The Day We Found the Universe - Marcia Bartusiak (2009)
Chapter 4. Such Is the Progress of Astronomy in the Wild and Wooly West
As Lick Observatory entered its third decade, life on Mount Hamilton continued to be a rustic adventure. Residents hiked the mountain trails, staged amateur theatricals, and read aloud around the roaring fireplace on frosty evenings. As long as the weather was favorable, the telescopes were scheduled for use nearly every night of the year. The lone exception was Christmas Eve, when operations were shut down for the holiday and the graduate students would sneak quietly into the cavernous dome to hang their stockings on the gear of the giant telescope.
There was a new addition to the observatory grounds, a tennis court, where, on Saturday afternoons, as one onlooker described it, “a spectacular performance is kept up, consisting of wild up-bursts of tennis balls, a la Roman candle, followed by hot chases down the canyons.” A molasses jug served as the loving cup for the annual Fourth of July tournament.
Those wanting to go into town often hitched a ride with one of the lucky few, such as Heber Curtis, who owned a car. The astronomer would load people into his Mitchell automobile, nicknamed Elizabeth, making sure to stash a bag of flaxseed in the trunk, so he could pour the seeds into the radiator whenever it started leaking. Pictures of Curtis in his later years, taken after an illness, typically depict a small and stern-looking man. But while he was at Lick, the students knew him as a “wonderfully kind, jolly person, always smiling, always happy.” His genial composure was only broken when he had to sneeze, a feat once described as “remarkable.”
By the 1910s the island-universe theory, dormant for many years, was slowly reemerging among a select group of scientists in both the United States and Europe. These astronomers were specifying that the spirals' sizes and the brightness of their novae only made sense if they were milky ways at great distance. The highly respected English astrophysicist Arthur Eddington was captivated by the vast breadth of this idea; it engaged his theoretical fantasies. “If the spiral nebulae are within the stellar system [the Milky Way], we have no notion what their nature may be. That hypothesis leads to a full stop,” he noted. “If, however, it is assumed that these nebulae are external to the stellar system, that they are in fact systems co-equal with our own, we have at least an hypothesis which can be followed up… [It] opens up to our imagination a truly magnificent vista of system beyond system … in which the great stellar system of hundreds of millions of stars (our galaxy)…would be an insignificant unit.” For Eddington, the heavens just seemed to make more sense viewed from this grander perspective.
The epicenter of this resurgence was located right at the Lick Observatory, where its director, W. W. Campbell, was at last persuaded by the mounting evidence and openly declared that thinking about the spirals as enormous distant bodies was “in best harmony with known facts.” And those facts were largely being gathered by Curtis, one of his most able staff members. Campbell was still focused on his monumental campaign, a virtual assembly line of stellar measurements systematically proceeding from target to target, to catalog the velocities of stars within the Milky Way. The survey was being done in hope that the data would reveal new clues on stellar evolution. It was left to Curtis to get back to the Crossley telescope and revive the observatory's investigation of the spiral nebulae, a program that had not been a top priority since Keeler's death. The compact reflector, however, was still one of the best tools around for imaging and analyzing the hazy celestial clouds.
Curtis, a gifted mechanic, right away made significant improvements to the telescope. First off, he erected a new observing platform that could be raised and lowered by an electric motor, installed a powered dome shutter, and devised a better mechanism for driving the telescope. The mirror had already been remounted in 1904 into a thick metal tube, whose rivets along the side made it resemble a beam on a naval battleship. This telescope remains in operation, now searching for extrasolar planets. It's possibly the oldest reflecting telescope still in use for professional research.
Heber Curtis standing by the renovated Crossley telescope
(Mary Lea Shane Archives of the Lick Observatory, University
Library, University of California-Santa Cruz)
When Curtis rekindled Keeler's pursuit of the spiral nebulae, the island-universe theory was regarded as just a good guess, an intuitive suspicion. Curtis was after more concrete proof. He started to dig deeper into the problem, in the same way that Keeler would likely have proceeded. But Keeler had a mere two years to work with the Crossley before his death; Curtis, fortunately, had more time, which allowed him to extend astronomy's knowledge of the spirals throughout the 1910s. This celestial quest became, in the words of a fellow astronomer, Curtis's “magnum opus.”
No one was more surprised perhaps at the zigs and zags in Curtis's career path than Heber Doust Curtis himself, who went to college at the University of Michigan in Ann Arbor, at the same time that Campbell happened to be teaching there. But their paths never crossed, for Curtis was a dedicated student of the ancient languages—Latin, Greek, Hebrew, Sanskrit, and Assyrian—earning first a bachelor's degree, then a master's. At this stage, Curtis voiced no interest whatsoever in science and never set foot inside an observatory. After teaching high school briefly in Detroit, he moved to California in 1894 to become a professor of Latin and Greek at Napa College, a small institution north of San Francisco. Curtis seemed destined for a life of quiet scholarship in the classics, until he came upon a small telescope at the college and on an impulse began to tinker with it.
His tiny college later merged in 1896 with the University of the Pacific, situated in the San Jose area, and he moved there, opportunely within the shadow of Lick Observatory. He continued his astronomical activities and got so caught up in his newfound hobby—and so adept at observing—that he was chosen to teach mathematics and astronomy at the small college. He was even able to spend some time on Mount Hamilton during the summers of 1897 and 1898 as a special student. The experience convinced him that he wanted to make astronomy his life's work. He hoped to continue at the Lick Observatory as a graduate student, but his inadequate preparation in science put up roadblocks. Keeler, Lick's director at the time, was looking for someone more professionally skilled in spectroscopy. Curtis was finally offered a fellowship at the University of Virginia, where for his PhD he reluctantly focused on a more mathematical topic, celestial mechanics, although along the way he made sure to get as much instrumental experience as possible. It was a risky move. He was resigning from a college professorship to start anew as a student in a field in which he had no prior training—and with a growing family to support as well.
Serendipity offered an assist. Just as Curtis was headed east in 1900 to begin his doctoral studies, Lick astronomers William Campbell and Charles Perrine were traveling to Georgia to scrutinize a solar eclipse, whose shadow was scheduled to cut across the southeastern United States. Curtis signed on as a helper, saying he was “ready and glad to be put at anything from a shovel up.” Given this opportunity, he proved to the Lick men that he could handle a telescope and spectrograph as if he had been using them all his life. Campbell took notice. As soon as Curtis finished his degree at Virginia in 1902, Campbell, by then Lick's director, hired him on as an assistant. To Curtis, having lived on a small mountain in Virginia was simply good training for a life on Mount Hamilton, where kids hunted rattlesnakes for fun in the summertime.
Curtis arrived at Lick covered in thick yellow dust from the long stagecoach ride, raring to begin his research straightaway. For the first few years, he focused on traditional Lick specialties, such as measuring stellar velocities, computing the orbits of binary stars, and going on solar-eclipse expeditions. Life was fairly routine, until one memorable April morning in 1906 when the mountain experienced a minor temblor. Damage was minimal at the observatory—a few coal-oil lamps overturned, loosened bricks on some of the buildings—but looking toward San Francisco, Lick residents saw an enormous tower of black smoke. They didn't realize how serious the disaster was until noon, when the daily stage from San Jose, which normally ran like clockwork, never showed up. By evening the astronomers turned Lick's 12-inch telescope completely horizontal and aimed it toward the Golden Gate, the strait connecting San Francisco Bay to the Pacific Ocean. Through the scope they saw three miles of fire-front, burning fiercely. “And, naturally, the lens inverted everything, so we saw buildings fall up and flames sweep down—which was a weird, weird sight…. It reminded me of… Dante's Inferno,” said Douglas Aitken, who had lived on the mountain at the time as a young boy.
Curtis missed all the excitement because two months earlier he had arrived in Chile to head up Lick's southern station on the summit of San Cristobal, on the outskirts of Santiago. With him were his mother, wife, and three small children. After a few years they became so comfortably settled in Chile that they contemplated an extended stay, having become fluent in Spanish and grown fond of the South American lifestyle. “Queer how completely we seem to have taken root here,” noted Curtis. But in 1909 Curtis received an unexpected invitation to return to Lick, not as an assistant or associate, but as a senior astronomer. Short on staff, the observatory needed an experienced hand to work with the Crossley reflector. In accepting the post, Curtis became Keeler's anointed successor, the next in line to tackle the mystery of the spiral nebulae.
Curtis first spent time getting to know the Crossley's strengths and weaknesses: What were the faintest stars it could photograph? How many hours of exposure were required? He had the good fortune to start his new venture just as a famous celestial visitor, Halley's Comet, visibly reappeared in the skies in 1910, as it did every seventy-six or so years, providing a superb target to test out the Crossley's photographic abilities. The comet this time passed relatively close to Earth, creating quite a stir throughout the world, so by the time it completely disappeared from telescopic sight in 1911, the Crossley and other Lick telescopes had taken nearly four hundred pictures of its spectacular passage.
With the Crossley checked out, Curtis at last turned his attention to the mysterious nebulae. Keeler and others at Lick had previously amassed a photographic library of around one hundred nebulae and clusters using the Crossley. By the summer of 1913, Curtis boosted that number to more than two hundred. “Many of these nebulae show forms of unusual interest,” he jotted down in his observatory report. “The great preponderance of the spiral form becomes more and more striking with the progress of the survey.” He was beginning the process of identifying and cataloging the nebulae, particularly the spirals, in hope of detecting patterns that would lead to revealing what they were. His descriptions conveyed the rich diversity in their appearance: A spiral could be either “patchy,” “branched,” “irregular,” “elongated oval,” or “symmetrical.” For the moment, he was merely recording what he saw, not venturing to discuss what they might be.
It was tiring work. “Crossley still has its old reputation of using up more energy than any other instrument on the hill,” Curtis told a colleague. Despite the improvements he had made on the telescope, it was still difficult to reach the eyepiece at certain positions. “If you got a little bit sleepy at night, it was dangerous, because it went down a great many feet [from the observing platform] to a floor in the basement,” said one of the telescope's later users. One wisecracker suggested the only way to observe with the Crossley in comfort was to fill the dome with water and observe from a boat.
When he first started his study, Curtis assumed that the spirals were comparable to the size of a modest cluster of stars, spanning no more than several hundred light-years in width. It was a reasonable assumption. Over at the Mount Wilson Observatory, with its new 60-inch reflector, George Ritchey had begun to photograph the spiral nebulae and was concluding they were a mix “of smooth nebulous material and also of soft star-like condensations or nebulous stars.” He surmised he was seeing a collection of developing stars—a good-sized cluster but certainly not an entire “island universe.”
But Curtis began to doubt this viewpoint as he gathered more evidence with the Crossley. Some of the first hints surfaced when he rephotographed a number of nebulae that Keeler had previously imaged. By comparing his most recent spiral pictures with those gathered years earlier, he hoped to see how the swirling clouds had rotated. The amount of motion measured was going to help him judge their distance. But Curtis didn't detect any sign of movement, not a smidgeon “rotatory or otherwise,” he reported. “As the spirals are undoubtedly in revolution—any other explanation of the spiral form seems impossible—the failure to find any evidence of rotation would indicate that they must be of enormous actual size, and at enormous distances from us.” It would simply be impossible to measure a shift by sight alone if the spiral were considerably larger and at the same time pushed far off into space.
An edge-on galaxy photographed by Heber Curtis in 1914,
showing the dark lanes of dust and gas within the disk
(Copyright UC Regents/Lick Observatory)
Even earlier Curtis started reporting that some of the spirals he photographed—the ones so tilted they were seen edge-on—resembled “the Greek letter Ф… for lack of a better term”: an oval ring crossed by a straight dark line. He expressly mentioned them in his research notes: NGC 891 “shows dark lane down center,” he jotted down. And NGC 7814 was described as small but with a dark lane “beautifully clear.”
This was at a time when Yerkes astronomer E. E. Barnard was also acquainting astronomers with myriad “dark nebulae” within the Milky Way. Barnard was gathering exquisite photographic evidence that the coal-black regions within the Milky Way that appeared to be devoid of stars (“holes in the heavens,” Herschel called them) were actually clouds of cosmic gas and dust—colossal streams of inky darkness without the hint of a glow. Curtis immediately connected this finding to his work: The dark lanes he was sighting in the spirals had to be “due to the same general cause that produces certain occulting effects in our own galaxy….” The dark bands were almost certainly matter—but matter that wasn't glowing.
This also explained why no spiral was ever seen in certain areas of the celestial sky, aptly named the “zone of avoidance.” Spiral nebulae were very exclusive objects; they tended to huddle around the north and south galactic poles, as if shunning the long white swath of the Milky Way. Astronomers had long scratched their heads over this peculiar distribution. If spirals were truly the birthplaces of new stars, why weren't they found in the richest star fields? Why were the spirals found in only those sectors of the sky where stars were scarce? Not one spiral had ever been spotted in the thick of the Milky Way. Curtis cleverly deduced that this cosmic quarantine was only an illusion: If his dark-banded spirals were truly distant galaxies, then the Milky Way, too, must have its own dark band. All the dark gaseous clouds within the Milky Way were collectively acting like an opaque wall, making it impossible to see the spirals that resided beyond this obstruction, keeping the spirals hidden. “[The] great band of occulting matter in the plane of our galaxy … serves to cut off from our view the distant spirals lying near the projection of our galactic plane in space,” explained Curtis. And that couldn't happen unless the spirals were very far off.
To Curtis this argument made perfect sense, but he was presenting the idea at a time when most astronomers still thought of the vast expanses between the stars as a pristine emptiness and the Milky Way as transparent as a glass window. His reasoning wasn't as readily accepted as he had hoped.
Curtis spent much of the 1910s on this fight—gathering data, giving lectures, coming up with fresh new arguments. He gathered clues as if he were a cosmic sleuth. “Were the Great Nebula in Andromeda situated five hundred times as far away as at present,” reasoned Curtis, “it would appear as a structureless oval…with [a] very bright center, and not to be distinguished from the thousands of very small, round or oval nebulae found wherever the spirals are found. There is an unbroken progression from such minute objects up to the Great Nebula in Andromeda itself; I see no reason to believe that these very small nebulae are of a different type from their larger neighbors.” But his mounting certainty that the spirals he photographed, both large and small, were all distant galaxies strewn through space was based solely on circumstantial evidence. He had convinced his colleagues at Lick, which came to be identified as a stronghold of island-universe supporters, but the majority of astronomers still preferred to think of all the stars and nebulae as inhabiting one great system, the Milky Way. Curtis was absolutely right, but convincing the wider community of astronomers was an entirely different matter.
And then something interesting…and very unusual…happened.
On July 19, 1917, some three hundred miles southeast of Mount Hamilton, George Ritchey was taking a routine photograph of a spiral nebula with the 60-inch reflector at the Mount Wilson Observatory. It was the fourth in a series of long-exposure photos he had been taking of NGC 6946 over the previous seven years. This time, though, he noticed a new pinpoint of light in the spiral's outer region. It had to be a nova, for this “new star” wasn't in any of his previous pictures. More important, this nova was distinctly different from the dazzling one that had flared up in Andromeda thirty-two years earlier. This one was very, very faint.
The unforgettable nova that briefly blazed within Andromeda in 1885 had reached a brightness that could be discerned by the naked eye (just barely); the nova in NGC 6946, on the other hand, was about sixteen hundred times dimmer. Ritchey knew he had caught the nova fairly early in its burst; a plate taken just a month earlier with another telescope showed no extra speck of light whatsoever. Telegrams announcing the new find were quickly sent out to other observatories.
Curtis likely received the report with a sinking heart, for he had sighted similar novae months earlier. On the very day that Ritchey's telegram reached Lick, Curtis was actually at his desk, drafting a paper on three faint novae he had discovered in other spiral nebulae. He had been sitting on the news since March, when he first observed the flare-ups. He was being very careful, holding off any announcement until he was sure that the outbursts were not simply variable stars reaching their maximum brightness. His caution kept him from the prize of first announcement.
The first nova that Curtis spotted was in NGC 4527, an elongated spiral located in Virgo. By checking plates of this region made earlier at the Harvard, Yerkes, and Lick observatories, Curtis confirmed that no star had been visible in the spiral over the previous seventeen years. The tiny dot on his photo reached around fourteenth magnitude (some sixty thousand times dimmer than the stars in the Big Dipper). And in the course of his plate search, he came upon two additional faint novae: this time in M100 (also known as NGC 4321), a spectacular spiral in Coma Berenices viewed face-on. One of these novae had flared in 1901, the other in 1914. “That both these novae should have appeared in the same spiral is especially worthy of note,” reported Curtis. By the time Curtis announced his finds in July 1917, though, all three of these novae had completely disappeared. But he made sure to point out in his bulletin that the new stars “must be regarded as having a very definite bearing on the ‘island universe’ theory.”
With such startling news from both Mount Wilson and Lick, nova hunting spread like wildfire among the top U.S. observatories. Going to old astronomical plates and searching for novae became the craze, and new candidates were found right away. The list was getting longer week by week. “Such is the progress of Astronomy in the wild and wooly West,” joked one Mount Wilson astronomer. Curtis was tremendously excited by all the discoveries. Every time he found a new nova in a spiral, he'd go through the observatory and show off the plate, like some proud papa in a hospital maternity ward.
Curtis soon had a big enough sample of novae to make a judgment call: He suspected that the 1885 outburst in Andromeda, as well as the 1895 one in Centaurus, were rare and exceptional celestial events. Curtis guessed that their spectacular radiance had misled astronomers into thinking the novae's host nebulae had to be close by. He suggested that nova bursts actually came in two varieties: The rarer ones were big and spectacular (now known to be stars blowing apart), while the ones seen more often were less energetic (determined later to be a flaring off the surface of a white dwarf star). And since the majority of the novae being sighted in the spirals more resembled the ordinary novae seen periodically within the Milky Way, he concluded that the spiral nebulae had to be millions of light-years distant, in order for those novae to appear so dim. He said as much to the Associated Press. He boldly told its reporter that the nova bursts he had discovered occurred some 20 million years in the past, meaning the nebulae had to be 20 million light-years distant for the light to be reaching us now. (With 1 light-year equaling about six trillion miles, that's more than a hundred million trillion miles.) For Curtis the faint novae were bona fide proof that the nebulae resided far beyond the borders of the Milky Way. But Curtis was championing this idea too early, before the physics could explain it. Many of his fellow astronomers were still fairly skeptical, unwilling to conjure up new celestial creatures willy-nilly. For them “Occam's Razor” prevailed, the long-standing rule of thumb established by the English philosopher William of Occam in the fourteenth century. “Pluralitas non est ponenda sine necessitate,” declared Occam, which can be translated as “plurality must not be posited without necessity.” Best to choose the simplest interpretation over an unnecessarily complex one—unless forced to do otherwise. One type of nova was far more preferable than two.
Arrows point to the novae discovered by Heber Curtis
in photos of NGC-4321 taken in 1901 and 1914.
(Copyright UC Regents/Lick Observatory)
Despite the lack of support for his creative hypothesis, Curtis was still gaining appreciable momentum on his endeavor, at least until World War I intervened. Just months after the United States officially joined the fight in 1917, Curtis went first to San Diego and then to Berkeley to teach officer recruits navigation. Afterward, he proceeded to Washington, D.C., to work for the Bureau of Standards on the design and development of military optical devices. Before taking his leave, though, Curtis made sure to compile a master list of the spiral nebulae he had photographed with the Crossley, by now more than five hundred. And as before, each of his photographs revealed ever more nebulae, pale and murky, surrounding the more consequential spirals that he officially cataloged. On one plate alone he counted 304 additional spirals. Keeler had estimated that 120,000 spiral nebulae were within observational range of the Crossley. Another Lick astronomer later upped that number to 500,000. Now Curtis was raising the figure even higher. “The great numbers of small spirals found on nearly all my plates of regions distant from the Milky Way, long since led me to the belief that [an earlier] estimate of half a million was likely to be under, rather than in excess of, the truth,” he reported. “[I] believe that the total number accessible with the Crossley Reflector with rapid plates and exposures of from two to three hours may well exceed 1,000,000.” This was an astounding hike in the spiral estimate.
While still in the U.S. capital wrapping up his work after the 1918 armistice, Curtis was invited to deliver a semipopular lecture on the spiral nebulae before the Washington Academy of Sciences and the Philosophical Society of Washington. His expertise on the topic was getting noticed. “Get up a collection of about 40 classy slides and send to me at once,” he wrote Campbell at Lick in great excitement. Curtis was thrilled at the opportunity, his first actually, to lay out before an influential scientific conclave all his hard-won evidence in support of the island-universe theory. He planned to use a lantern slide—the early-twentieth-century version of PowerPoint—to display the various types of spirals he had come across, to point out the dark lanes running through them, and to reveal the many fainter nebulae lurking in the background of his photographs of the spirals.
On the appointed day—March 15, 1919—a large audience gathered to hear Curtis in the new lecture room at Washington's prestigious Cosmos Club (then located at Lafayette Square), the traditional meeting place for the city's intelligentsia. Curtis opened with a tip of the hat to William Herschel. “The history of scientific discovery affords many instances where men with some strange gift of intuition have looked ahead from meager data, and have glimpsed or guessed truths which have been fully verified only after the lapse of decades or centuries,” he said. “We have now, as far as the spiral nebulae are concerned, come back to the standpoint of Herschel's fortunate, though not fully warranted deduction…that these beautiful objects are separate galaxies, or ‘island universes,’ to employ the expressive and appropriate phrase coined by Humboldt.” With these words, Curtis became the most outspoken and identifiable advocate of the island-universe theory.
Curtis at the time estimated that our stellar home was some 30,000 light-years wide and contained about a billion stars, with the Sun nicely situated right near the center. He was wrong about that: The Milky Way's dimensions were even then being revised upward, and the Sun was losing its front-row seat on galactic affairs. But Curtis was right about the spiral nebulae being far-off galaxies.
Over the course of that March evening, Curtis laid out his arguments point by point. First, there was the peculiar distribution of the spiral nebulae. If they are stars in the making, he asked, why are there no spirals in the very place where stars are most numerous, the Milky Way? “Occulting matter,” he answered, was masking our view, making it only appear as if the spirals were avoiding the plane of the Milky Way. And then there was the very light of a spiral to consider: The spectrum of a spiral revealed that its light was emanating from a massive assembly of stars, not just a cloud of gas.
His logic was impeccable. Going through the historical records, Curtis determined that nearly thirty “new stars” had made an appearance within the Milky Way over the last three hundred years, each suddenly rising to great luminosity and then sinking back into obscurity once again. But half that number had already been sighted in spiral nebulae in just a few years, making it all the more likely that the “spirals are themselves galaxies composed of hundreds of millions of stars.” Moreover, with the novae being so faint, they had to be situated millions of light-years away. “This is an enormous distance,” admitted Curtis, “but, if these objects are galaxies like our own stellar system, this is about the order of distance at which we should expect them to be placed.”
Curtis was fully aware of the magnitude and complexity of the new cosmic scheme he was proposing. “We know that the relative space in this, our galaxy, occupied by…our solar system … is about the same order as that occupied by a single drop of water in Chesapeake Bay,” he told his audience. “To go still beyond such a concept, the island universe theory forces us to consider a still mightier whole, a space containing hundreds of thousands of stellar universes like our own, each containing millions upon millions of suns… Awe-inspiring as are the concepts of astronomy, this newer concept surpasses them all; it staggers the imagination.” Curtis was plainly carried away by the allure of this astounding idea. His audience was enthralled as well. At the end they applauded with great enthusiasm and kept him long afterward for further discussion.
At the close of the war, officials at the Bureau of Standards had hoped that Curtis would stay with the agency, but he refused. “As to my staying here permanently, I have no idea whatever of doing that,” he assured Campbell. “[I'm] anxious to get back to my hill and the Crossley, and stay there…. I am more than ever of the opinion that men like you [and] Hale…get all the hard knocks, and not half the fun out of life that those of us lower down get.” An observer at heart, he was eager to return to his nebulae. By May 1919, he was back on Mount Hamilton gathering more evidence in support of distant galaxies.
Curtis already had a few converts to his cause. Astronomer Andrew Crommelin of the Royal Observatory in Greenwich favored the island-universe theory as well, but voiced caution: “The hypothesis of external galaxies is certainly a sublime and magnificent one,” he said. “[But] our conclusions in Science must be based on evidence, and not on sentiment.” His fellow astronomers were setting the bar high. Curtis had to provide more than logical arguments to win his case. He needed concrete evidence. Additional clues had been arriving, but they did not originate with Curtis. They instead turned up at the Lowell Observatory, Lick Observatory's long-standing competitor located in northern Arizona.