The Day We Found the Universe - Marcia Bartusiak (2009)
Chapter 6. It Is Worthy of Notice
First-time travelers to the southern hemisphere might mistake the clouds for high cirrus formations, somehow made luminous in the dark of night. Ancient Persians called the biggest one Al Bakr, or the White Ox. Europeans were introduced to the “two clouds of mist” from accounts of the first circumnavigation of the globe by Ferdinand Magellan and his crew in the early sixteenth century. And so the hazy pair came to be named in honor of the Portuguese-born explorer. The Large and Small Magellanic Clouds are each a chaotic collection of stars, richly diffused with glowing gas.
Such novel and fascinating sights were a compelling reason for European and American astronomers to set up observatories in the southern hemisphere. The Harvard College Observatory did just that in the 1890s, when it established a southern station in the highlands of Peru, just above the town of Arequipa. Before this, for more than a decade, Harvard had been carrying out a formidable task: to catalog every star in the northern sky and accurately gauge its color and brightness. Presented with a sizable endowment for a program in spectroscopy, observatory director Edward C. Pickering resolved to photograph and classify the spectra of all the bright stars as well. The Peruvian observatory allowed Harvard to extend the reach and sweep of this endeavor to the southern sky. By doing this, Pickering was helping astronomy move beyond just tracking the motions of stars across the sky to figuring out their basic properties. Though tedious and wearying, such astronomical surveys can often reveal a few surprises along the way. The Harvard survey was no exception, but it took many photographs to get there.
The Small and Large Magellanic Clouds (top left, bottom left) as seen from
Cerro-Tololo Inter-American Observatory, in Chile. The Milky Way
is on the right. (Roger Smith/NOAO/AURA/NSF/WIYN)
With the huge number of photographic plates of the northern and southern skies stacking up at the observatory on Garden Street in Cambridge, Massachusetts, Pickering shrewdly recognized the value of smart young women yearning to make contributions in an era that generally denied them full access to scientific institutions. Here was a ready workforce, he noted in one annual observatory report, entirely “capable of doing as much and as good routine work as astronomers who would receive much larger salaries. Three or four times as many assistants can thus be employed, and the work done correspondingly increased for a given expenditure.”
Williamina Fleming (standing) directs her “computers” while
Harvard Observatory director Edward Pickering looks on
(Harvard College Observatory)
These women “computers,” as they were called, many with college degrees in science, were situated in two cozy workrooms, pleasantly decorated with flowered wallpaper and star charts. Working at ma hogany writing tables, crammed together, each woman through the day might peer through a magnifying glass at her selected plate or industriously record her findings in a notebook. Resembling assembly-line workers in a factory, with their plain, unadorned dresses, these dedicated women—swiftly, accurately, and cheaply—numbered each star on a given plate, determined the star's exact position, and assigned it either a spectral class or photographic magnitude. Annie Jump Cannon, who established the stellar classification system adopted internationally in the course of this work, praised Pickering's modern outlook. “He treated [the computers] as equals in the astronomical world,” she claimed (somewhat Pollyannishly), “and his attitude toward them was as full of courtesy as if he were meeting them at a social gathering.” He was their gallant Victorian gentleman.
Pickering's first hire was his housekeeper, Williamina Fleming, who had displayed a keen intelligence in carrying out her duties. Frustrated one day by a male assistant's ineptitude, Pickering had declared that his maid could do a better job, and he found out she could. From the 1880s until Pickering's death in 1919, some forty women came to be employed in “Pickering's harem,” as it was jokingly known. One of his most brilliant choices was Henrietta Leavitt, who first began work at the Harvard Observatory as a volunteer.
Leavitt grew up in Massachusetts, within a big and supportive family (she was the oldest of five surviving children) that cherished education. Her father held a doctorate in divinity from the Andover Theological Seminary. The Leavitts moved to Cleveland when Henrietta was a teenager, where she eventually began her undergraduate studies at Oberlin College. In 1888, at the age of twenty, though, she returned to Massachusetts and entered the Society for the Collegiate Instruction of Women in Cambridge (what later became Radcliffe College). She primarily took courses in the arts and humanities but in her fourth year enrolled in an astronomy course. It must have been an inspiration, because after receiving her certificate in 1892, which stated that she had undertaken an education equivalent to a Harvard bachelor of arts degree, Leavitt remained in Cambridge to take some graduate courses and work as an unpaid helper at the college observatory.
According to those who knew her, she was a serious-minded woman, devoted to her family circle and to her friends. A photograph of her reveals a woman of quiet beauty with soulful eyes. “For light amusements, she appeared to care little,” said Harvard astronomer and colleague Solon Bailey. Yet, he went on, she was still “possessed of a nature so full of sunshine that, to her, all of life became beautiful and full of meaning.” Her good-natured disposition remained even after she experienced, sometime after her graduation, a serious illness that left her severely deaf.
As a volunteer she became an expert in stellar photometry, gauging the magnitude of a star by assessing the size of the spot it imprints upon a photographic plate. The brighter the star, the larger the spot made dark by the star's light upon the negative. In carrying out this work, she was also instructed to keep an eye out for variable stars, stars that regularly increase and decrease in brightness over a fixed span of time. These variables were found by comparing photographs of the same region of the sky taken at different times. Leavitt would place the negative of a photograph taken on one date directly over a positive photograph of the same region of the sky taken on another date. If the black and white images of a star did not exactly match, the star was likely changing its intensity and so was suspected to be a variable.
Henrietta Leavitt at her Harvard College Observatory desk
(AIP Emilio Segrè Visual Archives)
After writing up a draft of her initial research, Leavitt left Harvard in 1896 for a while, first traveling through Europe for two years and then moving to Wisconsin, where her father had a new ministry. But in 1902 she wrote Pickering for information on new job opportunities, at either Harvard or elsewhere. She obviously wanted to get back into astronomy and must have been overjoyed when Pickering made her an offer within three days for full-time employment. “For this I should be willing to pay thirty cents an hour in view of the quality of your work, although our usual price, in such cases, is twenty five cents an hour,” he wrote her. She replied that it was a “very liberal offer,” what today (taking inflation into account) would be a little over the U.S. minimum wage. A man would have gotten nearly twice that.
Not until the spring of 1904, though, did variable stars come back into her life in full force. Peering through a magnifying eyepiece at two photographic plates of the Small Magellanic Cloud, taken at different times, she noticed that several stars in the cloud had changed in brightness. On one plate a particular star was relatively luminous; on another plate that same star had turned far dimmer. It was as if the star were undergoing a slow-motion twinkle. Over the following year, she looked at additional images of the cloud and found dozens more. With each new delivery of plates from Harvard's station in Peru (and checks on old ones going back to 1893), she readily and meticulously updated her count, so much so that a Princeton astronomer described her as a “variable-star ‘fiend.’” Soon she included the Large Magellanic Cloud in her tally, and by 1907 she found a record-setting total of 1,777 new variable stars residing within the prominent, mistlike clouds (before that, only a couple of dozen variable stars had been detected in the Magellanic Clouds). She dutifully reported her findings in the 1908 Annals of the Astronomical Observatory of Harvard College, with thirteen pages taken up with listing every new variable she had discovered, its exact position in the sky, as well as its minimum and maximum brightness.
More intriguing was what she wrote at the end of this paper. Over the course of her painstaking examination of the Small Magellanic Cloud, she came to notice a special group of variable stars, sixteen in number. They were later identified as Cepheid variables, stars that are thousands of times more luminous than our Sun. Their name was derived from one of the first and brightest discovered, δ Cephei, located in the constellation Cepheus the King, a major landmark in the northern sky. These stars regularly vary their brightness in a matter of days or months. The shortest cycle Leavitt measured for these Magellanic variables was 1.2 days, the longest 127 days. Yet no matter if the Cepheid had a long or short period, each was as regular as a metronome in its variation. “As a rule, they are faint during the greater part of the time,” reported Leavitt, with the period of maximum brightness being fairly brief. The variable δ Cephei, for example, goes from dim to bright in just a day, then gradually fades back to its faintest magnitude over the next four days, until it suddenly brightens once again.
But it was the next sentence in Leavitt's report that turned into its most venerated statement. “It is worthy of notice,” she continued, “that… the brighter variables have the longer periods.” Since all her Cepheids were situated in the same celestial cloud, Leavitt could assume they were all roughly the same distance from Earth. And that meant she could trust that the Cepheids' periods were directly associated with their actual emission of light. Leavitt was in fact getting a first glimpse at astronomy's celestial Rosetta stone, a means for astronomers to solve the mystery of the spiral nebulae. The key was that link between a Cepheid's period—the steady rhythm of its oscillation—and its luminosity. She was on the brink of finding a new cosmic yardstick, one that would allow astronomers to determine the distances to far-off celestial objects that were formerly immeasurable by more traditional means.
Leavitt had chanced upon the celestial equivalent of lighthouses on Earth. A sailor, if he is familiar with the amount of light a particular lighthouse emits, could roughly estimate how far he is from land, given how bright the beacon appears to him from offshore. Similarly, a Cepheid's period labels it as having a particular brightness. The distance to the Cepheid is then obtained by figuring out how far away it must be to be viewed as the faint point of light we see from Earth. In this way, the Cepheid becomes a valuable “standard candle” (as astronomers call it) for gauging distances deep into space, when all other methods fail.
Bright and dim, bright and dim goes a Cepheid's cycle, but not endlessly. It was long believed that a Cepheid was an eclipsing binary star—one star regularly circling another like the Earth going around the Sun. But by 1914 it was recognized that this type of variable was actually a single, pulsating star, its atmosphere regularly ballooning out and then shrinking back in, over and over again. When the origin of a star's power was at last understood, astronomers came to see that a Cepheid is a star far more massive than our Sun, anywhere from five to twenty solar masses, that has reached a particular stage in its evolution. Having used up its main supply of hydrogen, the Cepheid becomes unstable for a while (about a million years) as it adjusts to burning new sources of nuclear fuel. When the star is compact, pressures build up, causing the star's outer atmosphere to expand and thus become more luminous. But once stellar pressures are reduced, gravity takes over and causes the star to contract back and become dimmer—that is, until stellar pressures build up once again. In this way, the Cepheid comes to pulsate in a regular fashion. More important, the brighter and more massive Cepheids oscillate more slowly than the fainter and smaller ones.
In 1908 Leavitt was wary that her initial sample of sixteen Cepheids was too small to secure a firm and predictable “period-luminosity” law. She needed more, but chronic illnesses and the death of her father delayed her a few years. Moreover, though very bright, allowing them to be seen over long distances, Cepheids are also very rare. Not until 1912 was Leavitt able to add nine more Small Magellanic Cepheids to her list. With twenty-five in hand, she could at last establish a distinct mathematical relationship between a Cepheid's blinking and its perceived brightness.
Science often involves discovering patterns, spotting regularity and order where none before had been noticed. And the pattern that Leavitt made plain, with such patient care and shrewd insight, in time opened up the universe. The connection was immediately apparent when Leavitt plotted her data on a graph. “A remarkable relation between the brightness of these variables and the length of their periods will be noticed,” she wrote, with a decided animation rare in scientific discourse. On a logarithmic scale, the visible brightness of her Cepheids rises steadily as the stars' periods get longer and longer. Her variable stars huddle along a sure, straight line from the bottom left to the upper right of the graph paper. This historic finding was published as Harvard College Observatory Circular, No. 173, a three-page paper titled “Periods of 25 Variable Stars in the Small Magellanic Cloud” and now considered a “masterpiece” of scientific literature.
Henrietta Leavitt's historic 1912 graph showing how a Cepheid's
brightness increases as the variable star's period gets longer (From
Harvard College Observatory Circular, No. 173 , Figure 2)
Cepheids stood ready to be the perfect standard candles, but first she needed to know the true brightness of at least one, the luminosity she would observe if she were essentially right next to the star. If she could determine the brightness of just one, her graph would let her know all the others. Once her graph was calibrated in this way, an astronomer could pick out a far-off Cepheid anywhere in the sky, measure its period, and infer its actual luminosity. The distance to the Cepheid then followed: By measuring the Cepheid's apparent brightness in the sky (a much fainter magnitude), you could figure out how far away it must be to appear that dim. Cepheids held the promise of being astronomy's handiest cosmic measuring tape. Astronomers could at last gauge the distance to celestial objects farther out than they ever conceived possible. Leavitt knew this, but she wasn't one to state things so daringly. Besides, Pickering chose his women computers “to work, not to think,” according to one Harvard astronomer. So, in a far quieter tone, Leavitt simply wrote at the close of her paper, “It is to be hoped, also, that the parallaxes [essentially, distances] of some variables of this type may be measured.”
What was needed was an indisputable distance to a bona fide Cepheid. But Leavitt's going to a telescope to pursue an answer was out of the question, not only because women were denied access to the best telescopes at the time (generally considered man's work) but because of her frail condition. Given her deafness and frequent illnesses, she had been advised by her doctor to avoid the chilly night air, an environment habitually faced by observers. She came to believe the cold aggravated her hearing condition. If she had the know-how, she could have carried out a calculation from her desk, using stellar data from previously published work, but Pickering held the strong conviction that his observatory's prime function was to collect and classify data, rather than apply it to solve problems. The accumulation of facts was Pickering's prime directive, so he quickly assigned Leavitt another task, a project on stellar magnitudes that he considered far more important. She respectfully carried out her boss's directive without objection for a number of years. Back in her room at the observatory, she continued to work on the stars photographed by others. Cecilia Payne-Gaposchkin, who came to the Harvard Observatory in the 1920s, called this “a harsh decision, which condemned a brilliant scientist to uncongenial work, and probably set back the study of variable stars for several decades.” Yet Leavitt's effort was not wasted. In the end her delegated work served as the basis for an internationally accepted system of stellar magnitudes.
But her desire to pursue the variables never left her; it only awaited the proper time to act on it. Soon after Pickering's death, Leavitt at last divulged her most cherished interest to the observatory's new director, Harlow Shapley. Once he arrived at Harvard in 1920, she lost no time in asking his advice on advancing her research on the stars in the Magellanic Clouds. By then Shapley had already calibrated the Cepheids, but he told Leavitt he would like to see a deeper investigation of the short-period variables, stars that pulse over a matter of hours instead of many days. “[It's] of enormous importance in the present discussions of the distances of globular clusters and the size of the galactic system,” he said. Moreover, does the same period-luminosity law also work for stars in the Large Magellanic Cloud? he asked. He wished her success on tackling these questions.
But just as she was on the verge of completing her prolonged stellar magnitude project—possibly when she would have at last gone back to her work on the Cepheids—Henrietta Leavitt passed away at the age of fifty-three. She had faced a long and grueling struggle with stomach cancer. By the time of her death, on December 12, 1921, she had discovered some twenty-four hundred variable stars, about half the number then known to exist. Her contributions at Harvard had been unique, making it difficult for them to replace her. “Miss Leavitt had no understudy competent to take up her work,” Shapley told a colleague the day after her death. Unaware of her passing, a member of the Royal Swedish Academy of Sciences four years later contacted the Harvard Observatory to inquire about her discovery, intending to use the information to nominate her for a Nobel Prize in Physics. But by the rules of the award, the names of deceased individuals could not be submitted.