The Day We Found the Universe

by Marcia Bartusiak

  Hubble first carried out an extensive study of the diffuse nebulae within the Milky Way, identifying the various types and describing the sources of their luminosity. But he also kept track of the “non-galactic nebulae” that he came across as he carried out this research. Hubble's sympathies certainly leaned toward the island-universe theory. When he was a graduate student at Yerkes he especially noted that the high velocities of the spiral nebulae “lend some color to the hypothesis that the spirals are stellar systems at distances to be measured often in millions of light-years.” But he became more circumspect once he became a staff member at Mount Wilson, at least in print. Caution became his byword. He emphasized in a 1922 Astrophysical Journal paper that the term non-galactic didn't mean the spirals were necessarily “outside our galaxy” but that these nebulae tended to avoid the galactic plane. At this point, Hubble's publications no longer contained grand references to island universes or other galaxies, as those of Heber Curtis and Vesto Slipher were doing. Hubble started to keep his words fairly neutral, adopting the guarded language that came to be a trademark of his research reporting. He was now consciously hiding his biases to avoid criticism.

  Hubble was far more vocal and forthright, though, about his observational plans. In February 1922 he sent a lengthy, typewritten letter to Slipher, a member of the Committee on Nebulae for the International Astronomical Union, on his long-term strategy for studying the nebulae. It was going to be an all-out attack. Hubble planned to determine their structure, peg their distribution across the heavens, and measure their dimensions. And as a stealth advocate of the island-universe theory, Hubble wanted to obtain undeniable proof that stars—vast collections of stars—resided in the spiral nebulae. He knew that finding novae were crucial in doing this and urged the IAU that “half a dozen of the largest spirals in addition to Andromeda should be followed carefully for novae.” Major Hubble was now applying his lessons on military tactics to conquering his astronomical targets.

  “I must confess that I am rather dazed by [Hubble's] letter,” said Lick astronomer William H. Wright, who had also received a copy of Hubble's agenda. “One can see that the nebulae will have no private life when he has his way. Hubble is a great lad, and I only hope that he will have the strength and energy to carry out a fraction of the work he would like to see done.”

  Hubble, who had just gained a seat on the committee, was particularly fired up about a nebula classification scheme he wanted the IAU to adopt. To Hubble, properly categorizing the nebulae was an essential first step in determining their physical nature. By 1923 he had divided the nongalactic nebulae into two categories: the ellipticals and the spirals. An elliptical was an amorphous blob shaped somewhat like an egg. The spirals, of course, were the stunning pinwheels. If the bright center of the spiraling disk was a round bulge, he called it a “normal spiral;” if elongated, a “barred spiral.” The nongalactic nebulae that didn't fit either class, like those resembling the chaotic Magellanic Clouds, were tagged “irregulars.” But the IAU committee dragged its feet on Hubble's naming system and desired some changes, a rebuke that may have had long-term effects. At one point in the long wait, Knut Lundmark published a similar scheme, which enraged Hubble. He accused the Swedish astronomer of plagiarism. Afterward, Hubble was never keen to work on committees, attend general astronomy meetings, or share in collaborations. With a few exceptions, he tended to work alone. There might have been another reason for this as well. Though displaying a commanding public presence, Hubble was actually “pathologically shy around colleagues with whom he had little… contact,” contends Allan Sandage, who knew Hubble in his later years. Hubble proceeded to classify the nebulae in his own way and over time his arrangement was eventually accepted by the astronomical community.

  Throughout 1923, over a total of forty-seven nights on the mountain, Hubble used both the 60-inch and 100-inch telescopes to survey a variety of nebulae around the celestial sky. He was on a reconnaissance mission. Though scarcely any nebulae were repeated, he did pay special attention to NGC 6822, a nebula in Sagittarius first discovered in 1884 by his former Yerkes colleague E. E. Barnard. The nebula stood out from the pack because it looked strikingly similar to the Magellanic Clouds in the southern celestial hemisphere.

  The 100-inch and 60-inch telescopes (left, right) side by side on Mount

  Wilson (Courtesy of the Archives, California Institute of Technology)

  By July Hubble found five variable stars in NGC 6822 and informed Shapley at Harvard, suggesting that Shapley investigate the object on the plates stored away at the Harvard observatory. “What a powerful instrument the 100-inch is in bringing out those desperately faint nebulae,” responded Shapley. “As for N.G.C. 6822, I think there is no doubt but that it is another star cloud like the Magellanic Cloud.” Although there was no love lost between Shapley and Hubble, the two astronomers maintained a courteous correspondence, perhaps adhering to that old adage, “Keep your friends close but your enemies closer.” In actuality, they needed each other. Shapley oversaw the world's foremost collection of astronomical photographs, while Hubble had ready access to its largest telescope.

  Shapley proceeded to estimate the distance of NGC 6822 by comparing its size and the observed magnitudes of its brightest stars to that of the Large Magellanic Cloud. Interestingly, he arrived at a distance of about a million light-years. “It appears to be a great star cloud that is at least three or four times as far away as the most distant of known globular clusters and probably quite beyond the limits of the galactic system,” reported Shapley in his observatory's December 1923 Bulletin. A news report by Science Service promptly called it “the most distant object seen by man, another universe of stars.” NGC 6822 wasn't a spiral nebula, but it certainly offered Shapley proof that large stellar systems existed beyond the Milky Way. Yet, for Shapley, his distance calculation for this stellar cloud simply had no bearing on the question of the spirals. For those he tenaciously held fast to his convictions and continued to spread the word in various publications that spiral nebulae were “neither galactic in size nor stellar in composition.”

  Hubble obtained more than fifty photographs of NGC 6822 over time and found fifteen variable stars. “Eleven…are clearly Cepheids,” he eventually reported two years later. Using them as standard candles, Hubble calculated a distance of some 700,000 light-years, which was undoubtedly beyond the borders of Shapley's newly supersized Milky Way. “N.G.C. 6822 lies far outside the limits of the galactic system,” stated Hubble, “and hence may serve as a stepping-stone for speculations concerning habitants of space beyond.” His early work on NGC 6822 likely gave Hubble the confidence that he could pursue Cepheids as distance markers in spiral nebulae, observations he was carrying out at the same time and would actually report on first.

  Observing with the 100-inch was a choreographed dance within the monumental dome a hundred feet high and nearly as wide. Sometimes Hubble could just lean back in a bentwood chair, his favorite, and serenely smoke his pipe in the darkness while taking a photograph. But other times he was perched high in the air on a platform that could adjust to any height via rails set on either side of the dome opening. With the telescope's clock drive shifting the telescope as the nighttime sky slowly moved overhead, he and his assistant made sure the advance stayed in synchrony with Earth's rotation. At the same time, they had to keep the dome rotated and the platform height adjusted, so that the telescope kept spying on the cosmos and not an inside wall. “This was the astronomical observing experience at its best,” noted Mount Wilson astronomer Allan Sandage, “a dark, quiet dome, a silently moving monster telescope, and mastery of the dangerous…platform, all in the interest of collecting data on a problem of transcendental significance.” Night after night, the cosmic waltz went on. If Hubble got clouded out, he had a backup: “You begin with deskwork, later you turn to heavy reading, and later, to a detective story,” he said.

  Edwin Hubble observing at the 100-inch, sitting on his

  favorite
bentwood chair (Reproduced by permission of the

  Huntington Library, San Marino, California)

  The only scheduled break was “lunch,” provided at midnight. In the early years, it was simply hardtack and cocoa (Hale considered coffee “unwholesome”), served in a concrete bunker beneath the 60-inch. Later on, at a shack built halfway between the 60-and 100-inch, astronomers were offered two pieces of bread, two eggs, butter and jam, and a single cup of coffee or tea, a repast purposefully kept skimpy by the observatory's notoriously frugal administrator, Walter Adams. Hubble gained the respect of the night assistants when he washed his own dish afterward, giving them a break from their cleaning responsibilities. The assistants also liked Hubble's no-nonsense attitude. Unlike some other Mount Wilson astronomers, he always arrived for his scheduled runs with a well-thought-out observing plan in hand. He delegated authority and expected a professional performance in return. “You knew where you stood with him,” said Humason.

  Hubble's observations at this time were fairly routine as he methodically went from target to target. He was noted for carrying a map of the heavens in his head; the hundred-odd Messier objects were as familiar to him as the alphabet. On July 17 he stopped to confirm a new and wispy nebula that Shapley had reported seeing on two occasions earlier that year in the Boötes constellation. But even after a 150-minute exposure, Hubble came up empty-handed. He saw nothing in that area of the sky. “Shapley object is probably an accident,” wrote Hubble in his logbook. From a photo he took on August 15, he spotted the track of an asteroid passing by. Week by week the routine continued.

  And then came the October surprise.

  Discovery

  Countless Whole Worlds…

  Strewn All Over the Sky

  October 4, 1923. The seeing was poor, but it was good enough (just barely) to stalk some celestial quarry that autumn evening. Hubble first pointed the 100-inch at NGC 6822, the far-off, Magellanic-like cloud of stars that he had long been studying. As the giant scope swung around, there was a whine, a series of loud clicks, and then a final clang as the instrument was secured into place. After taking an hour-long photographic exposure, Hubble went on to examine M32, a small and roundish nebula, for a spell. He then maneuvered the telescope just a fraction of a degree to photograph M31—the famous Andromeda nebula, the target of choice in the island-universe debate. By then the seeing had deteriorated to a point that other astronomers might have closed up shop. But Hubble persevered, and despite the mediocre viewing, soon noticed a new speck of light within Andromeda's cloudy veil. It was exactly what he was hoping to find one day as he conducted his extensive survey of the nebulae. Novae had been seen before in Andromeda; that wasn't startling. But Hubble was sure that additional sightings would help reveal Andromeda's secret. “Nova suspected,” Hubble neatly wrote in black ink in his logbook for Plate H 331 H. After photographing Andromeda for forty minutes, he went on to observe another nebula, a barred spiral, before ending his run.

  The very next night Hubble returned to the 100-inch to follow up. This time the atmosphere was better—clear and steady, at least for a while. When the sky was at its best, he aimed the telescope at Andromeda and again saw the new pinpoint of light. “Confirms nova suspected on H 331 H,” Hubble noted in his logbook.

  Everything, though, cannot be readily seen through the telescopic eyepiece or by a quick peek at a newly developed photograph. Plate H 335 H, the forty-five-minute exposure taken on October 5 to verify the nova, was analyzed in more detail later, back in Hubble's Pasadena office. There he confirmed not just one but rather three new pinpoints of light within Andromeda. He figured he was seeing two additional novae and wrote “N” beside each one on his plate to mark their location.

  From his earlier work on the Magellanic cloud–like NGC 6822, Hubble knew that he had to make sure his newly spied objects were truly novae and not some other phenomenon. For a further check, he turned to the massive collection of plates archived in a quakeproof vault at the observatory headquarters. He began perusing previous photos of the Andromeda nebula, taken by observatory astronomers as far back as 1909. By comparing his latest photographic plate with those from the past, he could easily see that two of his spots of light were indeed novae—never-before-seen stellar flares. But one spot, the one farthest out from the center of the nebula, had been around before. Going from plate to plate, Hubble could see that this tiny dot of light was brightening and dimming over time. It was not a nova at all, but instead some kind of variable star. At this point Hubble went back to plate H 335 H, crossed out the N beside this particular dot, and beneath it wrote “VAR!” instead. His exclamation point emphasized the significance of this discovery: He had struck celestial gold. Once he had this stellar nugget in his hand, he didn't let it go.

  Hubble more carefully tracked the ups and down of his variable's luminosity from the archival photographic plates. He also continued his survey of the heavens, making sure to check back on Andromeda again and again, as this was the time of year when Andromeda was in full view. He found more novae and another variable. He kept track of his finds, numbering each nova and variable and marking their positions in the spiral with a tiny red dot or circle on photos of Andromeda.

  Three nights in February 1924 proved especially crucial. Over the fifth, sixth, and seventh of that month he directly observed his first variable in Andromeda brightening by more than a magnitude, doubling its luminosity, a tremendous break. From the data he had on hand he could now sketch a reliable light curve. The variable star went through its complete cycle—from bright to dim and back to bright again—in a matter of 31.415 days. From the length of this period and the shape of the curve (sharp rise and slow decline), Hubble now comprehended that he had captured that elusive and rare celestial beast—a Cepheid variable, a star seven thousand times brighter than our Sun. But it appeared so dim—the barest smudge on his photographic plate—that Hubble knew it had to reside at a great distance. It was on average more than one hundred thousand times dimmer than the faintest stars visible to the unaided eye.

  The photographic plate of Andromeda (M31) on which Edwin Hubble

  identified a Cepheid variable star, mistaken at first for a nova, in a spiral

  nebula—the first step in Hubble's opening up the universe (Courtesy

  of the Observatories of the Carnegie Institution of Washington)

  At some point during these deliberations, Hubble went back to his logbook, page 157, and quickly scrawled an added note on the side of the page to amend the report of his October 5 observing run. Customarily reserved, Hubble at this moment is unmistakably restive. He didn't write his message in black ink, which he regularly did for his records, but instead in pencil. And his handwriting, usually so fluid and precise, was more hurried and askew. He was obviously elated: “On this plate (H 335 H), three stars were found, 2 of which were novae, and 1 proved to be a variable, later identified as a Cepheid—the 1st to be recognized in M31.” To highlight the addition, he drew a big arrow, pointing directly downward at his historic news. In its broad stroke, the arrow makes his excitement visible upon the page. For once Hubble dropped his guard and figuratively clicked his heels at this moment of discovery.

  Hubble couldn't help but notify his nemesis. On February 19 he wrote Harlow Shapley about his efforts over the previous months. Hubble didn't open with polite niceties or inquiries of health. He got straight to the point. “Dear Shapley:—You will be interested to hear that I have found a Cepheid variable in the Andromeda Nebula (M31). I have followed the nebula this season as closely as the weather permitted and in the last five months have netted nine novae and two variables.” His glee in communicating this news jumped off the page as he then provided Shapley with all the technical details on color index corrections and magnitude estimations. Shapley was, after all, the world's reigning Cepheid expert—not only in using them as standard candles but figuring out early on, soon after he arrived at Mount Wilson, that they were pulsating stars, their atmospheres repe
atedly ballooning in and out.

  Pages 156 and 157 of Hubble's 100-inch telescope logbook

  (Reproduced by permission of the Huntington Library,

  San Marino, California)

  Accompanying this legendary letter was a graph that Hubble had fastidiously drawn in pencil on paper torn from a notebook. It displayed the light curve for his “Variable No. 1” in M31—a roller-coaster ride that peaked at eighteenth magnitude, dipped a bit below nineteenth magnitude, and then rose once again to its maximum brightness over a period of thirty-one days, “which, rough as it is,” he told Shapley, “shows the Cepheid characteristics in an unmistakable fashion.” And here was the kicker. Hubble used the exact same technique for gauging a distance to the spiral that Shapley had devised for mapping the arrangement of globular clusters around the Milky Way. Applying the Cepheid period-luminosity formula that Shapley had derived, Hubble calculated a distance to Andromeda of around 1 million light-years (“subject to reduction if star is dimmed by intervening nebulosity,” he carefully noted). No more oblique evidence or convoluted reasoning, such as Heber Curtis was forced to use. The Cepheid provided a direct and indisputable yardstick out to the nebula. Andromeda was indeed an island universe.