich Observatory. (Johann Lamont, director of the observatory at the Bavarian Academy of Sciences, had made a perfunctory study of stellar spectra in 1836.)

  Both Huggins and Miller were predisposed to claim priority over their cohorts: Huggins, the nonacademic newcomer, keen to make his professional mark; Miller, the aggrieved laboratory veteran, disenfranchised by his rivals in Heidelberg (at least, in his own view). As Miller would have been loathe to acknowledge, credit for discovery occasionally sidesteps the first to reach the goal. The door to scientific fame is ever ajar for those who arrive later—those whose work surpasses the reigning threshold of completeness and exactitude, whose methodology and data instill trust among experts in the field, whose results sort out conflicting observations or theories.

  Whether or not Huggins and Miller began their spectroscopic work in early 1862 as they claim, one point is certain: In January 1863, spurred by a development on the Continent, the pace of their project accelerated from a canter into a full-fledged gallop. The Royal Astronomical Society released a translation of a paper by Florentine astronomer Giovanni Battista Donati, describing the dark “striae” or lines he had observed as early as 1860 in the spectra of fifteen stars. Donati, whose eponymous comet of 1858 was the first ever to be photographed, had cobbled together a telescope using a fifteen-inch solar-burning lens from the city’s Accademia del Cimento. (The great lens had been commissioned in 1690 by the Grand Duke of Tuscany. On a European tour in 1814, the celebrated chemist Humphry Davy and his assistant, a then-unknown Michael Faraday, had used the device to incinerate a diamond and infer its pure-carbon makeup.)

  In Donati’s setup, the focused stellar rays passed through a table-mounted prism, and the resulting spectrum was observed through a theodolite. A pair of metal indices—one fixed, the other movable by a micrometer screw—were superimposed onto the view-field, then adjusted to match the separation between any two spectral lines. To prepare for an evening’s observation, Donati sighted the Sun, draping a cloth over the burning lens to diminish the intensity of the spectrum to a tolerable level. He rotated the prism to bring a selected solar line into coincidence with the fixed metal index. Leaving the prism in this state until nightfall, he pointed the telescope toward the target star, adjusted the micrometer screw to align the second metal index with a chosen spectral line, and recorded the separation between the indices. Measuring each line in this fashion, Donati constructed a crude drawing of the star’s spectrum on the same scale as the Sun’s.

  Donati’s results were problematic. Many of the stellar lines were significantly displaced from those Joseph Fraunhofer had identified decades earlier. In the light of the star Sirius, for example, a dark line that Fraunhofer had noted in the green portion of the spectrum appeared to Donati in the blue. Fraunhofer’s famous D line of sodium was completely absent in Donati’s spectral drawings of four of his fifteen stars. Donati could not account for the inconsistencies, except to suggest that Fraunhofer might have mistaken one line for another.

  In the wake of Donati’s paper, Huggins and Miller rushed a preliminary report to the Royal Society on February 19, 1863, outlining their spectroscopic studies of the stars Sirius, Betelgeuse, and Aldebaran. That very day, Huggins learned that Lewis M. Rutherfurd in New York had recently completed visual observations of the spectra of twenty-three stars, the results to be published in a forthcoming issue of the American Journal of Science. Huggins and Miller hurriedly appended a statement to their report informing the Royal Society that they had viewed the spectra of at least thirty stars over the previous twelve months; measurements of the lines of these stars were in progress.

  A common complaint in this prephotographic era of stellar spectroscopy was the extreme difficulty sighting the ghostly spectral lines. The dispersed glimmer of a star is barely sufficient to stimulate the retina, much less ease measurement of lacunae strewn along a spectral-smudge of light. “On any but the finest nights,” Huggins and Miller wrote, “the numerous and closely approximated fine lines of the stellar spectra are seen so fitfully that no observations of value can be made. It is from this cause especially that we have found the inquiry . . . more than usually toilsome; and indeed it has demanded a sacrifice of time very great when compared with the amount of information which we have been enabled to obtain.”

  By mid-1864, Huggins and Miller had endured enough. Instead of the anticipated inventory of fifty stars, they had obtained detailed results for only four, of which a mere two, Aldebaran and Betelgeuse, had complete spectral-line maps. The spectrum of low-lying Sirius, the brightest star in the sky, had been muddled by turbulence
in the air. (The atmospheric roiling had also foiled attempts in January and March 1863 to produce a wet-collodion photograph of Sirius’s spectrum.) Other stars had received only cursory treatment. Yet what seems a meager outcome relative to what had been expected is, upon reconsideration, a veritable triumph of industry. With the fickle weather, seasonal cycling of the constellations, limited nightly observing times, and all manner of methodological complexities, the new science of stellar spectroscopy had proved to be a protracted venture. Complete investigation of even a single star, Huggins and Miller realized, would take months, if not years. Their accumulated observations, incomplete as they were, permitted inferences about the chemical nature of stars. Faced with the daunting prospect of more nighttime labor, they decided to publish.

  “On the Spectra of Some of the Fixed Stars,” read by Miller to the Royal Society on May 26, 1864, led off with a generous nod toward Gustav Kirchhoff as the discoverer of the connection between the dark lines of the solar spectrum and the bright lines of terrestrial flames. (On this occasion, Miller evidently set aside his challenge to Kirchhoff’s priority.) This tribute is followed by a stinging dismissal of Donati’s spectroscopic work: “[T]he positions which he ascribes to the lines of the different spectra relatively to the solar spectrum do not accord with the results obtained either by Fraunhofer or by ourselves. As might have been anticipated from his well-known accuracy, we have not found any error in the positions of the lines indicated by Fraunhofer.” The studies by Rutherfurd, Secchi, and Airy’s staff at Greenwich are politely acknowledged, without comment. (A public critique by Huggins convinced Airy to suspend his spectroscopic efforts. Rutherfurd also closed out his spectral examination of stars, using the spectroscope instead to assess the optical properties of telescope objectives.)

  Huggins and Miller’s broad conclusion echoes that of Fraunhofer many decades earlier: In their essential properties, stars are but distant suns, each encompassing an array of familiar chemical elements. The spectrum of Aldebaran revealed the presence of sodium, magnesium, hydrogen, calcium, iron, bismuth, tellurium, antimony, and mercury. Betelgeuse showed evidence of sodium, magnesium, calcium, iron, and bismuth. There was no doubt about the correspondences between the stronger stellar and elemental lines. Both sets of lines had been viewed simultaneously in the eyepiece: one originating in the fiery atmosphere of a remote star, the other from an incandescent spark across the room.

  The hydrogen lines in the spectra of white stars, such as Sirius, were visibly stronger than in yellow stars like the Sun, while lines of other elements were much weaker. Huggins and Miller suggest that the distinctive colors of stars, as well as their contrasting line patterns, might arise from differences in their chemical makeup. (Such variations would go unexplained until the twentieth century, when the physics of stars was elucidated. A star’s color and spectrum are much more sensitive to its surface temperature than its composition.)

  Drawings of the spectra of Aldebaran and Betelguese (α Orionis) by William Huggins, presented to the Royal Society on May 26, 1864.

  The symbolic core of the 1864 paper is its spectral-line maps of Aldebaran and Betelgeuse. Placed one above the other, each spans the width of three pages. Scores of fine lines crowd the graphical space, together as seemingly mundane as scratches in the dirt—until one notices the familiar Fraunhofer letters from the solar spectrum; and next, the assortment of italicized labels accompanying the stellar lines: H, for hydrogen; Hg, for mercury; Na, for sodium; Fe, for iron. Here, in what Huggins called the “strange cryptography of unraveled starlight,” was science of the most profound order: visual confirmation of the chemical unity of the Sun and stars—and, by extension, of Earth and life.

  In the early 1800s, William Herschel showed that Newton’s laws of gravity and motion, which fix the trajectories of planets around the Sun, also govern the orbital movements of binary stars around one another. This mechanical unity of the cosmos led scientists to speculate whether a chemical unity reigns throughout space. With their limited, but compelling, spectroscopic evidence, Huggins and Miller infer “that the stars, while differing the one from the other in the kinds of matter of which they consist, are all constructed upon the same plan as our sun, and are composed of matter identical, at least in part, with the materials of our system. The differences which exist between the stars are of the lower order, of differences of particular adaptation, or special modification, and not differences of the higher order of distinct plans of structure.”

  Extending the theme of cosmic unity to its logical conclusion, they suggest that the stars are not just structurally analogous to the Sun, but are “surrounded by planets, which they by their attraction uphold, and by their radiation illuminate and energize. And if matter identical with that upon the earth exists in the stars, the same matter would also probably be present in the planets genetically connected with them, as is the case in our solar system. . . . On the whole we believe that . . . at least the brighter stars are, like our sun, upholding and energizing centres of systems of worlds adapted to be the abode of living beings.”

  With surprising swiftness, the spectroscope had demonstrated the chemical commonalities of Earth, the Sun, and the stars—and perhaps of life itself, wherever it might exist. Having grown accustomed to—or at least resigned to—the operational hurdles of celestial spectroscopy, Huggins wondered where next to exert this powerful diagnostic lever. Now working alone (Miller had returned to his own research), should he increase the inventory of stellar spectra? Or, having addressed the similitude among these spectra, should he now confront the tantalizing differences in the presence or the relative strengths of certain lines?

  The idea of cosmic unity continued to stir Huggins’s inquisitive instincts. To the now-seasoned observer, the spectroscopic frontier lay, not necessarily farther into the void, but fainter into the depths of visual perception. What sort of celestial object radiates the dimmest spectrum perceivable through an eight-inch telescope, situated to its detriment underneath the leaden skies of outer London? If the pinpoint blaze of a star is reduced by the spectroscope to a wan glow, Huggins reasoned, the greater challenge would be to hunt for lines within the enfeebled spectrum of a diffuse object: a nebula.

  In 1864, little was known about nebulae beyond their ubiquity (some five thousand had been cataloged) and their visual appearance. And even that was an area of active disagreement. The stunning nebular images by Andrew Common and Isaac Roberts lay decades hence; the eye of the astronomer and the hand of the artist still reigned in the rendering of deep-space objects. The Royal Society had just published John Herschel’s General Catalogue of Nebulae and Clusters of Stars, an expansion of his father William’s eighteenth-century original. Descriptions were conveyed via numerical and alphabetical shorthand—L. for large, rr. for partially resolvable into stars, bn. for brightest toward the north side—with the rare triplet of exclamation points appended to objects whose splendor halts one’s breath. By this time, Lord Rosse’s six-foot-wide Leviathan reflector in Ireland had been applied to the study of nebulae for nearly two decades.

  To the eye, some nebulae were large and asymmetrical, while others (dubbed planetary nebulae by William Herschel) were compact and round. Rosse’s Leviathan revealed nebulae with a spiral shape, suggesting active coalescence of new solar systems, in the mode of Laplace’s widely held nebular hypothesis. Regardless of their form, the intrinsic nature of these diffuse objects was a mystery: Were they clouds of incandescent gas, thus inherently irresolvable; or were they remote star systems, rendered indistinct by distance? Or might the nebular population encompass both of these celestial species?

  Huggins believed that little of significance would come from further visual studies, no matter how large the telescope or acute the observer. But a spectroscope-equipped telescope was a different matter. “Prismatic analysis,” he offered, “if it could be successfully applied to objects so faint, seemed to be a method of observation specially suitable for determining whether any essenti
al physical distinction separates the nebulae from the stars, either in the nature of the matter of which they are composed, or in the conditions under which they exist as sources of light.”

  Of the many diffuse targets visible from his observatory, Huggins gravitated toward the disk-like planetary nebulae. With their relatively high surface brightness, planetaries shine a pale bluish-green, a hue not often encountered among stars. Indeed, Huggins was seeking a class of nebulae more likely to consist of diffuse gas than unresolved stars. The latter would presumably emit a collective spectrum of the sort Huggins had already studied for the past two years. But the spectrum of a cosmic gas cloud had never been observed and might take on any conceivable form. It was here, Huggins surmised, that a greater discovery might be made.

  On August 29, 1864, Huggins pointed his telescope—“armed with the spectrum apparatus,” he notes in martial metaphor—toward the Cat’s Eye, a bright planetary nebula in Draco discovered by William Herschel in 1786. The image in the eyepiece was confounding: “At first I suspected some derangement of the instrument had taken place; for no spectrum was seen, but only a short line of light perpendicular to the direction of dispersion. I then found that the light of this nebula, unlike any other extra-terrestrial light which had yet been subjected by me to prismatic analysis, was . . . monochromatic, and after passing through the prisms remains concentrated in a bright line.”

  Through Huggins’s small telescope, the spectrum of the Cat’s Eye appeared as a single, vivid stroke of green, never before seen in the laboratory or in the Sun. (Astronomers attributed the much-debated spectral line to a hypothetical element, nebulium; only in 1927 was it proved to originate in a rarified form of ionized oxygen.) Further examination of the nebular spectrum revealed a fainter emission in the blue, coincident with Fraunhofer’s F line of hydrogen, then a third, positioned between the others.