The Day We Found the Universe

by Marcia Bartusiak

  “I had not thought of making the very pretty use you make of Miss Leavitt's discovery,” Henry Norris Russell wrote Hertzsprung when this result came out. Russell had employed a similar technique around the same time, but his aim was to determine the average magnitude of a Cepheid. In the process, he concluded that they were giant stars, far bigger than our Sun. Inspired by Hertzsprung, Russell proceeded to make his own distance calculation to the Small Magellanic Cloud, arriving at 80,000 light-years. Both estimates were highly uncertain and turned out to be far less than current distance measurements (210,000 light-years), but each figure was still astoundingly huge for its day.

  Shapley soon adopted Hertzsprung's approach, although he used only eleven of Hertzsprung's thirteen Cepheids in his calibration, suspecting that two of them were peculiar. Just like Hertzsprung, he counted on a simple rule of perspective: The farther away a moving object is located from you, the slower it will appear to travel. A far-off plane seems to crawl along the sky, while a plane closer in going at the same speed would zoom right past you. After estimating an average velocity for a star, Shapley checked how his eleven Cepheids were journeying across the sky. The slower the apparent velocity, the more distant the Cepheid.

  It was at this point, though, that Shapley parted company with Hertzsprung. He didn't use Leavitt's period-luminosity relationship, which was based solely on stars in the Small Magellanic Cloud, but instead constructed his own relationship based as well on the Cepheids in the Milky Way, in order to obtain an “improved and extended” period-luminosity law combining both sets of variable stars. He then applied his new rule to the Cepheids found in the globular clusters. He would monitor a Cepheid to peg its period and then calculate the star's distance from his graph.

  This worked as long as Shapley could find Cepheids in his globular clusters. Some of the clusters had none at all, as far as he could observe. What they did harbor were variables that were not quite the same. These variables changed quite rapidly, in a matter of hours rather than days or months. There was no guarantee that they behaved in the same way as Leavitt's Cepheids.

  Shapley tried mightily to check with Leavitt on this question, writing several times to her boss, Edward Pickering, on whether she had detected fast variables in the Magellanic Clouds and found them to obey her rule. Pickering assured him that photographs were being taken. But progress on the question was occurring at a glacial pace. Pickering was keeping Leavitt busy with work he considered more important. “Routine stuff,” decried Russell to Shapley at one point. “I fear, however, that I am not the man who may justly raise my voice in criticism.”

  Eager to move forward, Shapley simply decided to treat his fast variables as if they did follow Leavitt's rule. He extended the Cepheids' period-luminosity relationship to include all these variables, both slow and fast. “This proposition scarcely needs proof,” he had boldly asserted in one early paper, though it was a very controversial decision. But by doing this, Shapley was able to determine the distances to the nearest globular clusters—a formidable task, as the stars were very faint. For clusters farther out, too remote to spot any variables, he resorted to using the brightest stars as distance markers. He just assumed that the brightest stars in a distant globular cluster had a similar magnitude on average to the brightest stars in a nearby cluster. And when the stars themselves could no longer be adequately resolved, he judged distance by the apparent size of the globular cluster in the sky. “The whole line of reasoning… was brilliant,” concluded astronomer Allan Sandage in a review of this technique decades later. Bailey, at Harvard, could have carried out this effort before Shapley, but he was overly cautious about the variables. To him there were too many uncertainties about their nature, so “definite conclusions from these data cannot be safely made,” he reported. Shapley had no such qualms.

  But it was yeoman's work, painstaking routines that took four years to complete. Shapley was securing the distance to every Milky Way globular cluster known at the time, sixty-nine in all. With the assistance of Edison Hoge, he took some three hundred photographs. Some exposures were only ten seconds in length, but others lasted up to two hours. Most took minutes. Afterward there was the brutal labor at the work-table analyzing what the images revealed. By 1917 he was writing a colleague that “the work on clusters goes on monotonously—monotonous as far as labor is concerned, but the results are continual pleasure. Give me time enough and I shall get something out of the problem yet.” By then the war was on, but Shapley didn't sign up. He claimed that Hale had convinced him to stay at his job.

  Some of the globular clusters (circled) surrounding the Milky Way

  (Harvard College Observatory, courtesy of AIP Emilio Segrè Visual Archives)

  Shapley didn't particularly enjoy his nights alone with the stars. What drove him back to the telescope month after month were his findings. With the first hint of dawn in the east, as the dome slit slowly closed with a noisome squeal, nearby coyotes would answer in kind with a serenade of high-pitched howls. At night's end, he and the other astronomers would walk back to the Monastery, sometimes whistling a merry tune if the viewing went well and forgetting that they might be disturbing the daytime observers—the solar astronomers—who were still fast asleep. Once in bed themselves, though, the nighttime observers could easily be wakened by the stirrings of the daytime crew. Both sides were together at noontime lunch, which offered the opportunity to settle any squabbles.

  Shapley was curious about nearly everything that came his way when on the mountain. “The most unwarranted fun of all comes from bugs,” he wrote a colleague while trapped in a snowstorm on Mount Wilson. “Not that I know much about them, but I am so interested that I would like to turn biologist.” In a way, he did. He began to study the travels of ants around the observatory, noticing that the higher the temperature, the quicker their pace. One species ran fifteen times faster once the Sun heated the insects by an additional 30°C. As he put it, he had discovered the “thermokinetics of ants.” Setting up “speed traps” to gauge the ants' pace precisely, he boasted he could estimate the day's temperature to within one degree by their perambulations. “Another method is to read your thermometer,” he wryly added. His findings were published in scientific journals. For further rest and relaxation, he and his wife climbed all the nearby mountains—little and big—collected plants, and killed any rattlesnakes that came their way.

  Between 1916 and 1919 Shapley published his growing body of data on the globular clusters in an extended series of papers, collectively titled “Studies Based on the Colors and Magnitudes in Stellar Clusters.” Each article progressively added another piece to the puzzle. Shapley was taking his reporting skills to a new beat. And in carrying out this endeavor he was ultimately forced to alter his original mental picture of the universe. It began to dawn on Shapley that the Milky Way was far larger than anyone had previously conceived. The first hints arrived when he estimated that some well-known star clusters within the Milky Way were at least 50,000 light-years distant. Later he was finding that the distances to the globular clusters ranged anywhere from 20,000 to 200,000 light-years.

  With the globulars acting in a way like surveyor posts, marking the boundaries of our galactic borders, the Milky Way was growing by leaps and bounds. As a result, the globular clusters could no longer be thought of as similar in size to the Milky Way, as Shapley once thought. The clusters were now far smaller by comparison. “This is a peculiar universe” was Shapley's reaction to this new cosmic landscape.

  So what did this mean for the spiral nebulae, which Heber Curtis and V. M. Slipher were now enthusiastically hawking as separate galaxies? Around this time Shapley's Mount Wilson buddy van Maanen was claiming to see some spirals rotate, an impossible feat if they were lying at a great distance. To perceive a rotation from so far away over a short period of time would mean the spirals had to be spinning at close to the speed of light!

  To understand why this would be so, imagine a kitchen clock sitting right by you
on the wall. The second hand is sweeping around the dial at a speed of about 1 centimeter per second. But then imagine the face of that clock covering the entire surface of the Moon, its apparent size looking just like the clock on your wall. Yet the clock in reality is now much bigger, so the second hand has to travel at a faster clip, about 110 miles per second, to make a full circuit over a minute's time. Now if that clock were as big as the Milky Way, the second hand would be moving at a demonic pace. If van Maanen's spiral nebula was truly a distant galaxy and he was able to detect its arms shift over a matter of years, then he was seeing it rotate at light-defying speeds.

  Not willing to tolerate such bizarre behavior, Shapley at first was doubtful of van Maanen's finding. In fact, he published an article in 1917 saying that “the minimum distance of the Andromeda Nebula must be of the order of a million light-years,” based on some dim novae detected within the nebula and the faintness of its brightest stars. “The difficulty is obvious,” he continued, “in reconciling van Maanen's measures of internal proper motion with the hypothesis of external galactic systems. We are not prepared to accept velocities of rotation of the order of the velocity of light.” The issue in question was not whether spiral nebulae truly rotate. In the 1910s Vesto Slipher had already detected evidence that they spin around. The proof was found within the spectra of the spirals that he was examining. Like a Frisbee thrown outward and spinning in flight, the rotation on one side is directed forward, adding to the measured velocity, while on the other side the spin is aimed back, subtracting from the overall speed. This difference manifests itself as a slight inclination in the spiral's spectral lines. But this motion was certainly not rapid enough to notice by eye alone when comparing photos taken just a few years apart. Moreover, the spectral signatures indicated that a spiral was closing up, wrapping its arms tighter around the nebula's center, “like a winding spring,” reported Slipher. But this clashed directly with van Maanen's claim that a spiral was opening up. Slipher, so modest and reticent, didn't make an issue of this contradiction. If he had made a clamor, loudly and persistently publicizing his proof, van Maanen's assertions might have been dismissed far earlier. But, as it turned out, Slipher's conflicting result was essentially neglected, occasionally discussed among astronomers privately but rarely singled out in print.

  Shapley soon asked his Princeton adviser, Russell, whether he too questioned van Maanen's rotations. “V. M. does a little, Hale a little more, and I much,” wrote Shapley. In time Russell replied that he was “inclined to believe in the reality of the [spirals'] internal proper motions, and hence to doubt the island universe theory. But if [the spirals] are not star clouds, what the Dickens are they?”

  Considering what happened soon after the receipt of this letter, Shapley likely took Russell's counsel very, very seriously. As Russell's protégé, he highly respected his former professor's opinion and would have had difficulty ignoring Russell's astronomical advice. It was well known in the community that Russell's “word was law,” which could “make or break a young scientist.” Shapley eventually had a change of heart, a transformation that was triggered not only by Russell's advice but also by the further evaluation of his immense pool of data.

  Starting in November 1917 Shapley fired off, with great rapidity, his next group of papers. In his ongoing series on the globular clusters, he completed articles six through twelve in just six months. It was as if he were back at his old newspaper job, pounding out an exclusive on his typewriter to meet a daily deadline. The first of these papers announced his grand goal straightaway: He intended to report on nothing less than “the general plan of the sidereal system…bearing on the structure of the universe.” Shapley made this bold claim because, while trying to make sense of all his data, he had a revelation. He came to believe that his observations were not only refashioning the Milky Way but the universe as well. Unlike many of his fellow astronomers, he was fearless at making extravagant leaps in speculation.

  At this stage Shapley had finished slogging through his many observations and calculations and had plotted the positions of the sixty-nine known globulars onto a graph. This provided him with a feel for how they were distributed through space in three dimensions. The result, he noted in paper number seven, was “striking.” Most of the clusters resided in one particular direction, over by the constellation Sagittarius. Like moths lingering by a streetlamp, they were symmetrically arranged about a spot rich in stars and nebulae within our galaxy. It was said the star clouds in this region were so thick that it was “impossible to count every star shown; the images of the faintest stars…merged into one another forming a continuous gray background.” The galactic coordinates for this spot did not match those for our solar system. The globular clusters were not arranged around the Sun at all (as might be expected). Good old Sol was situated off to the side—by Shapley's initial estimate around 20,000 parsecs, or 65,000 light-years away.

  Other astronomers had noticed this peculiar distribution of the globular clusters before. In 1909 the Swedish astronomer Karl Bohlin even dared to suggest that the center of the galaxy was in that direction, with the clusters all huddled around it. But no one at the time, including Shapley, took this idea seriously. It was just assumed that the solar system resided in the heart of the galaxy (or close to it). Now Shapley was confirming what Bohlin had suspected all along. His observations forced him to radically alter his original opinion.

  From this point on, Shapley's progress was swift. Papers eight through eleven, submitted for publication in December and January, provided the technical details on his methods, assumptions, and calibrations. Shapley knew his conclusion was going to be revolutionary, so he stacked his ammunition with orderly care. Page by page he was stepping toward his grand finale. The full-scale assault took place with paper number twelve, titled “Remarks on the Arrangement of the Sidereal Universe.” This particular article was not fully ready for submission to the Astrophysical Journal until April, in the waning days of World War I, but Shapley couldn't wait that long to spread the news. On January 8, 1918, he wrote the noted Arthur Eddington in England that “now, with startling suddenness and definiteness, [the cluster studies] seem to have elucidated the whole sidereal structure”—in other words, the architecture of the Milky Way. Not only were the globular clusters uniformly scattered around the center of the galaxy, with the Sun shoved off to the hinterlands, but the Milky Way was far larger than anyone had formerly presumed. Shapley now gauged it was an astounding 300,000 light-years from one end of the galactic disk to the other, ten times greater than previous estimates. “You may have been completely prepared for the result,” Shapley told Eddington, “but I was only partially successful as a prophet.”

  “While I cannot pretend to have anticipated the view of the stellar system that now seems to be emerging,” responded Eddington, “I do not feel any objection to it either.” This was a confidence booster for Shapley, who was still essentially a rookie in astronomical circles and assuredly grateful for support from such a renowned figure.

  Shapley didn't forget to give his boss, George Ellery Hale, then out of town on stressful war business, advance notice as well. “May I impose upon your time for a little while, with an off-hand talk about my astronomical work—divert your attention from earthly troubles to heavenly affairs?” wrote Shapley. The young staff astronomer hardly knew where to begin and for brevity's sake he cautioned Hale that he was leaving out the “probablies, perhapses, maybes, apparentlies, and other such necessary weaknesses in scientific exposition…. So my assumed surety … is neither over-confidence nor a whistling in the dark, but an agreement between us.”

  True to character, Shapley fashioned one of his ubiquitous tall tales to present his case to Hale: “The first man, away back in the later Pliocene, who knocked out a hairy elephant with his club, or saw his pretty reflection, or received a compliment, became suddenly conceited (it was a mutation) and there immediately evolved the first reflective thought in the world. It was: ‘I am
the center of the Universe!’ Whereupon he took himself a wife, transmitted this bigotry of his germplasm, and through hundreds of thousands of years the same thought without much alteration has been our heritage.” And now, he assured Hale, he would furnish the remainder of the story.

  Shapley reminded Hale that he was determining the distances to all the globular clusters then known and was getting ready to publish a series of papers announcing the results: twenty pages of tables, nearly a dozen figures, and around a hundred pages of text in all. For Hale, Shapley summarized his results in three, single-spaced typed pages. The bottom line, he said, was this: The Milky Way is huge, some 300,000 light-years in width, and the Sun is far away from the hub. “Start a messenger on a light-wave down the main highway from the center,” wrote Shapley, and he'd end up at Earth about sixty-five thousand years later. Moreover, he said, “there is no plurality of universes… The galaxy is fundamental in what we call the universe.” True to his impetuous nature, Shapley threw caution to the wind. The Milky Way was now so big, he figured, it had to be the dominant feature of the universe.