Herschel’s skill as an observer was equally refined; he had a way with telescopes. “Seeing is in some respect an art, which must be learnt,” he wrote.
I have tried to improve telescopes and practiced continually to see with them. These instruments have played me so many tricks that I have at last found them out in many of their humours and have made them confess to me what they would have concealed, if I had not with such perseverance and patience courted them.16
With the ardor of a man possessed, Herschel stayed at the telescope on virtually every clear night of the year, all night long, taking only a few minutes off every three or four hours to warm himself—or, as happened one night when the temperature dropped to 11 degrees Fahrenheit, to fetch a tool to break through the ice that had glazed over his inkwell. He rushed to the telescope to observe during intermissions in the concerts he conducted at Bath. When skies were cloudy he and Caroline waited up, hoping for a change in the weather. “If it had not been for the intervention of a cloudy or moonlit night I know not when he or I either would have got any sleep,” wrote Caroline in her diary.17 When they moved to Datchet, to a dank house so near the Thames that the yard was often flooded, Herschel waded through the water and climbed to the eyepiece of the telescope, staving off ague by rubbing onion on his hands and face. “He has an excellent constitution,” wrote Caroline, “and thinks about nothing else in the world but the celestial bodies.”18
Herschel’s preferred method of observing consisted of “sweeping” the sky. Wearing a black hood to keep any stray light from dazzling his dilated, dark-adapted eyes, he would move the telescope across a segment of sky, pausing to note the locations of interesting objects, then move the telescope slightly in the perpendicular and sweep back along an adjacent path. Ten to thirty such oscillations he called a sweep; and he registered each in what he called his “Book of Sweeps.” This was making a virtue of necessity; his telescope lacked the equatorial mountings and clock drives that are employed today to compensate for the earth’s rotation and to hold a single object effortlessly in view. Its great advantage was that it encouraged Herschel to memorize whole swathes of sky; the most significant northern hemisphere star map of the later eighteenth century may well have existed not on the pages of a celestial atlas but in Herschel’s mind.
It was to this familiarity with the sky that Herschel owed his discovery, on the night of March 13, 1781, of the planet Uranus. Uranus had been glimpsed dozens of times before, by Bradley, Flamsteed, and others, but always had been mistaken for a star. Herschel, his mind an encyclopedia of the night sky, realized as soon as he saw it that no star belonged there. At first he mistook the little green dot for a comet, but the Astronomer Royal, Nevil Maskelyne, calculated its orbit and determined that it must be a planet, one far beyond Saturn. In a single stroke, Herschel had doubled the radius of the known solar system. The resulting fame brought him a fellowship in the Royal Society, a pension, and an appointment as astronomer to King George III—who was being blamed for losing the American Revolution and was suffering a mental breakdown at the time, and must have felt grateful for a little good news.
Herschel received a royal grant of four thousand pounds to build and operate what would be the world’s largest telescope. Out of his own funds he had already managed to build a reflector twenty feet long, with a mirror eighteen and a half inches in diameter, but there were clear signs that he had pushed his private efforts about as far as they could go. Most ominous was the episode of the horse-dung mold. Herschel had wanted to cast a mirror fully three feet in diameter, with three times the light-gathering power of the eighteen-inch. No foundry would take on the unprecedented project, so Herschel resolved to do it himself, in the basement of his house at 19 New King Street in Bath. He constructed an inexpensive mold out of what the uncomplaining Caroline described as “an immense quantity” of horse dung. She, William, and their brother Alex took turns pounding the dung, assisted by their friend William Watson of the Royal Society. Finally came the day to, as Herschel put it, “cast the great mirror.” At first all went well, but then the mold cracked under the intense heat and molten metal flowed out across the floor, exploding flagstones and sending them caroming off the ceiling. The party fled into the garden, pursued by a rapidly expanding pool of liquid metal. Herschel took refuge on a pile of bricks and there collapsed. He had reached the practical limits of amateur telescope making.
The largest telescope in the world was built, therefore, with the king’s money, by a team of workmen under Herschel’s direction. It had a forty-eight-inch mirror that weighed a ton, housed in a tube forty feet long. To reach the eyepiece, Herschel had to climb a scaffolding that rose fifty feet into the air. Oliver Wendell Holmes described the instrument as “a mighty bewilderment of slanted masts, spars and ladders and ropes, from the midst of which a vast tube … lifted its mighty muzzle defiantly towards the sky.”19 At its dedication, the king took the archbishop of Canterbury by the arm with the words, “Come, my Lord Bishop, I will show you the way to Heaven.”20
With the forty-eight-inch reflector, Herschel discovered Enceladus and Mimas, the sixth and seventh satellites of Saturn, but ultimately the heroic telescope proved to be a disappointment. Training it on a given piece of sky was a taxing process that involved shouting instructions down to a team of laborers stationed in the rigging below, and its mirror tended to warp and mist over with changes in temperature and humidity. Herschel soon returned to working with smaller telescopes he had built by hand.
The nebulae continued to fascinate him. In 1781 he received a copy of Charles Messier’s new catalog of these glowing islands of light, promptly set to work observing them, and found that “most of the nebulae … yielded to the force of my light and power, and were resolved into stars.” He concluded, prematurely, that all nebulae were but star clusters, and could be resolved into their constituent stars once large enough telescopes were employed in observing them. His confidence in this comprehensive but erroneous hypothesis was shaken by his subsequent investigations of what he labeled the “planetary” nebulae—the ones now known to be shells of gas ejected by stars. When Herschel observed planetary nebulae in which the central star was too dim to be seen, he assumed that they were globular star clusters. But then, on the night of November 13, 1790, he came upon a planetary nebulae in Taurus with a clearly visible central star. He appreciated its significance immediately. “A most singular Phaenomenon!” he wrote in his journal. “A star of about the eighth magnitude, with a faint luminous atmosphere…. The star is perfectly in the center and the atmosphere is so diluted, faint and equal throughout, that there can be no surmise of its consisting of stars; nor can there be a doubt of the evident connection between the atmosphere and the star.” He decided that some nebulae must, after all, be composed not of stars but of “a shining fluid” of unknown constitution. “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,” he wrote, modifying his earlier hypothesis. “What a field of novelty is here opened to our conceptions!” he exclaimed, more delighted by the variety of the sky than bothered at having been wrong.21
Herschel could be astonishingly acute. He called the Orion Nebula, a knot of congealing gas sixteen hundred light-years from Earth, “the chaotic material of future suns,” which is exactly what it is.22 He argued that the sun belongs to a vast cluster of stars—a galaxy, as we would say today—and he tried to map its boundaries, by counting stars of various magnitudes in various directions in the sky. This effort failed, owing both to the fact that apparent magnitude is not a reliable index to the distance of stars and to the presence of dark, obscuring nebulae in the Milky Way that Herschel mistook for empty space. Nonetheless the inspiring fact remains that an oboe player with a handmade telescope undertook, in the eighteenth century, a scientifically defensible project aimed at charting the extent of the entire Milky Way galaxy.
Herschel studied other galaxies, too, notably the great n
ebula in Andromeda, which he assumed, correctly, to glow with “the united luster of millions of stars.” He even noted that the central part of Andromeda was of “a faint red color.” The central region of this giant galaxy is, indeed, warmer in hue than the surrounding disk—it consists of old red and yellow stars, while young blue stars predominate in the surrounding disk—but it seems incredible that this distinction, which was not fully established until the twentieth century, could have been detected by the eye of an eighteenth-century astronomer. And yet, Herschel being Herschel, one sometimes wonders.
In any case, Herschel’s legacy has less to do with the extent to which his conclusions were right or wrong than with his prophetically modern approach to deep-space astronomy. At a time when most astronomers were peering at planets through the narrow fields of refracting telescopes, Herschel was harvesting great swaths of ancient light from distant nebulae and galaxies. While they were refining their estimates of distances within the solar system to the second decimal place, he was endeavoring to chart the starshoals of intergalactic space. While they were using estimates of the velocity of light to adjust their calculations of the orbits of the satellites of Jupiter, he was, he realized, seeing so far into space as to be viewing the universe as it looked millions of years in the past. Herschel’s use of large reflecting telescopes to discern what he called “the construction of the heavens” may have been technologically precipitate, but it presaged the methods of the twentieth-century astronomers who were to realize his dreams. Cosmology for Kant and Lambert had been principally an indoor discipline; Herschel took it outdoors.
Sustained by his love for what he called “this magnificent collection of stars” in which we live, Herschel kept working until the end. “Lina, there is a great comet,” he wrote his sister Caroline on July 4, 1819. “I want you to assist me. Come to dine and spend the day here. If you can come soon after one o’clock we shall have time to prepare maps and telescopes. I saw its situation last night, it has a long tail.”23 He was eighty years old at the time, and he was still at work when he died, two years later.
William Herschel sought to chart our galaxy by counting stars of given apparent magnitudes in all quarters of the sky (top). The resulting chart (bottom), though extremely rough, hinted at the existence of the galactic plane.
9
ISLAND UNIVERSES
The light of the fixed stars is of the same nature [as] the light of the sun.
—Newton
Observations always involve theory.
—Edwin Hubble
Two schools of thought about the nature of the elliptical nebulae held sway in the nineteenth century.
One, the “island universe” theory of Kant and Lambert—the phrase is Kant’s—maintained that our sun is one among many stars in a galaxy, the Milky Way, and that there are many other galaxies, which we see across great gulfs of space as the spiral and elliptical nebulae. The other, the “nebular hypothesis,” maintained that the spiral and elliptical nebulae are whirlpools of gas condensing to form stars, and that they are nearby and relatively small. The nebular hypothesis also had originated with Kant, but was usually called “Laplacian,” after the French mathematician Pierre-Simon de Laplace, who had published a detailed account of how the sun and its planets might have congealed from a whirling nebula. Both theories were to some extent correct—some nebulae are, indeed, star-forming gas clouds, while the elliptical and spiral nebulae are galaxies of stars—but there was an understandable tendency to assume that a single theory would explain all types of nebulae, and this assumption bred confusion.
The observational evidence seemed to favor the nebular hypothesis. Most spectacular was the discovery by William Parsons, the third earl of Rosse, that some elliptical nebulae display a spiral structure. Lord Rosse, who employed a six-foot reflecting telescope that was at the time the largest in the world, actually was seeing spiral galaxies, but his observations were thought instead to support the nebular hypothesis, with its vision of stars condensing from whirlpools of gas. This impression was strengthened when photographs taken by Isaac Roherts in England in the 1880s revealed that most elliptical nebulae are spirals; when Roberts’s photographs were exhibited, at the Royal Astronomical Society in London in 1888, learned spectators were said to have gasped in recognition at the photographic evidence of “the nebular hypothesis made visible.”1 The hypothesis gained even more ground when time-exposure photographs made by James Keeler at Lick Observatory in California in the 1890s indicated that there are a great many spiral nebulae; Keeler estimated that over one hundred thousand spiral nebulae lay within the range of the Lick telescope. Hundreds of thousands of new solar systems seemed plausible, given the multitude of suns that bedeck the Milky Way, but it strained credulity to imagine that there could be hundreds of thousands of galaxies, each home to billions of stars.
The riddle ultimately was solved, not by the telescope or the camera alone, but by combining both with the spectroscope, which was to reveal what the stars and nebulae are made of—something that the philosopher Auguste Comte, as late as 1844, could cite as an example of knowledge forever denied the human mind.
The development of spectroscopy dates from 1666, when Newton noted that white sunlight directed through a prism produces a rainbow of colors. In 1802, the English physicist William Wollaston found that if he placed a thin slit in front of the prism the spectrum displayed a series of parallel dark lines, like the cracks between piano keys. But Wollaston set the experiment aside, and the elevation of spectroscopy to the status of an exact science was left to a skinny, impoverished teenager with a persistent cough, who when Wollaston made his discovery was in hospital, recuperating from injuries he had suffered in the collapse of the optics shop where he worked in the Munich slums. His name was Joseph Fraunhofer, and his fortunes were about to improve.
Optics in the early nineteenth century was a growth industry. Napoleon Bonaparte’s passion for maps and spyglasses had set surveyors and generals to writing orders for portable telescopes and theodolites, and the research of William Herschel and his son John, who charted the southern skies from an observatory at the Cape of Good Hope, had inspired interest in large telescopes among both enthusiasts who wanted to view the wonders of deep space for themselves and skeptics who were out to test the Herschels’ claims. A new breed of artisans prospered—the opticians, bitterly competitive, fiercely innovative, as hard as the brass and glass they worked with and as eccentric as the scientists and engineers they served. Emblematic of the breed was Jesse Ramsden of London, a perfectionist who toiled over his projects until he got them right, no matter how long it took; the eight-foot altitude-measuring circle that he crafted for Dunsink Observatory in Dublin, admittedly a masterpiece of precision, was delivered twenty-three years after the contract deadline.*
If the opticians expected to be treated like artists, that is just what many of them were. Alvan Clark, the great American telescope-maker, prospered as a portrait painter before he switched careers and built what are still regarded as the finest refracting telescopes in the world; keen-sighted, Clark was said to be able to fire six rifle bullets “through a distant board with such precision that one would say only a single shot had been fired,” and to detect tiny bubbles and ripples in glass that were invisible to lesser mortals.2
Fraunhofer was born into the steerage class of this flourishing profession. The eleventh son of an indigent master glazier, he had been orphaned at age eleven and apprenticed to one Philipp Weichselberger, a dull-witted Munich glasscutter who kept him overworked, underpaid, underfed, and uneducated. On July 21, 1801, the dilapidated building that comprised Weichselberger’s house and shop collapsed, and Fraunhofer, the only survivor, was at length pulled from its wreckage. His rescue made news, and his plight attracted the attention of Maximilian Joseph, the elector of Bavaria, who visited the injured boy in hospital and was impressed by his intelligence and cheerful disposition. The elector made Fraunhofer a present of eighteen ducats, enough to buy a g
lass-working machine, books, and release from what was left of his apprenticeship. Once free, Fraunhofer never looked back. He had an instinct for the essential, and his spirited research into the basic characteristics of various kinds of glass soon established him as the world’s foremost maker of telescope lenses.
Fraunhofer started out using spectral lines as sources of monochromatic light for his experiments in improving the color correction of his lenses, but soon became fascinated by the lines themselves. “I saw with the telescope,” he wrote, “an almost countless number of strong and weak vertical lines which are darker than the rest of the color-image. Some appeared to be perfectly black.”3 He mapped hundreds of such lines in the spectrum of the sun, and found identical patterns in the spectra of the moon and planets—as one would expect, since these bodies shine by reflected sunlight. But when he turned his telescope on other stars, their spectral lines looked quite different. The significance of the difference remained a mystery.
Fraunhofer died on June 7, 1826, at the age of thirty-nine, of tuberculosis, leaving the mysterious Fraunhofer lines as his legacy. In 1849, Léon Foucault in Paris and W. A. Miller in London found bright lines that coincided with Fraunhofer’s dark lines. Today these are known respectively as the emission and absorption lines, and they play a role in spectroscopy as potent as that of fossils in geology, producing information on the temperatures, compositions, and motions of gaseous nebulae and stars.
In the years 1855 through 1863, the physicists Gustav Kirchhoff and Robert Bunsen (the inventor of the Bunsen burner) determined that distinct sequences of Fraunhofer lines were produced by various chemical elements. One evening they saw, from the window of their laboratory in Heidelberg, a fire raging in the port city of Mannheim ten miles to the west. Using their spectroscope, they detected the telltale lines of barium and strontium in the flames. This set Bunsen to wondering whether they might be able to detect chemical elements in the spectrum of the sun as well. “But,” he added, “people would think we were mad to dream of such a thing.”4
Coming of Age in the Milky Way Page 16
