by Dava Sobel
In each group of stars, Mrs. Fleming measured the variable’s neighbors with a fly spanker, gave them alphabetical labels beginning with a for the brightest, and then calculated the brightness difference between a and b, b and c, and every other interval along the sequence. Then she judged the same set of stars, in the same way, on a second plate, a third, and a fourth. Although the pecking order of her sequences held fast, the size of the brightness intervals did not. Some of the plates had been taken with the 8-inch Draper telescope, others with the Bache. Differences between the telescopes, and also between the photographic emulsions on the plates, introduced inconsistencies. She got around them by averaging the four figures for each interval. These mean values gave her a set of stepping-stones from one end of each sequence to the other.
Assuming, for the moment, that every star labeled a equaled magnitude zero, she pegged the magnitudes of b onward by adding on the successive intervals. Next she nudged these interim values away from the arbitrary zero starting point by correlating them to visual magnitudes. The director and his assistants, both in Cambridge and in Arequipa, had repeatedly observed many of her comparison stars, and recorded their magnitudes. She pulled those numbers from published reports and paired them with hers. Subtracting the difference between the visual and the photographic for each star, she derived a mean difference for each sequence. In the final step, she added the mean difference to each star to arrive at its “adopted magnitude.”
Mrs. Fleming identified herself on the title page of her “Photographic Study” as “Curator of Astronomical Photographs.” Later, on her petition for U.S. citizenship, she shortened the title to “Astronomer,” since the form provided only a small space for stating one’s occupation. In another box, she crossed out “Wife,” typed “Husband” in its place, and put “deceased” in parentheses alongside James Orr Fleming’s name. As of September 9, 1907, she was officially an American.
Having established and disseminated her photographic standards, Mrs. Fleming began the slow work of applying them to her 222 variables. Many of these stars appeared on one hundred or more plates, and she intended to measure their magnitudes on every available image, in order to ascertain all 222 light curves. Along the way, or in the future, whenever the true magnitudes of her comparison stars became known, then the light curves of her variables could be adjusted accordingly.
Everything was relative in the realm of magnitude assessment. Mrs. Fleming’s photographic standards hinged on Pickering’s photometry, which hung in turn on decades of visual comparisons of one star to another. The great desideratum of “true” or “absolute” magnitude awaited the discovery of the distances to the stars and the dustiness of space: distance dimmed every sort of illumination; and stardust, if such a thing indeed littered the heavens, might obstruct the flow of starlight.
While Pickering praised Mrs. Fleming’s “Photographic Study” as “the first large collection of sequences of comparison stars for studying variables photographically,” he was in the midst of honing a lone stellar sequence to serve as a universal standard. Miss Leavitt contributed mightily to this effort. One day, Pickering anticipated, the great chain of forty or more stars constituting Harvard’s “North Polar Sequence” would underpin all photographic magnitudes.
At sixty-one, Pickering could still rely on his own keen vision for visual photometry. He was about to mount a new round of visual assessments of faint stars, using his latest photometer and a 60-inch reflecting telescope acquired from the estate of the late English astronomer Andrew Ainslie Common. The director’s “personal equation”—the way his eye connected to his brain and hands—naturally differed from the personal equations of his assistants, Wendell, Bailey, and Searle, and yet the decades of iteration ad nauseam had brought a gratifying consistency to their results. The Revised Harvard Photometry, published in 1908, made this manifest. It provided cumulative data on the magnitudes of nine thousand bright stars. Pickering hoped that astronomers everywhere would respect this compendium of his efforts since 1879 as the standard reference authority in the field.
In recognition of all Pickering had done to further photometry and spectroscopy, the Astronomical Society of the Pacific awarded him its 1908 Catherine Wolfe Bruce Gold Medal for lifetime achievement. Pickering might have felt even more pleased to see the honor bestowed on Mrs. Fleming, as he had often suggested, but her prospects of winning it seemed unlikely.
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THE REVISED HARVARD PHOTOMETRY, perused round the world, not only gathered and averaged information that had been scattered through several earlier volumes of the Annals, but also included the spectral type of each of the nine thousand stars, adjudged by Miss Cannon according to the Draper classification system. This useful addition soon elicited a criticism from a young Danish astronomer, Ejnar Hertzsprung of Copenhagen.
Hertzsprung shared Pickering’s ardor for hands-on photometry. For several years he had been trying to factor distance into the stellar magnitude equation, in order to determine stars’ intrinsic brightness. A number of stellar distances had been established by trigonometry for stars within one hundred light-years of the Sun. Relative distances of the farther stars could be wrung from their incremental movements, over time, across the line of sight, with the very farthest ones exhibiting the least so-called proper motion. By this yardstick, Hertzsprung revealed that some of the brightest stars lay at the greatest distances from the Sun. He could only imagine what blazing giants they must be, to beam such inordinate luminosity from the very depths of space.
In the spectra of the brightest faraway lights, Hertzsprung found very narrow, very sharply defined hydrogen lines. He recognized these traits as distinctions originally defined by Antonia Maury in describing the c division of her complex, two-tiered classification system.
As one of the first to see the wisdom of Miss Maury’s ways, Hertzsprung rued the use of Miss Cannon’s modified classification in the Revised Harvard Photometry. On July 22, 1908, he wrote to Pickering, complaining that the system adopted in the new volume was too simplistic. He compared it to a botanical classification based on the size and color of flowers instead of the morphology of plants. For emphasis, he reiterated the point with an animal analogy: “To neglect the c-properties in classifying stellar spectra, I think, is nearly the same thing as if the zoologist, who has detected the deciding differences between a whale and a fish, would continue classifying them together.”
Pickering, the original publisher of Miss Maury’s classification, appreciated its merits even while questioning its complexity. But Miss Maury had built her system on only a few hundred stars, and it might not hold true across tens of thousands. Likewise the conclusions that Hertzsprung drew from her work seemed to Pickering premature.
Miss Maury, who had never severed relations with Pickering, also wrote to him in mid-1908, to request another letter of reference. She was considering applying somewhere for an adjunct professorship in physics and astronomy. Without hesitation, the director again praised her “painstaking” investigations and “important” contributions. Presently she told him she would rather resume her research than pursue teaching. He assured her the observatory door stood open, though he could not promise her a full-time wage.
Miss Maury had long supplemented her income by giving freelance lectures, which she called “Evenings with the Stars.” Her promotional brochure boasted she had spoken at Cornell University, Wells College, the Brooklyn Institute of Arts and Sciences, the New York Academy of Science in the Museum of Natural History, and the New York City Department of Education, as well as to schools, lyceums, clubs, and parlor audiences. Her terms were ten dollars for a single talk, “Sun, Moon and Stars—A Brief Survey,” and thirty dollars for a four-part course on either “The Visible Universe” or “Evolution in the Heavens.” She illustrated the presentations with lantern slides she requested of Pickering and Mrs. Fleming, who also sent her the observatory’s circulars and other publication
s to keep her informed of scientific news through the years while she taught literature at girls’ schools in the towns near Hastings-on-Hudson.
In December 1908 Miss Maury returned to the observatory as an associate researcher. She reunited with the spectroscopic binaries that had made her reputation nearly twenty years before, and also with Beta Lyrae, the mystery variable that changed its light on an irregular, inscrutable timetable. Miss Leavitt, similarly intrigued by Beta Lyrae’s strange behavior, more than once opined to Miss Maury, “We shall never understand it until we find a way to send up a net and fetch the thing down!”
Miss Leavitt discovered another fifty-six new variables on the Harvard maps of the sky in 1908, maintaining her lead over Miss Cannon and Miss Leland by a wide margin. She also published her findings about the Magellanic Clouds. Through careful comparisons of many plates, she had observed the range of maximum to minimum brightness for all 1,777 of her variables, and listed these data in twelve dense pages of tables. Thus far, however, she had followed only a small number through their complete cycles of change. When she tabulated the periods alongside the magnitude ranges for these sixteen stars, a pattern emerged. “It is worthy of notice,” she wrote in her report, that “the brighter variables have the longer periods.” She wondered what that might mean, and whether the trend would persist. She was continuing to analyze more periods when illness interrupted her work about two weeks before Christmas. From her hospital room in Boston on December 20, she thanked Pickering for the pink roses and get-well wishes. Then she went home to Wisconsin to recuperate.
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THE IDEAL WEATHER CONDITIONS that smiled on Arequipa from May to October sometimes lasted all year long. Astronomers arriving from the north would remark on the stillness of the air that produced such optimal seeing. No radical day-to-night temperature changes unsteadied the dry atmosphere, and no predawn dew accumulated on telescope lenses. Those who stayed for many months almost welcomed the interruption of the brief cloudy season, which afforded them the time to make instrument repairs or tend to other neglected business. Of late, however, the off-season had lengthened. Cloud cover now spread a pall over the telescopes for extended periods between November and April. Harvard’s staff members at the Boyden Station had persisted in their observations through revolutionary gunfire and local epidemics of smallpox and yellow fever, but clouds could not be endured. Pickering got busy soliciting opinions about alternate sites in South Africa. As before in Colorado, California, and Peru, he needed someone to reconnoiter the potential locations. Once again he chose Solon Bailey.
At a farewell luncheon, President Eliot saluted Bailey as the observatory’s “foreign ambassador without portfolio.” Fortunately, the new venture would not require the fifty-four-year-old emissary to climb mountains or build roads. His entire African itinerary stretched across a plateau with an average elevation of 5,000 feet. Though only knee-high to the Andes, the Cape Colony’s Great Karroo plateau lay much farther south, and might facilitate the assembly of a South Polar Sequence to complement Pickering’s northern one.
Bailey left Cambridge for Africa by way of England on November 17, 1908, traveling solo, with two telescopes, a camera, and various meteorological equipment in tow. On the advice gained in London from Sir David Gill and Sir William Morris, both veterans of long service to the Royal Observatory at the Cape of Good Hope, Bailey planned to set up a principal station in Hanover. From that base, he could make excursions to test regions of the Orange River Colony, the Transvaal, and Rhodesia.
Bailey rode five hundred miles from Cape Town on a train that deposited him at Hanover Junction shortly after midnight, then covered the remaining nine miles wedged into the backseat of a horse-drawn, two-wheeled wagon called a Cape cart. He arrived at the only hotel in Hanover by 2 a.m. “The driver opened a door on the porch, lighted a candle and left me.” Bailey chose one of the room’s two beds. “On the following day the proprietor and his wife appeared and did all in their power to make my stay comfortable.”
In a ruled notebook with a blue moiré cover, Bailey graded the transparency of the African sky across a wide area over the course of a year. “The amount of clear sky and especially its distribution throughout the year,” he reported, “are much more favorable than at Arequipa.” On the other hand, the seeing, or steadiness of the atmosphere, was no better. Moving currents of air often made the stars jiggle in the telescope view. Temperatures rose and fell over a greater range than in Peru. There was more dew to contend with, not to mention the common occurrence of dust storms and violent thunderstorms.
“The vast stretches of plain, generally known as veldt, are parched and apparently dead in the dry season, but are often green and beautiful in the rainy season,” Bailey discovered. “Each farm must have its natural spring (fontein) of water for domestic and farm purposes.” Of all the places he surveyed in Africa, Bailey favored Bloemfontein, the capital of the Orange River Colony, as the best site for a permanent observatory. Its sky scored high grades for clarity on his scale, and the area “had much to offer in the way of social and educational advantages.”
While Bailey was abroad, the sky conditions at Arequipa deteriorated further. Smoke issued from the long-dormant El Misti volcano, and also the Ubinas volcano erupted, about forty miles east of the station. As a further aggravation, the condition of Mrs. Draper’s finances precipitated a drastic cut in funding.
“I have recently had to consider, very carefully, my financial condition and prospects,” she wrote to Pickering on January 24, 1909, “and find to my great regret, that I shall be unable to continue to furnish, to the Observatory, the amount I have been giving, for the past twenty-three years, for the carrying on of the work of the ‘Henry Draper Memorial.’” She set August 1 as the date she planned to reduce her monthly support to $400, less than half the accustomed sum. “I am exceedingly sorry to have to take this step, which will cause you as much surprise as it does regret to me—Fortunately, I believe that the special work for which I commenced my contribution, namely: cataloguing the stars by their spectra, is now in a fairly satisfactory state of completeness.”
Mrs. Draper’s generosity toward stellar spectroscopy had truly honored her husband’s name. But its “completeness” opened new avenues for further work. Just recently the 11-inch Draper telescope, the same one that furnished Miss Maury with detailed spectra of the bright northern stars, had been turned on fainter stars, opening them to more intense scrutiny and possible refinements in their classification.
“I have hesitated as long as is wise about making this change, but find now there is no alternative,” Mrs. Draper concluded. “I am happy that the necessity for reducing the allowance did not arise sooner, and that so much of value has been accomplished.” In this, her seventieth year, she was appreciating the retrospective view.
Pickering, now sixty-two, abandoned all hope of undertaking an expensive transatlantic relocation of the Boyden Station. At Mrs. Draper’s request, he summarized the results obtained to date with the Henry Draper fund, and projected how it would be used henceforward. He delivered the report in person.
“Since you were here I have more carefully looked over the paper,” she wrote on February 14, the anniversary of their alliance, “and feel, as I told you, that we had every reason to congratulate ourselves.” She regretted that her reduced payments would slow progress, but showed no diminution of her interest in the work or her affection for Pickering. “I enjoyed your little visit, so very much—It is always a pleasure to hear you talk of what is being accomplished at the Observatory. I wish you might run away more frequently.”
• • •
MISS LEAVITT’S CONVALESCENCE at her parents’ home in Beloit lasted longer than a year. When she finally felt ready to return to work, in January 1910, she was still not strong enough to travel to Cambridge. Pickering agreed to let her work remotely on determining magnitudes for the stars of the North Polar Sequence. Under the
special circumstances, he sent her a set of glass plates with all the accoutrements she needed—a wooden viewing frame, a magnifying eyepiece, a ledger. She worked only two to three hours a day at first, but increased her effort as her strength returned. In May she reappeared at the observatory in good health, and boarded again with the family of her uncle Erasmus Darwin Leavitt, the mechanical engineer and inventor, who lived in a large house on Garden Street near the observatory.
The summer of 1910 brought a cadre of some twenty foreign astronomers to Cambridge. Luminaries included Astronomer Royal Frank Watson Dyson, representing both Edinburgh and Greenwich; Oskar Backlund from the Pulkovo Observatory in Russia; and Karl Schwarzschild, director of the Astrophysical Observatory at Potsdam. All of them had been invited to the United States by astronomical impresario George Ellery Hale.
Hale, now founding director of the Mount Wilson Solar Observatory in California, had helped create the Astronomical and Astrophysical Society of America in 1898, and later conceived a global organization to unite investigators devoted to his own chosen specialty, the Sun. The International Union for Cooperation in Solar Research, or the “Solar Union,” met at Hale’s instigation in Oxford, England, in 1905, and in Paris in 1907. In preparation for a Pasadena meeting in 1910, Hale hoped to enlist Pickering as a member. The influential Pickering, Hale thought, could help widen the reach of the Solar Union to encompass stars beyond the Sun. Furthermore Pickering, as president of the Astronomical and Astrophysical Society of America, was perfectly positioned to host an East Coast open meeting of that organization, so timed as to bolster foreign attendance at the West Coast assembly of the Solar Union. Pickering agreed to gather the society’s members and overseas guests at Harvard in August, and then chaperone the visitors cross-country by rail for the Union get-together on Mount Wilson.
