The Glass Universe: The Hidden History of the Women Who Took the Measure of the Stars. Дава Собел
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.
The earliest known star counter, Hipparchus of Nicaea, catalogued a thousand stars in the second century BC, and later astronomers enumerated the content of the heavens to ever better effect. The projected Henry Draper Catalogue would be the first in history to rely entirely on photographs of the sky and to specify the “spectrum type,” as well as the position and brightness, for myriad stars.
Dr. and Mrs. Draper had gathered their spectra one by one, using a prism at the telescope’s eyepiece to split the light of each star. Pickering and his assistants, eager to increase the pace of operations, altered the Drapers’ approach. By installing prisms at the objective, or light-gathering end of the telescope, instead of at the eyepiece, they were able to capture group portraits containing two or three hundred spectra per plate. The prisms were large, square sheets of thick glass, wedge-shaped in cross section. “The safety and convenience of handling the prisms,” Pickering found, “is greatly increased by placing them in square brass boxes, each of which slides into place like a drawer.” Harvard’s picture gallery grew apace. When Mrs. Draper paid another visit soon after Thanksgiving, Pickering assured her that any star visible from Cambridge appeared on at least one of the glass plates.
Toward the end of December 1886, just when the staff had smoothed out most of the difficulties with the new procedures, Nettie Farrar’s beau proposed. Pickering was all in favor of marriage, of course, but he hated to lose Miss Farrar, a five-year veteran of the computing corps whom he had personally trained to measure spectra on the photographic plates. On New Year’s Eve, he wrote to inform Mrs. Draper of Miss Farrar’s engagement, and also to name Williamina Fleming, the former maid, as her replacement.
Since returning from Scotland in 1881, Mrs. Fleming had been assisting Pickering with photometry. Often she took the director’s penciled notations from the nightly observations with his assistants and applied the formulas he specified to compute the stars’ magnitudes. By 1886, when the Royal Astronomical Society awarded Pickering its gold medal for this work, he had already embarked on a parallel approach to photometry via photography. This change required Mrs. Fleming, well accustomed to reading lists of numbers scribbled in the dark, to judge magnitudes from fields of stars on glass plates.
Mrs. Fleming had let Pickering know that photography ran in her pedigree. Her father, Robert Stevens, a carver and gilder praised for his gold-leaf picture frames, had been the first in the city of Dundee to experiment with daguerreotyping, as the process was called in her childhood. She was still a child, only seven, when her father died suddenly of heart failure. Her mother and older siblings tried, for a time, to keep the business running without him, but without success. One by one, her older brothers sailed away to Boston, where she eventually followed them. Now, at twenty-nine, she had a seven-year-old child of her own to care and provide for. Edward would soon arrive; her mother was booking passage with him on the Prussian out of Glasgow.
Miss Farrar dutifully introduced Mrs. Fleming to the plates of stellar spectra, and taught her how to measure the hordes of tiny lines. Mrs. Fleming could have taught Miss Farrar a thing or two about marriage and childbirth, but on the subject of the spectrum she had everything to learn.
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THE YOUNG ISAAC NEWTON coined the word spectrum in 1666, to describe the rainbow colors that arose like ghostly apparitions when daylight passed through cut glass or crystal. Although his contemporaries thought glass corrupted the purity of light by imparting color to it, Newton held that colors belonged to light itself. A prism merely revealed white light’s component hues by refracting them at different angles, so that each could be seen individually.
The microscopic dark lines within the stellar spectra, to which Mrs. Fleming now directed her attention, were called Fraunhofer lines, after Joseph von Fraunhofer of Bavaria, their discoverer. A glazier’s son, Fraunhofer had apprenticed at a mirror factory and gone on to become a master crafter of telescope lenses. In 1816, in order to measure the exact degree of refraction in different glass recipes and lens configurations, he built a device that combined a prism with a surveyor’s small telescope. When he directed a beam of light from the prism through a slit and into the instrument’s magnified field of view, he beheld a long, narrow rainbow marked by many dark lines. Repeated trials convinced him that the lines, like the rainbow colors, were not artifacts of passage through glass, but inherent in sunlight. Fraunhofer’s lens-testing apparatus was the world’s first spectroscope.
Charting his finds, Fraunhofer labeled the most prominent lines with letters of the alphabet: A for the wide black one at the rainbow’s extreme red end, D for a dark double stripe in the orange-yellow range, and so on through the blue and violet to a pair named H, and ending farther along the violet with I.
Fraunhofer’s lines retained their original alphabetical designations through the decades following his death, gaining greater importance as later scientists observed, mapped, interpreted, measured, and depicted them with fine-nib pens. In 1859 chemist Robert Bunsen and physicist Gustav Kirchhoff, working together in Heidelberg, translated the Fraunhofer lines of the Sun’s spectrum into evidence for the presence of specific earthly substances. They heated numerous purified elements to incandescence in the laboratory, and showed that each one’s flame produced its own characteristic spectral signature. Sodium, for example, emitted a close-set pair of bright orange-yellow streaks. These correlated in wavelength with the dark doublet of lines that Fraunhofer had labeled D. It was as though the laboratory sample of burning sodium had colored in those particular dark gaps in the Sun’s rainbow. From a series of such congruities, Kirchhoff concluded the Sun must be a fireball of multiple burning elements, shrouded in a gaseous atmosphere. As light radiated through the Sun’s outer layers, the bright emission lines from the solar conflagration were absorbed in the cooler surrounding atmosphere, leaving dark telltale gaps in the solar spectrum.
Astronomers, many of whom had considered the Sun a temperate, potentially habitable world, were shocked to learn of its inferno-like heat. However, they were soon placated—even soothed—by the revelatory power of spectroscopy to expose the chemical content of the firmament. “Spectrum analysis,” Henry Draper told the Young Men’s Christian Association of New York in 1866, “has made the chemist’s arms millions of miles long.”
Throughout the 1860s, pioneering spectroscopists such as William Huggins discerned Fraunhofer lines in the spectra of other stars. In 1872 Henry Draper began photographing them. While the number of spectral lines in starlight paled in comparison to the rich tapestry of the Sun’s spectrum, several recognizable patterns emerged. It seemed that the stars, which had for so long been loosely categorized by brightness or color, could now be further sorted according to spectral features hinting at their true nature.
In 1866 Father