Keeler not only turned reflectors into astronomy's instrument of choice, he inspired astronomers to take a new look at the universe. Was the cosmos defined as simply the Milky Way, or was there more to the universe than met the unmagnified eye? Keeler took a problem previously tackled by amateurs, for the most part—the spiral nebulae—and turned it into a prime concern for professional astronomers. He gave traditional astronomy a good shake at the end of the nineteenth century and in the process reinvigorated a debate that had been going on for centuries. What was the true nature of those irresolvable nebulae—so mysterious, so enthralling—that pervaded the celestial sky? Could the universe possibly be far larger?
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 [1811]: 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.
The Day We Found the Universe Page 6