Coming of Age in the Milky Way - Timothy Ferris (2003)
Part I. SPACE
Chapter 9. ISLAND UNIVERSES
The light of the fixed stars is of the same nature [as] the light of the sun.
Observations always involve theory.
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 glass-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
Kirchhoff was mad enough to try, and by 1861 he had identified sodium, calcium, magnesium, iron, chromium, nickel, barium, copper, and zinc in the sun. A link had been found between the physics of earth and the stars, and a path blazed to the new sciences of spectroscopy and astrophysics.
In London, a wealthy amateur astronomer named William Huggins learned of Kirchhoff’s and Bunsen’s finding that Fraunhofer lines were generated by known chemical elements in the sun, and saw at once that their methods might be applied to the stars and nebulae. “This news came to me like the coming upon a spring of water in a dry and thirsty land,” he wrote.5 Huggins fitted a spectroscope to the Clark telescope at his private observatory, on Upper Tulse Hill in London. By carefully studying each spectrum until he could make sense of their many overlapping lines, he succeeded in identifying iron, sodium, calcium, magnesium, and bismuth in the spectra of the bright stars Aldebaran and Betelgeuse. This was the first conclusive evidence that other stars are made of the same substances that we find here in the solar system.
With mounting excitement, Huggins turned his telescope to a nebula. His journal for the year 1864 records the feeling “of excited suspense, mingled with a degree of awe, with which, after a few moments hesitation, I put my eye to the spectroscope. Was I not about to look into a secret place of creation?” He was not disappointed:
I looked into the spectroscope. No spectrum such as I expected! A single bright line only! … 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. Stars after the order of our own sun, and of the brighter stars, would give a different spectrum; the light of this nebula had clearly been emitted by a luminous gas.6
Because this first nebula Huggins observed with his spectroscope happened to be gaseous, he was led to the erroneous conclusion that all nebulae, the ellipticals and spirals included, were gaseous and that none was composed of stars.
But life is seldom simple, and misleading evidence for the nebular hypothesis continued to accumulate. The positions of hundreds of spiral nebulae were charted, and they were found to be most numerous in the parts of the sky that lie well away from the Milky Way—to “avoid” the Milky Way, in astronomical jargon. The avoidance effect suggested that the spiral nebulae were associated with our galaxy. (Actually, avoidance results from the fact that dark clouds along the plane of our galaxy obscure our view of the other galaxies, so that we see mostly those that lie away from the galactic plane.) The nebular hypothesis was strengthened on the theoretical front as well, when the astrophysicist James Jeans demonstrated, with considerable mathematical rigor, that a collapsing cloud of gas would tend to assume a disk shape much like that of the spiral nebulae. Jeans even managed to coax his model into generating spiral arms like those seen in the astrophotographs.
By now the nebular hypothesis was so successful that a bandwagon syndrome took over and astronomers began seeing what they thought they ought to see. One announced that he had measured the parallax of the Andromeda spiral. (Parallax is detectable only out to a few hundred light-years; the Andromeda galaxy is over two million light-years away.) Another found that by examining older photographs he could detect signs of circular motion in spiral nebulae. (In reality, galaxies are so large that to see a galaxy turn by as much as the second hand on a clock moves in one second would require taking two photographs separated by an interval of fully five million years.)
As the twentieth century began, then, several of the most stupefying aspects of the closed, pre-Copernican cosmology had been resurrected on a galactic scale. The sun was widely thought to be located at or near the center of a stellar system—the Milky Way—which embraced every star and nebula in the telescopic sky, and which, therefore, constituted nothing less than the entire observable universe. Beyond our galaxy might lie an infinite void, but this question remained as purely academic as had been the nature of space beyond the outer sphere of stars in Aristotle’s model.
But there is a self-correcting mechanism to science, and by the turn of the century it had begun to assert itself. The first cracks in the facade of the nebular hypothesis appeared on the theoretical side, when a fatal defect was identified in the Jeans theory of how the solar system had condensed. Were the hypothesis correct, the mathematicians calculated, the sun should have retained most of the angular momentum of the solar system, and be spinning very rapidly; instead, the solar “day” lasts a leisurely twenty-six days at the sun’s equator, and the planets harbor 98 percent of the angular momentum of the solar system.* The observational evidence began to turn against the nebular hypothesis as well. Huggins took a spectrum of the Andromeda nebula in 1888 but found it hard to interpret. Nine years later, Julius Scheiner in Germany published a spectrum of the Andromeda nebula, noting that the spectrum was not gaseous but starlike. Undoubtedly, at least some spiral nebulae were made of stars.
Exploding stars then came to the astronomers’ aid, as they had centuries earlier for Tycho, Kepler, and Galileo. Two or three supergiant stars explode in an average major galaxy every century, with such brilliance that they can be seen across the reaches of intergalactic space. Since thousands of galaxies (or elliptical and spiral nebulae, as they were then being called) lay within the reach of existing telescopes and cameras, it was only a matter of time before supernovae began to be detected in photographs of other galaxies. The first such extragalactic supernova to be noticed, in Andromeda in 1885, happened to be near the center of the spiral, and so could be explained away as the sputtering of a Laplacian protosun. But then, in 1917, George Ritchey, an optician at Mount Wilson, and Heber Curtis, an astronomer at Lick, announced that they had found several novae in old file photographs of spirals. Other astronomers started ransacking their plate files, and found scores more. The novae were not central, but occurred primarily in the spinal arms. This was extremely damaging to the notion that all nebulae were gaseous: Dozens of exploding stars in galaxies full of stars made sense; in Laplacian gas disks, they did not. As Curtis commented, “The novae in spirals furnish weighty evidence in favor of the well known ‘island universe’ theory.”7
The stage was set for the discovery of galaxies. What remained was the most expansive surveying project in the history of our planet—to chart the location of the solar system in the Milky Way, and to determine the distances of the other galaxies, if such they were, beyond.
The champion of this cause was the founder of observational astrophysics, George Ellery Hale. Hale’s early career reenacted the progression of spectroscopy from the sun to the stars. He became enchanted by the sun as a boy growing up in the Chicago suburbs, built a backyard observatory where he observed solar spectra, and by the age of twenty-four had invented the spectrohelioscope, a device that made it possible to examine the solar atmosphere in one wavelength of light at a time. Captivated by the realization that, as he kept repeating all his life, “the sun is a star,” he then turned his attention to the depths of space. He was responsible for building four telescopes, each in its day the world’s largest—the 40-inch refractor at Yerkes Observatory in Wisconsin and, in southern California, the 60- and 100-inch reflectors at Mount Wilson and the 200-inch reflector at Palomar. Mount Wilson in particular stood as a monument to Hale’s dual passions in spectroscopy: There, solar telescopes recorded the spectra of the sun by day, while by night giant reflecting telescopes were employed to probe the multitude of other suns scattered through the Milky Way and beyond.
Hardworking even by the hard-boiled standards of the opticians and astronomers of the day, Hale rode mules up the rocky, twisting road from Pasadena to Mount Wilson’s peak, and when no mules were available simply ran up the side of the mountain. He did a lifetime’s worth of research of his own and managed simultaneously to act as the observatory director, raising funds for ever larger telescopes and recruiting some of the world’s leading astronomers to Mount Wilson. One of the cleverest of his recruits was Harlow Shapley.
Shapley had studied at Princeton Observatory under Henry Norris Russell, where he specialized in Eclipsing Binaries. These are double stars, so close together in the sky that they look like single stars even through the most powerful telescopes, that happen to be oriented in space in such a way that they periodically eclipse each other. The resulting variations in the total brightness of the system bear a superficial resemblance to genuine variable stars, which change their brightness owing to internal pulsations. In this fashion Shapley came to study variable stars as well. The knowledge he gained in this somewhat backhanded way came in handy, for a class of variable stars—the Cepheid variables—were to provide astronomy with a means of measuring distances across interstellar space and even intergalactic. Thanks to the Cepheids, Shapley was to earn a place in history as the first human being to establish the location of the sun in the Milky Way galaxy.
Cepheids—as Shapley was the first to propose—pulsate, varying in brightness as they change in size. Astrophysically speaking, they are giant stars, three or more times the mass of the sun, undergoing a period of instability that occurs when they run low on hydrogen fuel and begin burning helium. The wonderful thing about them is that the period of each Cepheid—i.e., the time it takes to go through a cycle of variation in brightness—is directly related to its intrinsic brightness (i.e., its absolute magnitude). Once the absolute magnitude of any star is known, it is a simple matter to compute its distance: All the astronomer has to do is measure its apparent magnitude and then apply the formula that brightness decreases by the square of the distance. If, for instance, we have two Cepheid variables with the same period, we may assume that they have about the same absolute magnitude. If the apparent magnitude of one is four times that of the other, we conclude (barring complications such as the interference of an intervening interstellar cloud) that the dimmer star is twice as far away.
The relationship between the periodicity and the absolute magnitude of Cepheid variable stars was discovered in 1912 by Henrietta Swan Leavitt, one of a number of women hired at meager wages to work as “computers” in the Harvard College Observatory office in Cambridge, Massachusetts. Leavitt spent her days examining photographic plates taken through the twenty-four-inch refracting telescope at the Harvard station in Arequipa, Peru. One of her tasks was to identify variable stars. This involved comparing thousands of pinpoint star images on plates taken on different dates, looking for changes in brightness. It was painstaking toil, considered too menial to claim the time of a full-fledged astronomer. Leavitt spent thousands of hours at it, and in doing so acquired an unusual degree of familiarity with the southern sky.
She happened to be assigned to a region that includes the Magellanic Clouds. So named because they attracted the attention of Magellan and his crew on their voyage around the world, the Magellanic Clouds are two large, shaggy patches of softly glowing light that resemble detached swatches of the Milky Way. We know today, as Leavitt and her contemporaries did not, that the Clouds are nearby galaxies, and that the stars in each Cloud therefore all lie at about the same distance from us, like fireflies in a bottle viewed from across a field at night. This means that any significant difference in the apparent magnitudes of stars in a Magellanic Cloud must result from genuine differences in their absolute magnitudes and not from the effect of differing distances. Thanks to this happy circumstance, Leavitt in studying Cepheid variable stars in the Magellanic Clouds was able to notice a correlation between their brightness and their period of variability—the brighter the Cepheid, the longer its cycle of variation. The period-luminosity function Leavitt discovered was to become the cornerstone of measuring distance in the Milky Way and beyond.
Shapley, out to chart the Milky Way galaxy, seized on the Cepheids with great enthusiasm. Using the big sixty-inch Mount Wilson telescope, he photographed globular star clusters—spectacular assemblages of hundreds of thousands to millions of stars —identified Cepheid variable stars in each of them, then employed the Cepheids to calibrate the distances of the clusters. “The results are continual pleasure,” he wrote the astronomer Jacobus Kapteyn in 1917. “Give me time enough and I shall get something out of the problem yet.”8 The payoff came sooner than Shapley had hoped, and within a matter of months he could write, to the astrophysicist Arthur Stanley Eddington: “Now, with startling suddenness and definiteness, they [the globular clusters] seem to have elucidated the whole sidereal structure.”9
Shapley had found that the globular clusters are distributed across a spherical expanse of space, as if they were part of an enormous metaglobular cluster, and that the center of this sphere is nowhere near the sun, but lies far away to the south, past the stars of Sagittarius. In a superbly daring intuitive leap, Shapley then conjectured—accurately, as it turned out—that the center of the realm of the globular clusters was also the center of the galaxy itself. As Shapley put it, “The globular clusters are a sort of framework—a vague skeleton of the whole Galaxy—the … best indicators of its extent and orientation.” If so, the sun lies far from the center of things: “The solar system can no longer maintain a central position,” Shapley asserted.10
Shapley’s triumph was marred only by problems with his calculation of distances. The diameter of the Milky Way galaxy previously had been reckoned—by various investigators, Shapley among them—at some fifteen to twenty thousand light-years. Now, with his Cepheid variable work in hand, Shapley concluded that the correct figure was three hundred thousand light-years—more than ten times larger than the dimensions entertained by his contemporaries, and three times the most generous estimates we have today.*
Various errors contributed to Shapley’s inflated picture of the Milky Way galaxy. Like many of his contemporaries, he underestimated the extent to which clouds of interstellar gas and dust dim the images of distant stars, making them appear farther away than they really are. Moreover, he assumed that the Cepheid variable stars he observed in globular clusters were essentially identical to those Henrietta Leavitt had found in the Magellanic Clouds; actually, as Walter Baade and other astrophysicists were to find, the cluster variables are less massive and intrinsically less bright, and therefore by implication less distant, than a straightforward comparison of their periods with those of their younger cousins would imply. Inaccuracies of this sort are routine on the cutting edge of science, but they had the dolorous effect of misleading Shapley into thinking that the Milky Way, rather than being but one galaxy among many, was a system of unique grandeur. He began to think of the Milky Way as more or less the entire universe, and to regard the spiral nebulae but its subjects or its satellites.
For these and perhaps for subtler psychological reasons as well, Shapley came to take a proprietary interest in defending what he called “the enormous, all-comprehending” dimensions of the galaxy that he had charted. He called this view his “big galaxy” hypothesis. Those who agreed with him tended to think of the word “big” in terms of its Norse etymology, from bugge, meaning “important.” Those who disagreed preferred to emphasize the word’s Latin etymology, from buccae, for “puffed up.”
Among the dissidents was Heber Curtis of Lick Observatory, an advocate of the “island universe” theory. Shapley reacted to Curtis’s arguments with the abhorrence of a patient contemplating surgery: Curtis, he noted, “must shrink my galactic system enormously to have much luck with island universes.”11 The issue was formally debated by Shapley and Curtis under the auspices of the National Academy of Sciences, in Washington, D.C., on April 26, 1920. Shapley was generally judged the loser, but, as is usually the case in science, the debate settled little and the last word belonged not to men but to the sky.
The hypothesis defended by Curtis, that the spiral nebulae were galaxies of stars, would be confirmed if a spiral could be unambiguously resolved into stars. That vital step was accomplished in 1924 by Shapley’s colleague and nemesis, Edwin Hubble. A tall, elegant, and overbearing man with a highly evolved opinion of his potential place in history, Hubble made everything he did look effortless—he had been a track star, a boxer, a Rhodes scholar, and an attorney before turning astronomer—and one of the things he did most effortlessly was to infuriate Shapley. Hubble took scores of photographs of M33 and its neighbor M31, the Andromeda spiral, and found there what he later called “dense swarms of images in no way differing from those of ordinary stars.”12
Whether the pinpoints of light on Hubble’s photographic plate really were stars, however, was open to contention; Shapley dismissed them as curds in a Laplacian nebula. Here, again, Henrietta Leavitt’s Cepheid variable stars provided the needed mileposts. Cepheids are bright enough to be discernible across intergalactic distances. Using the new one-hundred-inch telescope at Mount Wilson, Hubble photographed the spirals again and again, comparing the plates to find stars that had varied in brightness. His efforts soon bore fruit, and on February 19, 1924, he wrote Shapley, who by then had left Mount Wilson to become director of Harvard College Observatory, a laconic note containing one of the most momentous findings in the history of science: “You will be interested to hear that I have found a Cepheid variable in the Andromeda Nebula.”13
Hubble deduced that Andromeda lies about one million light-years away, an estimate half the distance of later ones but clearly sufficient to establish that the spiral was well beyond even Shapley’s “big galaxy.” Shapley replied sourly that he found Hubble’s letter to be “the most entertaining piece of literature I have seen for a long time.”14 Later he complained that Hubble had given him insufficient credit for his priority in using Cepheid variables to chart distances. But the game was over. Hubble’s paper announcing that he had found Cepheids in spirals—read (in his Olympian absence) at a joint meeting of the American Astronomical Society and the American Association for the Advancement of Science in Washington, D.C., on New Year’s Day, 1925—initiated the final decline of the nebular hypothesis, the ascendancy of the island universe hypothesis, and humankind’s realization that we live in one among many galaxies.
Hubble went on to identify not only Cepheids but novae and giant stars in Andromeda and other galaxies. These studies helped allay his fear that the laws of physics might break down beyond our home galaxy, rendering his distance measurements invalid. Newton, too, had wondered whether “God is able … to vary the Laws of Nature, and make Worlds of several sorts in several Parts of the Universe.”15 Hubble, in his short paper announcing the finding of Cepheids in M31, took time to caution that his results depended upon the assumption that “the nature of Cepheid variation is uniform throughout the observable portion of the universe.” When he found Cepheids and other familiar stars in the galaxy NGC 6822, he wrote with evident relief that “the principle of the uniformity of nature thus seems to rule undisturbed in this remote region of space.”16
Some astronomers have a gift for making lovely, sharp photographs of galaxies with large telescopes. Hubble was not one of them, though he was adept at extracting essential data from the generally flawed plates he did obtain. Nor was he especially skillful at taking spectra, but in this he was soon aided by one Milton Humason, a resourceful young man with an inquiring mind who started out on Mount Wilson as a muleteer and observatory janitor, began assisting the astronomers in their work at the telescope, and eventually became an expert observational astronomer in his own right. Throughout the 1930s and 1940s, Hubble and Humason pushed back the frontiers of the observable universe, charting and cataloging ever more distant galaxies. Eventually, Hubble was taking photographs that were strewn with the images of more remote galaxies than foreground stars.
In 1952, the year before Hubble died, Walter Baade announced at a meeting of the International Astronomical Union in Rome that he had discovered an error in the calibration of the Cepheid period-luminosity value, the correction of which doubled the cosmic distance scale. Further refinements in the distance scale were attained by Hubble’s former assistant Allan Sandage, later in collaboration with the Swiss astronomer Gustav Tammann, and it became possible for astronomers to measure, with some confidence, the distance to galaxies hundreds of millions to billions of light-years away.
At these distances, time commands a significance equal to that of space. Inasmuch as it takes time for the light from a distant galaxy to pass through space, we see the galaxy as it was long ago: The galaxies of the Coma cluster, for instance, appear to us as they looked seven hundred million years ago, when the first jellyfish were just appearing on Earth. Owing to this phenomenon, called lookback time, telescopes probe not only out into space but back into the past. It should, therefore, be possible to determine, by looking far into deep space, whether the universe was once different than it is today. Evidence that this is indeed the case came in the 1960s, when Sandage and radio astronomer Thomas Matthews discovered quasars, and Maarten Schmidt determined that they were extraordinarily far away. Quasars appear to be the nuclei of young galaxies, at distances of a billion light-years and more. There is nothing quite like them in the universe today. And so the exploration of space opened the pages of cosmic history.
The work of charting our place in the universe goes on, and today we can say with some confidence that the sun is a typical yellow star that lies in the disk of a major spiral galaxy, about two thirds of the way out from the galactic center. The disk contains not only stars and their planets but also vast, rarefied lakes of hydrogen and helium gas, denser knots of gas where atoms have been able to find one another and bind together as molecules, and giant thunderheads of soot given off by smoky stars. Waves generated by harmonics in the gravitational interaction of the myriad stars move across the disk in a graceful, spiral pattern, plowing the interstellar material into globules dense enough to collapse under the attraction of their own gravitational force. In this way new stars are formed, and it is the light of the most massive and shortest-lasting of the young stars that illuminates the spiral arms, making them visible. The spiral arms, then, are not objects but processes—as transitory, by the bounteous spatiotemporal standards of the Milky Way, as the back-blowing veils of froth that whitecap the waves of earthly oceans.
Beyond the Milky Way lie more galaxies. Some, like the Large Magellanic Cloud and the Andromeda galaxy, are spirals. Others are ellipticals, their stars hung in pristine, cloudless space. Others are dim dwarfs, some not much larger than globular clusters. Most belong, in turn, to clusters of galaxies. The Milky Way is one of a few dozen galaxies comprizing a gravitationally bound association that astronomers call the Local Group. That group in turn lies near one extremity of a lanky archipelago of galaxies called the Virgo Supercluster. If we could fly the sixty million or so light-years from here to the center of the supercluster, we would encounter many sights worth seeing along our way—the giant cannibal galaxy Centauras A, an elliptical busily gobbling up a spiral that blundered into it; the distended spiral M51, with its one outflung arm stretching after a departing companion galaxy; the furiously glowing spiral M106, with its bright yellow nucleus and its shoals of blue-white stars; and, at the supercluster core, the giant elliptical Virgo A, wreathed in thousands of globular star clusters, harboring some three trillion stars, and adorned by a blue-white plasma jet that has been spat from its core with the velocity of a bolt of lightning.
Beyond Virgo lie the Perseus, Coma, and Hercules clusters, and beyond them so many more clusters and superclusters of galaxies that it takes volumes just to catalog them. There is structure even on these enormous scales; the superclusters appear to be arrayed into gigantic cosmic domains that resemble the cells of a sponge. Beyond that, light from faraway galaxies, riding the contours of curved space, becomes as dappled as the moon’s reflection on a pond in a gentle breeze. Out there, awaiting some future Hubble or Herschel, lie many a tale of things past, or passing, or to come.
*Equally unpunctual in his social commitments, Ramsden once arrived for a party at Buckingham Palace at the hour and day inscribed on an invitation sent him by the king, but one year late.
*By the 1980s, theoretical astrophysicists using computer models had derived a general theory of the origin of the solar system that, though more sophisticated than those of Kant, Laplace, or Jeans, resembles them at least superficially. The new model envisions the sun congealing from a nebular cloud, the remnants of which formed a flattened disk that condensed, as it cooled, into a multitude of little chunks of material, or “planetesimals,” which in turn collided to form the planets. Indirect confirmation of the theory came when an orbiting infrared telescope detected cold, planetesimallike disks around Vega and several other bright, young stars. The details of the theory, however, are quantitatively difficult, and still have not been worked out. It is one of the humbling truths of contemporary science that, while we theorize about the origin of the entire universe, we do not yet fully understand how our own little planetary system began.
*Modern estimates put the diameter of the Milky Way disk at seventy to one hundred thousand light-years. There probably are dim stars much farther out, however, as well as stray halo stars and “tramp” globular clusters orbiting the galaxy at distances of over three hundred thousand light-years from the galactic center.