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 [1861]: 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 [1850]: 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 telesc
opes. 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. Chamber
lin 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.
The Day We Found the Universe Page 7