The Glass Universe: The Hidden History of the Women Who Took the Measure of the Stars

When the thirty-year-old Pickering took over as director on February 1, 1877, his primary responsibility had been to raise enough money to keep the observatory solvent. It received no support from the college to pay salaries, purchase supplies, or publish the results of its labors. Aside from interest on its endowment and income from its exact-time service, the observatory depended entirely on private bequests and contributions. A decade had passed since the last solicitation for funds. Pickering soon convinced some seventy astronomy enthusiasts to pledge $50 to $200 per year for five years, and while those subscriptions trickled in, he sold the grass cuttings from the six-acre observatory grounds at a small profit. (They brought in about $30 a year, or enough to cover some 120 hours’ worth of computing time.)

Born and bred on Beacon Hill, Pickering navigated easily between the moneyed Boston aristocracy and the academic halls of Harvard University. In his ten years spent teaching physics at the fledgling Massachusetts Institute of Technology, he had revolutionized instruction by setting up a laboratory where students learned to think for themselves while solving problems through experiments that he designed. Pursuing his own research at the same time, he explored the nature of light. He also built and demonstrated, in 1870, a device that transmitted sound by electricity—a device identical in principle to the one perfected and patented six years later by Alexander Graham Bell. Pickering, however, never thought to patent any of his inventions because he believed scientists should share ideas freely.

At Harvard, Pickering chose a research focus of fundamental importance that had been neglected at most other observatories: photometry, or the measurement of the brightness of individual stars.

Obvious contrasts in brightness challenged astronomers to explain why some stars outshone others. Just as they ranged in color, stars apparently came in a range of sizes, and existed at different distances from Earth. Ancient astronomers had sorted them along a continuum, from the brightest of “first magnitude” down to “sixth magnitude” at the limit of naked-eye perception. In 1610 Galileo’s telescope revealed a host of stars never seen before, pushing the lower limit of the brightness scale to tenth magnitude. By the 1880s, large telescopes the likes of Harvard’s Great Refractor could detect stars as faint as fourteenth magnitude. In the absence of uniform scales or standards, however, all estimations of magnitude remained the judgment calls of individual astronomers. Brightness, like beauty, was defined in the eye of the beholder.

Pickering sought to set photometry on a sound new basis of precision that could be adopted by anyone. He began by choosing one brightness scale among the several currently in use—that of English astronomer Norman Pogson, who calibrated the ancients’ star grades by presuming first-magnitude stars to be precisely a hundredfold brighter than those of sixth. That way, each step in magnitude differed from the next by a factor of 2.512.

Pickering then chose a lone star—Polaris, the so-called polestar or North Star—as the basis for all comparisons. Some of his predecessors in the 1860s had gauged starlight in relation to the flame of a kerosene lamp viewed through a pinhole, which struck Pickering as tantamount to comparing apples with oranges. Polaris, though not the sky’s brightest star, was thought to give an unwavering light. It also remained fixed in space above Earth’s north pole, at the hub of heavenly rotation, where its appearance was least susceptible to distortion by currents of intervening air.

With Pogson’s scale and Polaris as his guides, Pickering devised a series of experimental instruments, or photometers, for measuring brightness. The firm of Alvan Clark & Sons built some dozen of Pickering’s designs. The early ones attached to the Great Refractor—the observatory’s premier telescope, a gift from the local citizenry in 1847. Ultimately Pickering and the Clarks constructed a superior freestanding model they called the meridian photometer. A dual telescope, it combined two objective lenses mounted side by side in the same long tube. The tube remained stationary, so that no time was lost in repointing it during an observing session. A pair of rotating reflective prisms brought Polaris into view through one lens and a target star through the other. The observer at the eyepiece, usually Pickering, turned a numbered dial controlling other prisms inside the instrument, and thus adjusted the two lights until Polaris and the target appeared equally bright. A second observer, most often Arthur Searle or Oliver Wendell, read the dial setting and recorded it in a notebook. The pair repeated the procedure four times per star, for several hundred stars per night, exchanging places every hour to avoid making errors due to eye fatigue. In the morning they turned over the notebook to Miss Nettie Farrar, one of the computers, for tabulation. Taking Polaris’s arbitrarily assigned magnitude of 2.1 as her base, Miss Farrar arrived at relative values for the other stars, averaged and corrected to two decimal places. By these means, it took three years for Pickering and his crew to pin a magnitude on every star visible from the latitude of Cambridge.

The objects of Pickering’s photometry studies included some two hundred stars known to vary their light output over time. These variable stars, or “variables,” required the closest surveillance. In his 1882 report to Harvard president Charles Eliot, Pickering noted that thousands of observations were needed to establish the light cycle of any given variable. In one instance, “900 measures were made in a single night, extending without intermission from 7 o’clock in the evening until the variable had attained its full brightness, at half past 2 in the morning.”

Pickering needed reinforcements to keep watch over the variables. Alas, in 1882, he could not afford to hire even one additional staff member. Rather than dun the observatory’s loyal subscribers for more money, he issued a plea for volunteers from the ranks of amateur observers. He believed women could conduct the work as well as men: “Many ladies are interested in astronomy and own telescopes, but with two or three noteworthy exceptions their contributions to the science have been almost nothing. Many of them have the time and inclination for such work, and especially among the graduates of women’s colleges are many who have had abundant training to make excellent observers. As the work may be done at home, even from an open window, provided the room has the temperature of the outer air, there seems to be no reason why they should not thus make an advantageous use of their skill.”

Pickering felt, furthermore, that participating in astronomical research would improve women’s social standing and justify the current proliferation of women’s colleges: “The criticism is often made by the opponents of the higher education of women that, while they are capable of following others as far as men can, they originate almost nothing, so that human knowledge is not advanced by their work. This reproach would be well answered could we point to a long series of such observations as are detailed below, made by women observers.”

Pickering printed and distributed hundreds of copies of this open invitation, and also convinced the editors of several newspapers to publish it. Two early responses arrived in December 1882 from Eliza Crane and Mary Stockwell at Vassar College in Poughkeepsie, New York, followed by another from Sarah Wentworth of Danvers, Massachusetts. Pickering began assigning particular variables to individuals for observation. Although his volunteers lacked any equipment as sophisticated as the meridian photometer, they could compare their variables with other nearby stars, and estimate the brightness changes over time. “If any of the stars become too faint,” he advised them by letter, “please send word, so that observations may be attempted here” with the large telescope.

Some women wrote to request formal instruction in practical or theoretical astronomy, but the observatory provided no such courses, nor could it admit curious spectators, male or female, at night. During the day, the director would be only too pleased to show visitors around the building.

Pickering’s daytime duties as director required him to correspond regularly with other astronomers, purchase books and journals for the observatory’s library, attend scientific meetings, edit and publish the Annals of the Astronomical Observatory of Harvard College, oversee finances, answer inquiries by mail from the general public, host visiting dignitaries, and order supplies large and small, from telescope parts to furnace coal, stationery, pens, ledgers, even “water closet paper.” Every bit of observatory business demanded his personal attention or at the very least his signature. Only when a blanket of clouds hid the stars could he find a night’s sleep.

• • •

MRS. DRAPER’S GLASS PLATES demanded examination by daylight. Although Pickering had heard much about these images, and even discussed them with the doctor the night of the Academy dinner in November, he had not seen them till now. He was accustomed to looking at spectra—the separated rays of starlight—through the telescope, using attachments called spectroscopes that former director Joseph Winlock had purchased in the 1860s, when spectroscopy came into vogue. The live view through the spectroscope turned a star into a pale strip of colored light ranging from reddish at one end through orange, yellow, green, and blue to violet at the other. The spectroscope also made visible many black vertical lines interspersed at intervals along the colored strip. Astronomers believed that the breadth, intensity, and spacing of these spectral lines encoded vital information. Though the code remained unbroken, a few investigators had proposed schemes to classify the stars by type, according to the similarities in their spectral line patterns.

On the Draper plates, each spectrum looked like a gray smudge barely half an inch long, yet some contained as many as twenty-five lines. As Pickering viewed them under a microscope, their detail stupefied him. What skill their capture demonstrated, and what luck! He knew of only one other person in the world—Professor William Huggins of England—who had ever succeeded in capturing a stellar spectrum on a photographic plate. Huggins was also the only man of Pickering’s acquaintance, aside from Dr. Draper, to have discovered an able astronomical assistant in his own wife, Margaret Lindsay Huggins.

Mrs. Draper agreed to leave her plates in Pickering’s care for a complete analysis, and returned to New York. She promised Mrs. Pickering, who was considered one of Cambridge’s most accomplished gardeners, to visit again in spring or summer, in the hope of seeing the observatory grounds in full bloom.

Pickering measured each spectrum with a screw-thread micrometer. By February 18, 1883, he could report to Mrs. Draper that he was finding “much more in the photographs than appears at first sight.” The computers had plenty to do in graphing the readings from his every half-turn of the screw, then applying a formula and computations to translate them into wavelengths. It became clear that Dr. Draper had demonstrated the feasibility of studying the stellar spectra by means of photography, instead of by peering through instruments and drawing a record of what the eye saw.

Pickering again pressed Mrs. Draper to publish an illustrated account, not merely to establish priority for her husband, but, more important, to show other astronomers the great promise of his technique.

For help with the preparation of the paper, Mrs. Draper asked a noted authority on the solar spectrum, Charles A. Young of Princeton, to contribute an introduction outlining Henry’s methods. Meanwhile she catalogued all seventy-eight plates in the spectra series, relying on Henry’s notebooks to specify the date and time of each photograph taken, the star name, the length of every exposure, the telescope used, and the width of the spectroscope slit, plus incidental remarks about observing conditions, such as “There was blue fog in the sky” or “The night was so windy that the dome was blown around.”

Pickering summarized the twenty-one plates he had scrutinized in ten tables with explanations. He reported the distances between spectral lines, stating the methodology and mathematical formulas employed to translate line positions into wavelengths of light. He also commented on the similar work being done by William Huggins in London, and ventured to categorize some of Draper’s spectra by Huggins’s criteria. When he sent his draft to Mrs. Draper for approval, she balked at the mention of Huggins.

“Dr. Draper did not agree with Dr. Huggins,” she wrote Pickering on April 3, 1883, concerning two of the stars in the series. Their nearly identical spectra both showed wide bands, which had made Huggins classify the two stars as a single type, but the Draper photographs revealed that one of these stars also had many fine lines between the bands, which set it apart from the other. “In view of this I should not like to accept Mr. Huggins’ classification as the standard when Dr. Draper did not agree with it.” Although Pickering had seen the abundance of fine lines she described, he found them too delicate for satisfactory measurement.

“You will not I hope be annoyed at my criticism,” Mrs. Draper added, “but I feel in publishing any of Dr. Draper’s work that I want his opinions represented as nearly as possible, now that he is not here to explain them himself.”

The Drapers had met William and Margaret Huggins while visiting London in June 1879, at the Hugginses’ home observatory on Tulse Hill. Mrs. Draper recalled Mrs. Huggins as a petite woman with short, unruly hair that stuck straight out from her head as though galvanized. She was half the age of her husband, but a full participant in his studies, both at the telescope and in the laboratory.

The two couples seemed destined to become either rivals or intimates. William gave Henry the benefit of his lengthier experience by offering helpful advice about spectroscope design. He also recommended a new type of dry, pretreated photographic plate that had lately come on the market. There was no need to paint liquid emulsion on these plates just prior to exposing them, and consequently they allowed for much longer exposure times. Before leaving England, the Drapers purchased a supply of Wratten & Wainwright’s London Ordinary Gelatin Dry Plates, which proved a boon indeed. They were particularly sensitive to the ultraviolet wavelengths of light, beyond the range of human vision. Unlike the old wet plates, the dry ones created a permanent record suitable for precision measurement. The dry plates gave the Drapers the wherewithal to photograph the spectra of the stars.

• • •

THE PAPER ANNOUNCING the stellar spectra findings, “by the late Henry Draper, M.D., LL.D.,” appeared in the Proceedings of the American Academy of Arts and Sciences in February 1884. Pickering mailed copies to prominent astronomers everywhere. By return mail dated March 12, he received William Huggins’s indignant reaction. Huggins found some of Pickering’s measurements “very wild,” the letter said with emphasis. “I should be glad if you could see your way to look into this, because it would be better that you should discover the error & publish the correction, than that the matter should be pointed out by others. … My wife unites in kind regards to you and Mrs. Pickering.”

Pickering was certain he had not erred. And, as Huggins had never explicated his measurement procedures, Pickering stood firmly by his own. As they traded charges, Pickering forwarded Huggins’s letters to Mrs. Draper.

Now it was her turn to grow indignant. “I felt very sorry,” she wrote Pickering on April 30, 1884, “that you should have been subjected to such an ungentlemanly attack, through your interest in Dr. Draper’s work.” Before returning the letters to Pickering, she took the liberty of copying one, since “it is worth preserving as a curiosity of epistolatory literature.”

During this same time, Pickering was seeking assistants who might help Mrs. Draper advance her husband’s work to the next stage. He considered former director Joseph Winlock’s son, William Crawford Winlock, currently employed at the U.S. Naval Observatory, to be a very likely prospect, but Mrs. Draper rejected him. To her regret, she could not induce her preferred candidate, Thomas Mendenhall, to leave his professorship at Ohio State University. She channeled some of her frustration into the creation of the Henry Draper gold medal, to be awarded periodically by the National Academy of Sciences for outstanding achievements in astronomical physics. She gave the Academy $6,000 to endow the prize fund, and spent another $1,000 commissioning an artist in Paris to fashion a medal die featuring Henry’s likeness.

The spring of 1884 brought Pickering new money worries. The successful five-year subscriptions from generous astronomy enthusiasts had run their course, ending the accustomed annual stipend of $5,000. The director was covering various operating expenses out of his own salary, and even so was forced to let go five assistants. In a touching show of solidarity, observatory colleagues took up a collection to retain one of those who had been dismissed, and furnished “part of the required sum,” Pickering told his circle of advisers, “from their own scanty means.” He appreciated the “extraordinary efforts on the part of the observers, who have performed without assistance the work in which they were previously aided by recorders. This has required an increase in the time spent in observation, and has rendered the work much more laborious. While this evidence of enthusiasm and devotion to science is most gratifying, it is obvious that it cannot long be continued without injury to health. Indeed, the effects of over-fatigue and exposure during the long, cold nights of last winter were manifest in more than one instance.”

The motto on the Pickering family coat of arms, “Nil desperandum,” plus the lifelong habit of his own thirty-seven years, obliged the director to substitute resourcefulness and resilience for despair. He began formulating a means of combining Mrs. Draper’s wishes and wealth with the capabilities and needs of his observatory.

“I am making plans for a somewhat extensive piece of work in stellar photography in which I hope that you may be interested,” he informed her in a letter of May 17, 1885.

Pickering intended to redirect most of the observatory’s projects along photographic lines. His predecessors the Bonds had recognized the promise of photography, and achieved the first photograph of a star in 1850, but the limitations of the wet plates had impeded further attempts. With the new dry plates, possibilities multiplied. Determinations of stellar brightness and variability would surely prove easier and more accurate on photographs, which could be examined, reexamined, and compared at will. A methodical program for photographing the entire sky would transform the painstaking process of zone mapping. As a bonus, these photographs would reveal untold numbers of unknown faint stars, invisible even through the world’s biggest telescopes, because the sensitive plate, unlike the human eye, could gather light and aggregate images over time.

Pickering’s younger brother, William, a recent graduate of MIT, was already teaching photographic technique there and testing the limits of the art by trying to photograph objects in motion. The twenty-seven-year-old William had consented to assist Edward in a few photographic experiments with the Harvard telescope. One of their pictures yielded 462 stars in a region where only 55 had been previously documented.

The part of Pickering’s plan with the greatest potential interest for Mrs. Draper concerned a new approach to photographing stellar spectra. Rather than focus on one target star at a time, à la Draper or Huggins, Pickering anticipated group portraits of all the brightest stars in a wide field of view. To achieve these, he envisioned a new instrument setup combining telescope and spectroscope with the type of lens used in the studios of portrait photographers.

“I think there will be no difficulty in carrying out this plan without your aid,” he assured Mrs. Draper. “On the other hand, if it commends itself to you, I am confident that we could make it conform to such conditions as you might impose.”

“Thanks for your kindness,” she replied on May 21, 1885, “in remembering my desire to be interested in some work with which Dr. Draper’s name could be associated, and his memory kept alive. I will be glad to cooperate, if I can, in what you suggest, for its bearing on stellar spectrum photography appeals to me very strongly.” More than two years had passed since Henry’s death. Still unable to make his observatory productive, she saw no harm in lending his name to Harvard.

Pickering proceeded slowly and with caution, apprising her of his progress until he could send her some sample images of stellar spectra taken through his new apparatus. She found them “exceedingly interesting.” On January 31, 1886, she said, “I would be willing, if the plan could be carried out satisfactorily, to authorize the expenditure of $200 a month or somewhat more if necessary.” Pickering thought more would be needed. They settled terms on Valentine’s Day for the Henry Draper Memorial—an ambitious photographic catalogue of stellar spectra, gathered on glass plates. Its goal was the classification of several thousand stars according to their various spectral types, just as Henry had set out to do. All results would be published in the Annals of the Harvard College Observatory.

On February 20, 1886, Mrs. Draper sent Pickering a check for $1,000, the first of many installments. Pickering publicized the new undertaking in all the usual places, including Science, Nature, and the Boston and New York newspapers.

Later that spring Mrs. Draper decided to increase her already generous gift by donating one of Henry’s telescopes. She visited Cambridge in May to make the arrangements. Since the instrument needed a new mounting—something Henry had meant to build himself—she asked George Clark of Alvan Clark & Sons to fabricate the parts, at a cost of $2,000, and to oversee the transfer of the equipment from Hastings to Harvard. Once arrived, it would require its own small building with a dome eighteen feet in diameter, and Mrs. Draper meant to cover that expense as well. Together with the Pickerings, she strolled among the plantings of rare trees and shrubs around the observatory to select a site for the new addition.

CHAPTER TWO

What Miss Maury Saw

THE INFUSION OF FUNDS for the Henry Draper Memorial made the Harvard College Observatory hum with new people and purpose. Construction of the small building to house Dr. Draper’s telescope started in June 1886 and continued through the summer while Mrs. Draper toured Europe. In October the instrument was mounted in the new dome. Now there were two telescopes outfitted for nightly rounds of spectral photography—the Draper 11-inch and an 8-inch purchased with a $2,000 grant from the Bache Fund of the National Academy of Sciences. The illustrious Great Refractor, through which the first-ever photograph of a star had been taken in 1850, later proved unsuitable for photography. Its 15-inch lens had been fashioned for visual observing; that is, for human eyes most attuned to yellow and green wavelengths of light. The lenses of the two new instruments, in contrast, favored the bluer wavelengths to which photographic plates were sensitive. The 8-inch Bache telescope also boasted a wide field of view for taking in huge tracts of sky all at once, rather than homing in on single objects.

In less than a decade at the helm, Edward Pickering had shifted the observatory’s institutional emphasis from the old astronomy, centered on star positions, to novel investigations into the stars’ physical nature. While half the computing staff continued to calculate the locations and orbital dynamics of heavenly bodies, a few of the women were learning to read the glass plates produced on-site, honing their skills in pattern recognition in addition to arithmetic. A new kind of star catalogue would soon emerge from these activities.