By February Slipher came to trust both his instrument and his expertise (which in hindsight was truly incredible; today, with far better equipment, astronomers measure Andromeda approaching us at 301 kilometers per second, a difference from Slipher's rate of less than a third of a percent). Slipher informed Lowell that the plates “agree as closely as could be expected and I can not doubt the reality of the displacement.” Andromeda had to be moving at an astounding clip. Instead of announcing the result in a major astronomical journal, though, Slipher chose to publish his brief account—just nine paragraphs—in the Lowell Observatory Bulletin. True to form, Slipher held off on any grander statement until he had secured some confirmation.
Yet even one spiral nebula velocity was an exceptional accomplishment. Many were thrilled for Slipher. “It looks to me as though you have found a gold mine,” wrote Miller, “and that, by working carefully, you can make a contribution that is as significant as the one that Kepler made, but in an entirely different way.”
Max Wolf at the Königstuhl Observatory in Heidelberg admired the spectrum's “beauty.” Edwin Frost, then editor of the Astrophysical Journal, wrote his sincere congratulations at the revelation of such an “incredible” velocity. “It is hard to attribute it to anything but Doppler shift,” he said. “Your success on this object indicates the value of elevation above the sea…. It is a pity that someone cannot try other objects of this sort at elevations of 12,000 to 15,000 ft.” Astronomers would, but only decades later.
Then there were others, such as Campbell at Lick (predictably), who were highly skeptical. “Your high velocity for [the] Andromeda Nebula is surprising in the extreme. I suppose…the error of [your] radial velocity measurement may be pretty large. I hope you have more than one result for velocity.”
To be fair to Campbell, an extraordinary finding like this needed extraordinary proof, and Slipher knew that as well. He had already put out the call for others to try to confirm it. Within a year, Wolf was able to follow up. His spectrum was cruder but still in fair agreement. Soon after, even persnickety Lick Observatory came to confirm Andromeda's fleetness. Lick astronomer William H. Wright obtained a velocity that nearly matched Slipher's. “I had planned to get at this work years ago when Fath got his big displacement… but you seem to have beaten me to it,” Wright told Slipher.
Lowell was enormously pleased. “It looks as if you had made a great discovery,” he wrote, right after Slipher's initial finding. And then the director added, “Try some more spiral nebulae for confirmation.” Slipher took up the challenge with great enterprise, for he was better at following directions than initiating his own scientific pursuits.
Working on Andromeda, though, was a holiday compared to gathering the spectral light from other spirals. Though its center is barely discernible to the naked eye, Andromeda is still the biggest and brightest spiral in the nighttime sky. The others only get progressively smaller and dimmer, which made it even harder for Slipher to obtain their velocity. “Spectrograms of spiral nebulae are becoming more laborious now because the additional objects observed are increasingly more faint and require extremely long exposures that are often difficult to arrange and carry through owing to Moon, clouds and pressing demands on the instrument for other work,” he noted in his work papers. The job for him was “heavy and the accumulation of results slow.”
Slipher's first target after Andromeda was M81, a spiral that is brighter than most, and then he looked at a peculiar nebula situated in the Virgo constellation known as NGC 4594. In his notes, he described it a “telescopic object of great beauty.” It's now popularly known as the Sombrero galaxy for its distinctive resemblance to a Mexican hat viewed from the side. Slipher eventually saw that NGC 4594 was moving at a speed “no less than three times that of the great Andromeda Nebula.” This time, however, the nebula was not traveling toward Earth but instead was whisking away at some 1,000 kilometers per second. Slipher was greatly relieved. Finding a nebula that was racing outward rather than approaching removed any lingering doubts that the velocities might not be real. “When I got the velocity of the Andr. N. I went slow for fear it might be some unheard-of physical phenomenon,” he wrote his mentor Miller. Now, by the spring of 1913, he was reassured that the spectral shifts on his plates reliably meant movement.
At this stage, with just a few measurements in hand, Slipher began to think of the nebulae as drifting by the Milky Way—coming toward us on one side of the galaxy, and wandering away on the other side. He was reluctant to speculate publicly on what the spiral nebulae might be, but he did share some of his pet theories in private correspondence with his astronomer friends. At first he thought they might be dust clouds illuminated by reflected starlight, much the way he had already proven, to great acclaim, how the famous Pleiades star cluster shines. Or maybe, he went on to muse, the spirals were very old stars “undergoing a strange disintegration, brought about possibly by their swift flight through stellar space.” But, even then, he was beginning to have reservations about such interpretations. If the spirals were indeed single stars surrounded by fine matter, Slipher posed in one 1913 letter, why are spirals not “more numerous in, rather than outside, the Galaxy?” That was the very same question Curtis was starting to ask over at the Lick Observatory.
Throughout the succeeding months Slipher kept expanding his list, one spiral at a time. His accomplishment was all the more amazing, considering the relative crudeness of his instrument. Lowell Observatory's 24-inch telescope had only manual controls, ones that weren't yet sophisticated enough for fine guiding. Yet he had to hold the tiny image of each spiral nebula on the slit of the spectrograph with utmost care and steadiness for hours on end as the heavens progressively rotated above him. When asked years later how he was able to do this, Slipher replied dryly, “I leaned against it.” Given the faintness of his targets, his exposures often ran twenty to forty hours, which meant they extended over several nights, even weeks if there was unfavorable weather. And nothing could be done when the Moon was brightly shining. “With such prolonged exposures the accumulation of plates is not very rapid,” he informed Lowell, “but the results are worth while and encouraging,” so much so that Slipher was beginning to feel uncharacteristically possessive of his findings. “It is our problem now and I hope we can keep it,” he told his boss.
Slipher need not have worried. No one else could catch up to him. By the summer of 1914 he had the velocities of fourteen spiral nebulae in hand. And with this bounty of data, an undeniable trend was at last emerging: While a few nebulae, such as Andromeda, were approaching us, the majority were rapidly moving away.
For island-universe devotees this was great news. “My harty [sic] congratulations to your beautiful discovery of the great radial velocity of some spiral nebulae,” wrote Danish astronomer Ejnar Hertzsprung. “It seems to me, that with this discovery the great question, if the spirals belong to the system of the milky way or not, is answered with great certainty to the end, that they do not.” The speeds were simply too great for them to stay put within our home galaxy. But Slipher at this stage was still on the fence. “It is a question in my mind to what extent the spirals are distant galaxies,” he responded.
For most of his career Slipher published few detailed papers of his work, outside of his observatory's in-house bulletin. He either sat on his data until he was absolutely sure of the results or generously sent his findings to others to use in their analyses. Part of this might have been a reaction to the rumpus the observatory faced whenever Lowell defended his more sensational findings. Slipher inwardly feared that the unwelcome publicity was affecting astronomers' opinions on all other research coming out of Flagstaff. So, he preferred to keep his head down, out of the line of fire, adopting the philosophy, Let the work speak for itself. The singular exception for Slipher was his work on the spiral nebulae velocities. He had worked on so many stellar and planetary spectra that he was absolutely confident of what he was seeing—so confident that he for once overcame his ho
mebound nature and traveled to Northwestern University in Evanston, Illinois, to present his results in person.
In August 1914 sixty-six astronomers from around the United States gathered at Northwestern for their annual meeting, four days of scientific talks, official business, concerts, and social excursions to Lake Michigan. It was the conference when the astronomers unanimously voted to change their title from the Astronomical and Astrophysical Society of America to simply the American Astronomical Society. At the same time, a young man named Edwin Hubble, a graduate student at the Yerkes Observatory, in Wisconsin, was elected for membership.
The presentations were made in the lecture room of the university's Swift Hall of Engineering. Slipher's paper, one of forty-eight read at the meeting, was titled “Spectrographic Observations of Nebulae.” At the start of his talk, Slipher told the audience that he began his investigations simply to obtain a spiral nebula's spectrum, but went on to say that the exceptional velocity of the Andromeda nebula made him shift his attention to the velocities themselves. The average speed of the spirals, he reported, was now “about 25 times the average stellar velocity.” Of the fifteen spiral nebulae he had observed so far, three were approaching Earth, the rest were moving away. The velocities ranged from “small,” as it was recorded on his list, to an astounding 1,100 kilometers per second. That was the greatest celestial speed ever measured up to that time.
When Slipher finished delivering this remarkable news, his fellow astronomers rose to their feet and gave him a resounding ovation. No one had ever before witnessed such a spectacle at an astronomical meeting. And with good reason: Slipher had alone climbed to the top of the Mount Everest of spectroscopy. Even Campbell, his relentless competitor, came to both accept the finding and respect the tremendous effort behind it. “Let me congratulate you upon the success of your hard work,” he wrote Slipher after the meeting. “Your results compose one of the greatest surprises which astronomers have encountered in recent time. The fact that there is a wide range of observed velocities—some of approach and some of recession—lends strong support to the view that the phenomena are real.”
Astronomers at the 1914 American Astronomical Society meeting
in Evanston, Illinois. Vesto Slipher is circled on the left,
Edwin Hubble on the right. (From Popular Astronomy,
“Report of the Seventeenth Meeting,” 1914)
Soon after, Slipher was notified that the National Academy of Sciences in the United States was about to begin publication of a periodical titled Proceedings, aimed at displaying the nation's best scientific work. Slipher was asked to contribute an account of his groundbreaking research. “I am…glad to have your kind offer to present my papers to the Academy,” he replied. “It only remains for me to do something worth sending.” Slipher, as usual, was being modest to a fault.
Over the next three years, after he had gathered more spectra, Slipher at last came around to Hertzsprung's view. He, too, began to envision the Milky Way as moving among other galaxies just like itself. He first made this view public before the American Philosophical Society, when he was invited to give a key address at its 1917 annual meeting, one of the nation's most important scientific gatherings. Keen to report on his most up-to-date findings, Slipher even enlisted the help of a mathematician—Elizabeth Williams, in Boston, who had long worked as Lowell's top computer—two weeks before the lecture to help him double-check the direction and magnitude of his full complement of spiral nebulae, now numbering twenty-five. She telegraphed her results in the nick of time.
“It has for a long time been suggested that the spiral nebulae are stellar systems seen at great distances,” said Slipher at the April conference in Philadelphia. “This is the so-called ‘island universe’ theory, which regards our stellar system and the Milky Way as a great spiral nebula which we see from within. This theory, it seems to me, gains favor in the present observations.” With all but four of his twenty-five spiral nebulae racing outward, Slipher speculated at one point that the spirals might be “scattering” in some way, a precocious intimation of the cosmic expansion that took many more years to fully recognize.
Though other astronomers were confirming a few of Slipher's results, the Lowell Observatory astronomer was the absolute ruler in this new celestial realm. He dominated the field for years. By 1925, forty-five spiral nebulae velocities were pegged with assurance, and it was Slipher who had measured nearly all of them. As early as 1915, researchers in Germany, Canada, the United States, and the Netherlands began to look for a pattern in Slipher's growing mound of data. It was an extremely difficult task, though, as the speeds measured for the spiral nebulae were entangled with other velocities, such as Earth's orbital travels and the Sun's journey through the galaxy. It was like trying to determine the exact speed of a train off in the distance, while you yourself are in a car racing down a highway.
The investigators began by subtracting out the extra factors—first the Earth's motion, then the Sun's—to see how fast the spiral nebulae were truly moving. Once these secondary velocities were removed, the astronomers saw that the nebular speeds continued to be enormous, far higher than the average velocity of a star within our galaxy. More important, they confirmed that the mistlike disks were indeed generally headed away from us. A few nebulae, such as Andromeda, were exceptions (they didn't yet know that Andromeda and the Milky Way were gravitationally bound together and so wouldn't be flying away from each other), but all in all the spiral nebulae were primarily moving outward into space in all directions. The German astronomer Carl Wirtz went even further in 1922 by looking at a nebula's size and luminosity to roughly judge which of the nebulae were closer to us and which were farther out. By making this assumption, he noticed a particular progression to the stampede outward: The more distant the nebula, the faster it was receding. That was intriguing.
But perhaps this relationship between speed and distance was a false impression. Maybe the effect would disappear as the velocities of more and more nebulae, especially those found in the southern celestial sky, were measured. It could all average out: half of the nebulae moving toward us, the others away. Astronomers began to worry that what looked like an overall recession might turn out to be a temporary illusion. To take care of this, they began to insert a special component into their equations, a term they labeled K, which kept track of the trend. Maybe this term would eventually fade away, but maybe not.
Despite these loose ends, by the time of the 1917 American Philosophical Society meeting, the island-universe theory was rousing from its slumber. Heber Curtis had begun to publish his findings on the spiral nebulae in the major journals, and his cogent arguments in support of distant galaxies were already convincing the top astronomers who counted, such luminaries as Eddington at Cambridge University, in England, Campbell at Lick, and Hertzsprung, then at the Potsdam Observatory, in Germany. The swift velocities that Slipher was finding only strengthened the idea that the spirals were indeed situated far beyond the Milky Way's borders. But success could not be fully grasped until astronomers figured out a method for determining how far away Andromeda and its sister spirals truly were. Nothing could ever be settled in this ongoing debate until someone determined the distances to these exasperating nebulae, in a way that every astronomer had confidence.
What Slipher and Curtis did not yet know was that a novel way to carry out such a celestial measurement had been budding even as they were beginning their researches on the spiral nebulae. It involved a gifted woman with a keen eye, who came upon some intriguing stars while examining photos of an alluring feature in the southern nighttime sky.
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.”
The Day We Found the Universe Page 11