Solar-spectrum mapping reached an essentially modern standard of accuracy during the 1880s with the invention of the concave reflection grating by Henry Rowland, a physics professor at Johns Hopkins University. Replacing the flat diffraction grating with one of the proper curvature eliminated the need for focusing lenses, producing a sharper, brighter, and higher-resolution spectrum. The omission of lenses from the optical path also allowed the spectrograph to reach into the ultraviolet part of the spectrum, wavelengths ordinarily absorbed by glass. And with the new dry-plate photographic process, segments of the solar spectrum could be recorded in mere minutes.
In 1888, Johns Hopkins published Rowland’s map of the solar spectrum, featuring some twenty thousand absorption lines and an easy-to-read wavelength scale. The dispersion in wavelength was so high that the spectrum, pieced together from its segments, stretched forty feet. As to its merits relative to its predecessors, an advertising circular for the map crowed that the “superiority is so great there is no possibility of comparison.” Rowland’s map and his subsequent compilation of spectral-line wavelengths, intensities, and chemical identifications were an immediate hit among solar astronomers and laboratory spectroscopists, and would serve as the standard reference for researchers well into the twentieth century.
By the 1890s, line coincidences on the various solar maps had raised the number of chemical elements identified in the solar atmosphere to around forty. Among the more curious tales of spectroscopic discovery is that of helium, the second most abundant element in the universe. Astronomers had realized that by attaching a spectroscope to the eyepiece-end of a telescope, they could zoom in on specific solar features, such as sunspots or flares. During the solar eclipse of August 18, 1868, French astronomer Pierre-Jules-César Janssen trained his spectroscope on a flamelike outburst called a prominence, which protruded beyond the Sun’s occulted disk. As expected, the prominence’s spectral lines were bright instead of dark, because its diffuse gas appears against the blackness of space, not the blinding solar surface. However, the emission-line array did not fully mirror the ordinary Fraunhofer spectrum; evidently, the physical conditions in a prominence differ from those in the region underneath, where line absorption takes place. The prominence spectrum included species familiar from the laboratory, but also a delicate yellow line whose placement was almost, but not quite, coincident with the well-known D doublet of sodium. The new line, Janssen realized, had no match in the spectral patterns of known elements.
In the aftermath of the eclipse, Jules Janssen and Norman Lockyer independently found that the circumsolar spectrum, ordinarily effaced by the luminance of the Sun’s disk, was, in fact, bright enough to observe in full daylight. Lockyer confirmed the presence of the yellow line in the Sun’s chromosphere, the red-tinged atmospheric layer that surrounds the solar photosphere (whose boundary roughly demarcates the Sun’s “surface”). When laboratory tests failed to support astronomers’ initial suspicion that the line arose from some form of hydrogen, the mystery element was dubbed helium, after the Greek god of the Sun.
Lockyer was loathe to suggest that a new element had been found on the Sun before being isolated on Earth. At the time, a host of spectroscopically theorized substances, like jargonium, nigrium, coronium, and nebulium, were jostling for scientific acceptance. All proved to be fictitious or else manifestations of known elements. A similar cautionary tale erupted in the late 1870s with Henry Draper’s notorious “discovery” of oxygen in the Sun. Draper’s assertion that the telltale pattern of oxygen appeared in emission among the Fraunhofer absorption lines contradicted the inviolable rule of line formation: spectral lines generated in the photosphere must be dark, as the ambient matter absorbs light from the hotter regions below. The five-year-long controversy, in which Norman Lockyer wielded the opposing cudgel, was stilled only by Draper’s death in 1882. (While Draper’s bright lines proved illusory, three dark lines of oxygen were identified in the solar spectrum in 1896.)
Helium remained a notional element until 1895, when Scottish chemist William Ramsay incinerated a chunk of cleveite, a uranium ore, hoping to isolate a sample of argon, the gas he had codiscovered with Lord Rayleigh the previous year. Peering through a hand-held spectroscope, Ramsay applied an electrical discharge to the recovered gas. He was perplexed to see, in addition to the telltale spectral pattern of argon, a secondary array of unfamiliar lines. After further research, Ramsay announced to his wife, “There is argon in the gas; but there was a magnificent yellow line, brilliantly bright, not coincident with, but very close to the sodium yellow line. I was puzzled, but began to smell a rat. . . . Helium is the name given to a line in the solar spectrum, known to belong to an element, but that element has hitherto been unknown on the earth.” Within months, the terrestrial helium line was found to have a weak companion alongside, a feature that sent spectroscopic astronomers scurrying to their telescopes. The solar helium line proved to be double as well, proving beyond doubt that Ramsay’s gas was the same as the gas resident in the Sun. From argon and helium, Ramsay went on to discover the noble gases krypton, neon, and xenon, and received the Nobel Prize in chemistry in 1904.
Henry Draper’s 1876 photographic spectrum of the sun, depicting his spurious “solar emission lines” of oxygen.
In addition to revealing the chemical composition of the Sun, the spectroscope confirmed the existence and physical extent of the Sun’s light-absorbing envelope. In Kirchhoff’s model, dark lines are impressed upon the otherwise continuous solar spectrum when light is selectively absorbed by chemical elements in the Sun’s atmosphere. Were this absorbing layer viewed without the back-illumination of the Sun’s interior, an emission-line version of the Fraunhofer absorption spectrum would appear instead. Astronomers realized that such a spectrum reversal might be glimpsed at the Sun’s limb during an eclipse; in the brief interval before the Moon fully occults the Sun, the light of the solar disk is nearly absent and the incandescent photosphere might be seen unimpeded against the blackness of space.
In the campus newspaper, Dartmouth College astronomer Charles A. Young described the first-ever sighting of the aptly named “flash spectrum” during the December 22, 1870, solar eclipse expedition in Jerez, Spain:
The slit of my spectroscope was placed tangential to the sun’s limb, just at the base of the chromosphere, the 1474 line on the cross-wires. As the crescent of the sun (or decrescent, rather) grew narrower, this line, and the magnesium lines close by, as well as some others in the same neighborhood which I am accustomed to see bright in prominences, gradually increased in brilliancy, when suddenly, as the last ray of the solar photosphere was stopped out by the moon, the whole field of view was filled with countless bright lines—every single dark line of the ordinary spectrum, so far as I could judge in a moment, was reversed, and continued so for perhaps a second and a half, when they faded out, leaving only those I had at first been watching.
Young added in a subsequent report, “The phenomenon was so sudden, so unexpected, and so wonderfully beautiful as to force an involuntary exclamation.” The brevity of the line reversal indicated that the Sun’s light-absorbing stratum spans some five or six hundred miles, a mere one-thousandth of the solar radius. The existence of the flash spectrum confirmed Kirchhoff’s rough model of solar structure, although he had never conceived that such a thin layer of gas would suffice to generate the profusion of Fraunhofer lines.
Spectroscopic studies of the Sun during the latter decades of the nineteenth century illustrate why astrophysics rose so slowly after its nominal birth in 1859 at the hands of Bunsen and Kirchhoff. Although spectroscopists were gratified by their success in fixing the chemical composition, and to a much lesser extent, the physical layout of the Sun, they were keenly aware that their instruments barely scratched its surface. The solar interior was the province of theoretical physicists, who had yet to formulate the mathematical means to probe the opaque depths of our central star. Yet even in the Sun’s periphery, physical theory was sidet
racked by preconceived notions about how matter should behave in the high-temperature, low-pressure, gravitationally governed solar environment. In retrospect, we know that at least two erstwhile assumptions about the solar atmosphere—that its temperature decreases radially outward and that its atoms occupy different atmospheric strata according to their weight—are simply wrong. (Temperatures rise steadily above the photosphere, and atoms distribute themselves freely throughout the gas.)
Even as the solar data accumulated, significant questions remained in the realm of spectroscopic practice: To what extent do temperature and pressure affect the creation and appearance of spectral lines? Which of the purported solar spectral lines arise instead from absorption of sunlight in Earth’s atmosphere? To the perplexing pile of numerical and visual information were applied a host of speculative, often contradictory, physical theories. Benighted by their foundational ignorance, astronomers sparred uselessly in private correspondence, professional journals, and conferences over interpretations of one or another observation. Without a realistic model of the atom, for instance, the origin of spectral lines cannot be explained: How does an atom emit or absorb light in the doses and at the specific wavelengths it does? Why does the very same atom ignore light of other wavelengths? What, structurally, differentiates one element from another?
Superlatives abounded in the professional and public press about the evolving fusion of solar photography and solar spectroscopy. Yet the artifacts of this fertile coupling embodied, for the most part, technological advancements whose scientific value remained latent. The astrophysics of the Sun could not be teased out until the fundamental physics of both matter and energy marched into the twentieth century.
While solar researchers haggled over the nature of our nearest star, a handful of adventurous astronomers aimed their spectroscopes in the opposite direction, toward deep space. The challenge to see, much less photograph, a stellar spectrum using 1860s technology seemed ludicrous to most telescopic observers. Even the brightest star in the night sky is one ten-billionth the brilliance of the Sun. To steer a star’s feeble glimmer into the guts of a spectroscope, disperse its aggregated wavelengths to the point of near invisibility, and then seek to extract any datum of scientific value strains the very definition of optimism. To seek such an outcome, not just for a star, but for a faint, diffuse nebula, enters the realm of delusion. Yet every uncharted realm draws its explorers. And the more distant that realm, the more intrepid the explorer.
Chapter 17
A STRANGE CRYPTOGRAPHY
[W]hen a molecule of hydrogen vibrates in the Dog-Star, the medium receives the impulses of these vibrations; and after carrying them in its immense bosom for three years, delivers them in due course, regular order, and full tale into the spectroscope of Mr. Huggins, at Tulse Hill.
—James Clerk Maxwell, “On Action at a Distance,” discourse at the Royal Institution, London, 1873
IN 1855, THIRTY-YEAR-OLD ENGLISH SILK MERCHANT and amateur astronomer William Huggins sold the family business, moved with his elderly parents to the upscale London suburb of Lambeth, and observed the night sky the way it had been done since Galileo’s time: peering into the eyepiece of a telescope and letting the cosmic light flood his retina.
Astronomy was the perfect hobby for Huggins, given his affinity for the sciences and his financial means. (It is said he dropped an interest in microscopy because of his distaste for dissection.) Huggins acquired his first telescope when he was eighteen. He replaced that instrument in 1853 with a pricey Dollond equatorial refractor of five-inches aperture. Like many neophytes, his observing runs were routine: peering at and sketching the Moon, planets, and the occasional comet. His drawings of Mars from April 1856 appear to be a hash of what he saw through the eyepiece plus what he had seen in existing illustrations. Although never formally trained in astronomy—what advanced academics he had came from private tutors—Huggins pursued his new avocation as an entrepreneur might a fledgling business: to gain one’s market share of fame required hard work, self-belief, and persistence.
Huggins was elected in 1854 to the Royal Astronomical Society, where he enjoyed fellowship with veteran cosmic seekers like Warren De La Rue, George B. Airy, and William “Eagle-eye” Dawes. (The Dawes’ limit is still used as a gauge of a telescope’s ability to resolve close-together double stars.) To his already substantial cottage on Lambeth’s Upper Tulse Hill, Huggins added a twin-story, twelve-by-eighteen-foot observatory, whose iron columns raised the working floor above the surrounding trees. In 1858, on Dawes’s recommendation, he swapped out his five-inch telescope for a top-quality, eight-inch Alvan Clark refractor. The telescope rested on a massive, pyramidal, brick pier, which was anchored in concrete at ground level. The original hemispherical dome, twelve feet wide at the base and sheathed in zinc, revolved atop three iron balls along a circular, iron channel. A doorway from the upper level of the house led straight into the observatory.
At the May 1856 meeting of the Royal Astronomical Society, Huggins presented a model and description of his new facility. He had fully entered the small army of independent amateur astronomers: men of means whose scientific aspirations superseded their more grounded ambitions in commerce or society. Often lacking the validation of an advanced degree, they delved into areas of research and invention spurned by their institutional counterparts. Over the next few years, Huggins received from these night-sky specialists a thorough education in the practical and theoretical aspects of astronomy. With time, he hoped he might garner the sort of acclaim bestowed upon his contemporary Richard Carrington, who cataloged the places of 3,735 stars from his private observatory at Redhill, Surrey. An 1858 encomium to Carrington captures the dynamism Huggins would come to share with his fellow amateur scientists: “Talent and zeal, untiring devotion and industry, and an unsparing but prudent application of private resources, have equally combined to produce the work in question. . . . The work was a labour of love; and the results are such as might be expected,—of unquestioned excellence, and a standing memorial of his ability and love of science.”
William Huggins’s observatory after its 1870 expansion. At front, Huggins’s wife Margaret.
In January 1862, having marked time in desultory studies of planets and double stars, Huggins attended a lecture at London’s Pharmaceutical Society by his neighbor, William Allen Miller, Chair of Chemistry at King’s College. It was here, presumably, that Huggins learned of the momentous discovery by German scientists Robert Bunsen and Gustav Kirchhoff: the perplexing array of dark lines in the solar spectrum—the Fraunhofer lines—might be used to deduce the chemical constituents of the Sun. No longer did the Sun’s remoteness place it frustratingly out of reach of laboratory-type scrutiny; to the contrary, its relentless stream of light delivers its chemical signatures right to Earth.
Miller’s lecture was nominally about the science of spectroscopy, but was also a retort to Bunsen and Kirchhoff’s claim of priority in spectrochemical analysis. Thus, Huggins heard the subject’s entire time line of development, from Newton’s seventeenth-century experiments to present-day research in England and on the Continent. The saga would no doubt have inspired the autodidact Huggins, who privately sought validation by his peers. How might one establish one’s own research credentials among the crowded roster of visual observers?
Decades later, in a much-read retrospective titled The New Astronomy, William Huggins described in heroic prose his response to the genesis tale of cosmochemistry:
This news was to me like the coming upon a spring of water in a dry and thirsty land. Here at last presented itself the very order of work for which in an indefinite way I was looking—namely, to extend [Bunsen and Kirchhoff’s] novel methods of research upon the sun to the other heavenly bodies. A feeling as of inspiration seized me: I felt as if I had it now in my power to lift a veil which had never before been lifted; as if a key had been put into my hands which would unlock the unknown mystery of the true nature of the heavenly bodies.
Whether this latter-day account portrays a genuine eureka moment or else the self-congratulatory embellishment of an aging man, Huggins did set his sights—and a spectroscope—on the stars. He stocked his household observatory with the trappings of the Victorian spectroscopist until it resembled a Frankenstein’s laboratory: prisms, batteries, electrical spark coils, Bunsen burners, and chemical powders and fluids. And in William Allen Miller, his neighbor, Huggins found a close-at-hand and knowledgeable (if initially hesitant) collaborator.
William Huggins in his observatory.
Like their contemporaries Bunsen and Kirchhoff, Huggins and Miller were a complementary pair, whose collective productivity far exceeded the sum of what either might have accomplished on his own. Miller was the spectroscopic expert of decades standing, who provided the materials and the know-how to carry laboratory practice into the observatory. Huggins was the astronomer and inveterate maker, whose stamina, manipulative skill, and visual acuity secured results. Lacking the deep state and institutional pockets available at Heidelberg, Huggins bankrolled every aspect of his outsized hobby.
From the start, in early 1862, Huggins and Miller set themselves an ambitious agenda for a technology so new and so tangled. The spectroscope had proven its worth in the energy-lavish domain of the Sun, but was virtually untried on the dim luminal inflow of stars. Like other researchers, Huggins and Miller wished to cross-check the elemental composition of the Sun against those of the stars, based on coincidences of spectral lines. That meant building a spectroscope with sufficient dispersive power to portray the full complement of stellar lines, but not so dispersive as to dilute the already-faint spectrum to the point of invisibility.
Starlight Detectives Page 22
