The Day We Found the Universe - Marcia Bartusiak (2009)
Chapter 3. Grander Than the Truth
Contemplating a universe of magnificent vastness has not been a recent affair. In the first century B.C., the Roman poet and philosopher Lucretius approached the question with cunning logic: “Let us assume for the moment that the universe is limited,” he posed. “If a man advances so that he is at the very edge of the extreme boundary and hurls a swift spear, do you prefer that this spear, hurled with great force, go whither it was sent and fly far, or do you think that something can stop it and stand in its way?” For Lucretius and a few Greek thinkers before him, it was hard to imagine that an impenetrable cosmic barrier existed. It seemed ludicrous.
But Lucretius's reasoning never flourished. It was overshadowed by the authoritative cosmology espoused by Aristotle in the fourth century B.C. The noted Greek philosopher preferred a motionless Earth poised in the center of a celestial sphere of set dimensions, a concept of such influence that it endured for centuries. Over that time scholars only occasionally reflected on the possibility of a universe significantly bigger. In the sixteenth century, for example, Thomas Digges in England imagined the stars scattered throughout a boundless space, while in Italy Giordano Bruno presciently declared that “the center of the universe is everywhere, and the circumference is nowhere.” Even Isaac Newton had a good scientific reason to prefer a cosmos without end. If the universe had a border, gravity would gradually draw all its matter inward, and ultimately the universe would collapse. To keep the cosmos stable—immutable and immovable—required that the stars be spread infinitely outward in all directions. “If the Matter was evenly disposed throughout an infinite Space,” wrote Newton to a friend, “it could never convene into one Mass.”
Yet most found such enormity difficult to grasp and horrifying to ponder. A character in Thomas Hardy's nineteenth-century novel Two on a Tower, an astronomer named Swithin St. Cleeve, gave splendid voice to this apprehension: “There is a size at which dignity begins; further on there is a size at which grandeur begins; further on there is a size at which solemnity begins; further on, a size at which awfulness begins; further on, a size at which ghastliness begins. That size faintly approaches the size of the stellar universe. So am I not right in saying that those who exert their imaginative powers to bury themselves in the depths of that universe merely strain their faculties to gain a new horror?”
Even as late as the eighteenth century, most celestial observers still backed away from questions of the universe's true size and nature, for professional astronomers at this time were primarily mathematicians who used Newton's laws to predict the motions of the Moon, planets, and comets. Stars themselves, as distinct celestial objects, were not yet as interesting or provocative to them as determining with utmost precision their coordinates (in essence, their heavenly latitude and longitude) for celestial atlases. As a result, cosmological conjectures on the universe's size, shape, and destiny were largely thrashed out by those on the fringe, such as Thomas Wright, a dilettante and schemer who clawed his way up the social ladder from a rather modest background as a carpenter's son. After serving as a watchmaker's apprentice, seaman, and then teacher of mathematics and navigation, he went on to make a comfortable living in England giving private lessons on architecture and science to noble families. He tutored Lord Cornwallis's daughters (sisters of the Revolutionary War general), hunted with the Earl of Halifax, and dined regularly for a time with the Duke and Duchess of Kent.
With the backing of his wealthy benefactors, Wright published a lavish book in 1750 titled An Original Theory; or, New Hypothesis of the Universe, which attempted to explain the structure of the Milky Way. Then thirty-nine years old, the Englishman applied his self-taught expertise in surveying and geometry to the question he had been pondering, off and on, for many years: Why does the Milky Way appear as a misty streak that stretches across the celestial sphere? Galileo with his telescope had revealed that this cloudlike band was composed of innumerable stars, but why should the stars arrange themselves in such a streamlike fashion?
Thomas Wright of Durham
(From Thomas Wright's An Original Theory; or,
New Hypothesis of the Universe, 1750)
Limited in formal education, Wright filled his book with arcane theological digressions, as was the style of his time, but in the midst of his ramblings he introduced the startling idea, now deemed obvious, that our position in space affects how we perceive our celestial environment. He proposed that the Milky Way could be “no other than a certain Effect arising from the Observer's Situation, I think you must of course grant such a Solution at least rational, if not the Truth; and this is what I propose by my new Theory.” Hedging his bets, he offered a couple of explanations for the Milky Way's appearance. One model pictured the stars moving in a vast ring, much like the rings of Saturn, around a central point. But, strongly guided by his religious views, he preferred to think of the Milky Way as a thin spherical shell of stars—essentially a bubble—with the solar system on the surface and the Eye of Providence, the “agent of creation,” residing in the center.
Thomas Wright's engraving of the Milky Way,
depicting it as a disk of stars (From Thomas Wright's
An Original Theory; or, New Hypothesis of the Universe, 1750)
Wright included a number of lush illustrations, thirty-two in all, which conveyed his seminal ideas better than the text itself. One engraving—the one still found in textbooks today—displays the Milky Way as a flat layer of stars. This was a first step in imagining his huge spherical shell. “I don't mean to affirm that [the disk] really is so in Fact,” he wrote, “but only state the Question thus, to help your Imagination to conceive more aptly what I would explain.” Looking along the plane of Wright's big, gently curving shell, in which the Sun is embedded, Earth's inhabitants would readily perceive a disklike structure. The Milky Way appears as a band, mused Wright, because we observe this thin layer of stars edge-on; when looking away from the plane, stargazers see fewer stars.
Wright went on to consider whether certain cloudy spots, then being observed in the heavens in greater numbers, might be additional creations, bordering upon us but “too remote for even our telescopes to reach,” countless spheres with many “Divine Centres.” He seemed to be echoing the Swedish philosopher Emanuel Swedenborg, who in 1734 also wondered if “there may be innumerable other spheres, and innumerable other heavens similar to those we behold, so many, indeed, and so mighty, that our own may be respectively only a point.”
If left there, Wright's imaginative ideas and dazzling illustrations would have likely generated hardly a footnote in astronomical history. He even reverted to a more medieval cosmic model, outrageous in its fires-of-hell imagery, some years later. But as British historian Michael Hoskin first pointed out, Wright managed to achieve a degree of acclaim when others widely disseminated what they thought he meant. A few months after the publication of An Original Theory, its key ideas were summarized in a Hamburg journal. The review selectively stressed Wright's concept of the Milky Way as a flat ring, rather than a sphere. This ring was compared to our solar system, with the stars moving around much like the planets circling the Sun. Inspired by this brief journal account, a young Prussian tutor in 1755 wrote his own book on the subject. Like Wright, he described the nebular patches in the nighttime sky as “just universes and, so to speak, Milky Ways… These higher universes are not without relation to one another, and by this mutual relationship they constitute again a still more immense system.” These words were virtually ignored until the author—Immanuel Kant—achieved fame as one of the world's great philosophers. Even then his ideas on the universe's design almost didn't survive. Kant's manuscript was destroyed when his printer went bankrupt. Fortunately, a shorter version was tucked away in the appendix of another book that he published in 1763.
Kant, trained in science, imagined that Wright's ring of stars was actually a continuous disk. This was more than wishful thinking; he was inspired by the latest astronomical evidence. Pierre-Louis de Maupertuis in France had been observing dim objects in the sky, what he called “nebulous stars,” that appeared elliptical in shape, the very way a disk would appear when tipped at an angle. “I easily persuaded myself,” wrote Kant, “that these stars can be nothing else than a mass of many fixed stars… On account of their feeble light, they are removed to an inconceivable distance from us.” With such reasoning, Kant arrived at the correct image of a galaxy's basic structure. Kant was astonished that previous observers of the heavens had not figured out the structure of our galaxy earlier. The Milky Way resembled a flat plate. Moreover, it was just one of many star-worlds scattered throughout the heavens. The German scientist Alexander von Humboldt later dubbed them Kant's “island universes,” a phrase that would resonate throughout the astronomical community like a mantra—some championing Kant's vision, others deriding it. Johann Lambert, a former tailor's apprentice in Alsace who had learned some science on his own, independently arrived at a similar conclusion in 1761 with his Cosmological Letters on the Arrangement of the World-Edifice. With the publication of these works, the “mystery of the nebulae” came to vex both philosophers and astronomers for more than a century.
From the days of Ptolemy, astronomers talked about certain stars in the sky that appeared “cloudy” to the eye. The most famous is in the northern constellation Andromeda, the mythical princess situated in the sky near her parents, Cassiopeia and Cepheus, and her husband, Perseus. At her waist is an oval patch of light, best seen on the darkest of nights. As early as the tenth century, astronomer Al-Sufi of Persia noted it as a “little cloud” in his catalog of the heavens. With the invention of the telescope more nebulae were sighted, and by the early 1700s Edmond Halley (of comet fame) counted six in all. To some observers, these pale entities were breaks in the celestial sphere, through which the light of the Empyrean—the highest heaven—came shining down. Others suggested that they were the hazy atmospheres surrounding distant stars. Halley, however, thought of them as unique celestial objects, unlike anything else in the heavens. They “appear to the naked Eye like small Fixed Stars,” he wrote, “but in reality are nothing else but the Light coming from an extraordinary great Space in the Ether; through which a lucid Medium is diffused, that shines with its own proper Lustre.”
Gradually found in greater numbers, these celestial objects took on even more importance in 1781 when the celebrated comet hunter Charles Messier published in France his list of more than one hundred nebulae, a directory that is still used today. The Andromeda nebula, for example, is commonly known as M31 because it's the thirty-first nebula in Messier's catalog. Messier, though interested in the nebulae themselves, primarily wanted to let his fellow observers know that these celestial regulars, the most prominent of their kind, should not be mistaken for comets. He was putting up cosmic road signs for his colleagues, pointing out those nebulae visible above the horizon from the latitude of Paris.
No one was more excited by Messier's list than William Herschel, England's soon-to-be crown prince of astronomy. As soon as Herschel received a copy of Messier's catalog, he immediately aimed a telescope at the celestial clouds. “I…saw, with the greatest pleasure, that most of the nebulae, which I had an opportunity of examining in proper situations, yielded to the force of my light and power, and were resolved into stars,” he wrote a few years later. He was the first to make this discovery, using a telescope twenty feet in length with a mirror then twelve inches wide. It was the most powerful in its day, allowing him to see that many of the nebulae (what we now call open clusters and globular clusters) were actually comprised of hundreds and thousands of stars. This led him to the belief that all nebulae were far-off systems of stars. Any nebula still appearing cloudlike through his eyepiece, he figured, was simply too distant to behold individual stars clearly.
Herschel promptly initiated a grand hunt for nebulae, literally sweeping the heavens with his giant reflector. Previous endeavors to spot nebulae paled beside this effort. By 1786 he had sighted a thousand new nebulae and star clusters; three years later he added hundreds more. “These curious objects, not only on account of their number, but also in consideration of their great consequence, [are] no less than whole sidereal systems,” he wrote. He even boasted at one point that he had discovered fifteen hundred new universes. Each, he excitedly reported, “may well outvie our milky-way in grandeur.”
Herschel had come late to this pursuit. Raised in the Duchy of Hanover (now part of Germany) within a family of musicians, he fled as a teenager to England, Hanover's ally, in the midst of war. There he supported himself by copying musical manuscripts, composing, giving private lessons, and performing in local concerts. Eventually he obtained financial security by becoming a choral director in the city of Bath. Yet he was restless for more intellectual stimulation.
Inspiration arrived on May 10, 1773. On that day Herschel, then thirty-four years old, bought a copy of a popular astronomy textbook. “When I read of the many charming discoveries that had been made by means of the telescope,” said Herschel, “I was so delighted with the subject that I wished to see the heavens and Planets with my own eyes thro' one of those instruments.” By the autumn he was beginning to handcraft metal mirrors for a reflecting telescope. He became obsessed with his new hobby, soon shifting his interests from the music of the Earth to the music of the heavens. So passionate was his commitment to astronomy that his younger sister, Caroline, who had earlier joined him in England, fed him morsels of food by hand, so that he would not have to pause while grinding and polishing. Pointing his home-built instruments toward the sky, he came to memorize the heavens and in 1781 climactically spotted Uranus, the first planet discovered since the dawn of history. He was promptly elected a fellow of the Royal Society and procured an annual stipend from England's King George III, a pension that at last allowed Herschel to devote himself to his astronomical interests, especially building ever-larger telescopes (the largest he ever constructed was forty feet long).
Herschel was far ahead of his time, as he used his telescope to examine the universe much the way an astronomer would today. While other astronomers in his day focused solely on the motions of the stars and planets, he was determined to discern nothing less than the “construction of the heavens,” the title of one of his most notable papers. He wanted to reach out into distant space, far beyond the realm most studied by his contemporaries. Wright and Kant had done the same, but they were merely theoretical speculators, not practicing astronomers. Herschel insisted that his ideas be “confirmed and established by a series of observations.” Photography was still decades away, so to do this he had to spend hours at his eyepiece, awkwardly perched on a platform at the top of his telescope. So skilled did he become at fashioning telescopes that his instruments were the only ones at the time capable of seeing out to cosmological distances. His tireless assistant Caroline was often with him, jotting down the positions and descriptions of the many nebulae he came upon during his scans of the heavens.
Drawings of nebulae by astronomer William Herschel, 1811
(From Philosophical Transactions of the Royal Society of London
101 : 269-336, Plate IV)
“I have seen double and treble nebulae, variously arranged; large ones with small, seeming attendants; narrow but much extended, lucid nebulae or bright dashes; some of the shape of a fan, resembling an electric brush, issuing from a lucid point, others of cometic shape, with a seeming nucleus in the center;…when I came to one nebula, I generally found several more in the neighbourhood,” he reported. At one point, Herschel even imagined other beings residing within those nebulae, looking back at us: “The inhabitants of the planets that attend the stars which compose them must likewise perceive the same phænomena. For which reason they may also be called milky-ways by way of distinction.” He seemed to be confirming the Wrightian and Kantian visions: that the universe is vastly larger and more complex than previously imagined. The Milky Way was a cohesive system of stars and beyond that was a limitless universe, populated by other stellar systems, comparable to our own.
Astronomers might have become quite comfortable with and accepting of the idea that other galaxies existed, more than a century before Hubble proved it conclusively, if not for the fact that Herschel abruptly changed his mind about those hundreds of “new universes.” A new observation forced him to reconsider his previous assertions. It happened on a cold November evening in 1790 when Herschel came upon an eighth-magnitude star that was surrounded by a faintly luminous atmosphere of considerable extent. “A most singular phænomenon!” he jotted down in his notebook. He called this haze a “planetary nebula” because of its resemblance to a planetary disk (as noted earlier, now known to be an aging star shedding its outer envelope of gas). “Cast your eye on this cloudy star,” he wrote, “and the result will be no less decisive…that the nebulosity about the star is not of a starry nature… Perhaps it has been too hastily surmised that all milky nebulosity, of which there is so much in the heavens, is owing to starlight only.” In Herschel's mind, nebulae had to be comprised of either stars or a “shining fluid”—not both. So he decided that any nebulae that remained unresolved through his telescope were no longer distant stellar systems, but instead collections of luminous matter, likely the stuff out of which stars ultimately condensed.
Herschel's telescopes were so much better than the equipment of any other astronomer at the time that his colleagues trusted his judgment on this matter. They simply didn't have the telescopic power to confirm his findings. As a result, Herschel's pronouncement became the accepted wisdom. The universe swiftly shrank back to the borders of the Milky Way. We were alone in the universe once again … at least for a while.
Throughout the nineteenth century, the two explanations for the unresolved nebulae went through a relentless tug-of-war, one side winning the hearts of astronomers for a time, then the other. Some insisted they were nearby clouds of gas, while others championed them as far-off islands of stars. Each faction was seeking a solitary explanation, simple and elegant—and that meant choosing between the two possible options.
Cosmology at this time continued to be of more interest to independent astronomers than the professionals who toiled at university-or government-sponsored observatories, and it was one of these self-directed observers who gave renewed hope to those who favored the idea that the dim nebulae were similar to the Milky Way, separate galaxies whose individual stars over the vast distances melted into a uniform pool of light. The excitement arose when William Parsons, the third Earl of Rosse, constructed a giant telescope on the grounds of his ancestral home, Birr Castle, in central Ireland, seventy miles west of Dublin. So big was the telescope tube that at the observatory's opening ceremony, a dean of the Church of Ireland walked right down the huge cylinder wearing a top hat and an open umbrella.
Young Rosse (then Lord Oxmantown, prior to succeeding his father in the earldom) served in the British Parliament, but his passion was telescope-building, with his decided aim, according to those who knew him, “to make a telescope of the largest dimensions possible with the resources of his time.” In 1834, at the age of thirty-four, Rosse left politics to devote himself to a newfound career as a gentleman scientist. He had long wanted to surpass Herschel's instruments in size and devised the methods himself for casting and polishing the metal mirror in his own workshops, personally training the laborers on his estate to assist him. Though an aristocrat, he put on no airs; a British reporter once caught him working at a vise, his shirtsleeves rolled up, displaying brawny arms. The mirrors he constructed were made out of a tin and copper alloy, a blend that resulted in a reflectivity almost as high as silver. Rosse's first big success was a three-foot-wide mirror mounted in a tube twenty-six feet long. “It is scarcely possible to preserve the necessary sobriety of language in speaking of the moon's appearance with this instrument,” reported a friend.
The triumph gave Rosse the confidence to construct a mirror twice the size, taking no notice that the Irish weather was more infamous for rain than clear skies. First put into operation in 1845, this reflector, when erect, was said to resemble one of the ancient round towers of Ireland and was dubbed the “Leviathan of Parsontown.” “Sweeping down from the moat towards the lake, stand two noble masonery walls,” reported a houseguest. “They are turreted and clad with ivy, and considerably loftier than any ordinary house. As the visitor approaches, he will see between those walls what may at first sight appear to him to be the funnel of a steamer lying down horizontally.” It was the telescope's immense wooden tube, which was more than fifty feet long and held a polished metal mirror six feet in diameter. This mirror provided fourteen times more surface area for collecting light than Herschel's most productive telescope. A pulley system, attached to the top of the tube, allowed the telescope to be pointed by two men on the ground. A series of staircases and galleries provided the observer access to the mouth of the great tube. It was an astounding telescope size for its time and wouldn't be matched for another seven decades.
The Leviathan's prime targets were the “strange stellar cloudlets that fleck the dark vault of the heavens.” Rosse was determined to see if he could resolve the nebulae—those that remained stubbornly cloudlike—into stars. But what he turned up was something even more intriguing.
In the spring of 1845, Rosse and his assistant Johnstone Stoney began to study the fifty-first nebula, M51, in Messier's famous catalog. When William Herschel viewed it years earlier, he saw only a bright round nebula; his son later observed it as a ring with two branches. But Rosse, to his amazement, detected a distinct coiling, arms of gas wrapped around M51's center like a whirling pinwheel. No one had ever anticipated something like this. Some nebulae were shaped like spirals, “a structure and arrangement more wonderful and inexplicable than anything which had hitherto been known to exist,” reported Great Britain's Royal Astronomical Society.
In these days before astrophotography, Rosse sketched a picture of the configuration with painstaking care. “With each successive increase of optical power, the structure has become more complicated and more unlike anything which we could picture to ourselves,” Rosse reported. “That such a system should exist, without internal movement, seems to be in the highest degree improbable.” This is when M51 came to be called the Whirlpool because of the striking swirl of its appearance. Rosse went on to discern more than a dozen such spiral nebulae in the celestial sky.
A drawing of Lord Rosse's Leviathan (From Philosophical Transactions of
the Royal Society of London 151 : 681-745, Plate XXIV)
Despite Rosse's gorgeous drawings, a few believed the spiraling lanes of nebulous matter “existed only in the imagination of the astronomer.” Rosse's mirror was so large—its light-gathering power so great—that no other telescope could verify his find. But for others, the discovery revived and energized Herschel's earlier speculation that other systems of stars resided outside the borders of the Milky Way. Scottish astronomer and science popularizer John P. Nichol was certainly thrilled, for he had long been pushing the idea that “numerous firmaments, glorious as ours, float through immensity, doubtless forming one stupendous system.” He was a Kantian. Nichol thought of a galaxy (what he called a “grand group”) as the chief feature in the universe. “It is indeed wholly unlikely that our group, as a single instance of a species, should rest alone and forlorn amidst desert untenanted Space,” he wrote. The universe, to Nichol, was “thronged with similar clusters, separated far from each other as islands in the great Sea.” Some “are situated so deep in space,” he went on, “that no ray from them could reach our Earth, until after travelling through the intervening abysses, during centuries whose number stuns the imagination.” He even imagined some so far distant that their light left “at an epoch farther back into the Past than this momentary lifetime of Man, by at least THIRTY MILLIONS OF YEARS!” This was a brave estimate for someone to make in 1846, a time when many in the public still held to a biblical age for creation of only six thousand years and scientists over the previous fifteen years were just beginning to find evidence (then still controversial) that it was much longer.
Lord Rosse's drawings of M51 (top) and M99 (bottom),
which in the mid-1840s were the first nebulae found to have a spiraling
structure (From Philosophical Transactions of the Royal Society of
London 140 : 499-514, Plate XXXV)
It was said that Rosse's telescope was “poised so skilfully that a child could guide its movements.” By one astronomer's reckoning, it could gather twenty thousand times the light of the unaided eye. But the Leviathan did possess one blatant shortcoming: “It does not present objects in a perfectly distinct manner,” said Richard Proctor, a contemporary of Rosse's who once had the opportunity to peek at the sky with the giant telescope. “It used to be remarked of the great four-feet reflector of Sir William Herschel, that it ‘bunched a star into a cocked hat.’” Proctor believed the same was true for Rosse's great instrument. The sheer weight of the telescope's mirror—a truly massive four tons—distorted its images at times. Views of planets through the Rosse scope, judged Proctor, were “perfectly wretched.” Although the metal reflector had its good days as well as bad, criticism like this dampened enthusiasm for further advancement on mirrored telescopes. Herschel and Rosse had made great strides with their big reflectors, but most astronomers still preferred gathering their celestial light with lenses. Not until James Keeler got the Crossley reflector up and running at Lick Observatory in the 1890s did astronomers at last change their minds on their instrumental preference.
Rosse, an engineering wizard, was always more attracted to constructing a telescope than to using it. His astronomical work continued for some twenty years, but most of the measurements were carried out by associates. His greatest contribution to astronomy was his discovery of the spirals, revealed when the Leviathan first went into operation. In doing this, he introduced an entirely new celestial creature, a novel species of nebula that would tantalize and frustrate astronomers for decades to come.
Popular interest in astronomy grew immensely in the nineteenth century, likely fueled by the rising use of photography, which at last allowed the general public to view and admire gorgeous pictures of the celestial heavens at their leisure. The first known daguerreotype of a celestial object, the Moon, was taken by the American physician John Draper in the 1840s. Later, the brightest stars were imaged. But the process became more routine with the introduction of more sensitive plates in the 1870s, which allowed fainter and more diaphanous objects, such as nebulae, to be photographed.
At the same time, the invention of the spectroscope offered a novel means for astronomers to pursue the mystery of the nebulae. Widely known as the “new astronomy” or astrophysics, spectroscopy was particularly favored by the enthusiasts who lacked formal mathematical training in classical astronomy. Professional astronomers were slow to appreciate the power of the new instrument. Indeed, they were distraught to see their telescopes, once set up like grandiose metal sculptures within towering domes, now surrounded by a chaotic array of chemical and electrical contraptions required to carry out spectroscopic work. But nonprofessionals astutely perceived that spectroscopy, despite its inelegance, opened up virgin astronomical territory. Rather than dully peg the positions of stars to stupefying accuracies, they were going to discern the very nature of celestial objects—what they are instead of where they are.
No one was more dedicated or persistent in this new enterprise than William Huggins. At the age of thirty he sold his textile business in England and erected a private observatory at Tulse Hill, then a rural area about four miles south of central London. Soon tiring of routine astronomical observations, he was reinvigorated when he heard about the latest spectroscopic discoveries. He compared it to “coming upon a spring of water in a dry and thirsty land.” By 1862 he was able to show that the elements found both on the Earth and in the Sun also dwelled in the distant stars. “The chemistry of the solar system prevailed,” said Huggins, “wherever a star twinkled.”
Then, on the evening of August 29, 1864, he shifted his attention from stars to nebulae. He aimed his telescope at a bright planetary nebula in the Draco constellation. He recalled in a memoir years later that he felt “excited suspense, mingled with a degree of awe” as he put his eye to the spectroscope. The spectrum he beheld was a surprise: “A single bright line only!” he noted. “At first I suspected some displacement of the prism, and that I was looking at a reflection of the illuminated slit… This thought was scarcely more than momentary; then the true interpretation flashed upon me… The riddle of the nebulae was solved. The answer, which had come to us in the light itself, read: Not an aggregation of stars, but a luminous gas.” A star was simply too complex to be emitting a single spectral line; the emitter had to be a gaseous cloud, he thought, readying itself for stellar construction. In light of this and other findings, it became more popular to think of all nebulae as embryonic stars and planetary systems in the making. This idea was strengthened in 1888 when the English celestial photographer Isaac Roberts captured a full picture of the Andromeda nebula, an astounding feat at the time because of its faintness. When it was displayed at a Royal Astronomical Society meeting, murmurs could be heard in the audience: “The nebular hypothesis made visible!” The photo displayed a bright core surrounded by a wide, hazy cloud. When Huggins saw the image, he exclaimed that it had to be “a planetary system at a somewhat advanced stage of evolution; already several planets have been thrown off.”
With the great weight of his opinion, Huggins helped force the pendulum the other way. The island-universe theory was no longer a viable contender; it became passé. In his late-nineteenth-century A Text-book of General Astronomy for Colleges and Scientific Schools, a classic in its day, astronomer Charles Young stressed that astronomers no longer considered a spiral nebula as “a ‘universe of stars,’ like our own ‘galactic cluster’ to which the sun belongs…. In some respects this old belief strikes one as grander than the truth even. It made our vision penetrate more deeply into space than we now dare think it can.” To Young, the Milky Way was some 10,000 to 20,000 light-years wide. “What is beyond the stellar system, whether the star-filled space extends indefinitely or not, no certain answer can be given,” he said.
The island-universe theory had already been shaken in 1885 when a nova—a new pinpoint of orange-yellow light—was sighted near the center of the Andromeda nebula. At its brightest, around the sixth magnitude, this nova was nearly as luminous as the entire nebula. “This strange and beautiful object has broken silence at last, though its utterance may be difficult to interpret,” said the Greenwich Observatory astronomer E. Walter Maunder.
If Andromeda were a distant external universe, it was reasoned, the nova had to be shining with the energy of some fifty million suns, “a scale of magnitude such as the imagination recoils from contemplating,” said Agnes Clerke, a nineteenth-century historian of astronomy. That was actually a stupendous underestimate of the nova's power, but even that tally was too preposterous to consider in any serious fashion in 1885. The idea that a star could totally obliterate itself as an explosive supernova was not even a fantasy at the time. There was no physics to explain it. Stars were regarded as stable and enduring. It seemed more likely that the nova was an infant sun condensing and turning on within a vast collection of luminous matter on the edge of the Milky Way or perhaps a dark star running into nebulous matter and provoking an incandescent outburst.
Astronomer Edwin Frost, then nineteen and entering his senior year at Dartmouth College when the nova appeared, recalled the event with great vividness: “[The nova] was in the heart of the Great Nebula…and was a star of about the seventh magnitude. It thus became the only individual star distinguishable in this nebula, which at that time we supposed to be a purely gaseous body… The distance of the nebula was then not regarded as greater than that of the stars in our portion of the Milky Way… Among astronomers, as well as the public generally, it was thought that we might be observing the sudden transformation of the nebula into a star,” and perhaps a planetary system as well. Another great nova, dubbed Z Centauri, appeared in the spiral nebula NGC 5253 a decade later, reinforcing the belief that spiral nebulae were relatively close by. Given what astronomers then knew about stars, there was no other explanation.
So, by the turn of the twentieth century, most astronomers had settled on this common story for the spiral nebulae—that they were new stars and planets emerging. This idea gained momentum when Thomas Chamberlin, a respected geologist, joined up with Forest Ray Moulton, an expert on celestial mechanics, on modeling how the solar system came to be formed. The Chamberlin-Moulton theory suggested that a nomadic star passed near our Sun long ago, drawing out streams of gas. This material eventually became a rotating nebula with spiraling arms, from which the planets slowly condensed. Chamberlin, while working on this idea at the University of Chicago, had heard about the amazing images of spiral nebulae that James Keeler was obtaining with his reflector atop Mount Hamilton, which seemed to suggest that he and Moulton were on to something: The spirals might be the gas, just recently torn off and ready for condensation into the planets that would eventually orbit the star, the bright center of the spiral nebula. Chamberlin wrote Keeler, saying, “[I would deem] it a very great favor to be able to make use of your great harvest of new forms.” Keeler obliged.
“The question whether nebulae are external galaxies hardly any longer needs discussion. It has been answered by the progress of discovery,” declared Clerke with confident finality in her influential book The System of the Stars. “No competent thinker, with the whole of the available evidence before him, can now, it is safe to say, maintain any single nebula to be a star system of coordinate rank with the Milky Way.” To Clerke, such contemplations were “grandiose” and “misleading.” Our galaxy and the universe were one and the same—synonyms in the dictionary of the heavens.
But soon after Clerke wrote her comments, new observations were beginning to suggest something very different. At the Potsdam Observatory, in Germany, Julius Scheiner spent seven and a half hours in January 1899 gathering a spectrum of the Andromeda nebula. What he saw was unexpected. The spectrum did not resemble a cloud of gas, such as the Orion nebula, at all. Instead, it resembled the light emitted by a vast collection of stars. “That the spiral nebulae are star clusters is now raised to a certainty,” reported Scheiner. He began to imagine that the Milky Way itself was a spiral nebula, very similar to Andromeda. But at that point Scheiner was effectively a lone voice in the cosmic wilderness. At the Lick Observatory, Keeler took special note of the German's finding but died before he could follow up.
A further investigation was not undertaken until 1908, when Edward Fath, a graduate student at Lick, used the Crossley telescope to confirm Scheiner's findings on the spectrum of Andromeda (M31), as well as several other spirals, for his dissertation. It was wearying work, as Fath had to sustain a photographic exposure over several nights. One plate was exposed for a total of eight hours and forty-seven minutes. Another took more than eighteen hours. But the tediousness of the procedure paid off. To Fath, the results were unmistakable: from its spectral signature, Andromeda appeared to consist of myriad stars, many of them similar to our own Sun. As a double check, he took the spectra of some globular clusters, which were known to be assemblies of stars. Each spectrum looked exactly like Andromeda's.
“The hypothesis that the central portion of a nebula like the famous one in Andromeda is a single star may be rejected at once,” reported Fath, “unless we wish to modify greatly the commonly accepted ideas as to what constitutes a star.” He suspected the spirals were very remote, as the stars could still not be resolved into individual pinpoints of light, but he had no definitive proof—no slam dunk—to back up that guess. In 1908 there was as yet no way to measure the distance out to Andromeda directly.
As a result, Fath didn't promote his conclusion, perhaps because he was still a lowly graduate student and not in a position of authority to overturn the nebula-as-new-solar-system belief. Or perhaps because Lick Observatory director W. W. Campbell, a careful and conservative man of science, instructed him to downplay his speculations. Whatever the reason, Fath took a particularly cautious tone in the close of his official report. He said that his interpretation “stands or falls” on the question of determining a true distance to a spiral.
The response to Fath's report was like the sound of one hand clapping. Aside from a few outliers, hardly anyone else cared. Fath was soon offered a post at the Mount Wilson Observatory, where he did some follow-up work for a number of years but arrived at no breakthroughs. He eventually settled into a teaching job at Carleton College in Minnesota.
And that's where the matter stood until a man, whom Keeler had once rejected for a Lick graduate fellowship, took over the Crossley reflector in 1910 and continued the groundbreaking work of both Keeler and Fath. And in doing so, Heber Curtis challenged the conventional wisdom with single-minded determination. With great industry and zeal, he took on the problem of the spiral nebulae and made it his own.