Starlight Detectives

by Alan Hirshfeld

  Mount Wilson’s one-hundred-inch reflector was the culminating element in a triad of astrophysical innovation that evolved over the latter half of the nineteenth century. With its completion, the finely honed tools of celestial photography and celestial spectroscopy could be applied to a host of heavenly objects, primary among them the spiral nebulae. Nearly eight decades had passed since Lord Rosse’s Leviathan telescope revealed this pinwheel species of space cloud, and two decades since James Keeler’s deep-sky pictures exposed their incredible ubiquity. The convergence of photographic, spectroscopic, and telescopic technologies during the early twentieth century promised to resolve the heated debate about the nature of spiral nebulae: Are they aggregations of gas and infant stars within our Milky Way, or full-fledged galactic islands that populate the entire universe?

  With the one-hundred-inch reflector nearing completion, Hale looked to expand the Mount Wilson staff. Among the candidates was a recent doctoral recipient from Yerkes whose photographic study of variations in a faint nebula had been published in the Astrophysical Journal. In the spring of 1917, Hale offered the young man a position, with full access to the big telescope. The reply arrived shortly afterward in the form of a telegram: “Regret cannot accept your invitation. Am off to the war.” Signed, Edwin Hubble.

  Chapter 26

  SIZE MATTERS

  Once one has exhausted all possibilities for error, one is finally forced to abandon a prejudice, and redefine what one means by ‘correct.’ So painful is this experience that one does not forget it. The subsequent replacement of an old prejudice by a new one is what constitutes a gain in real knowledge. And that is what we, as scientists, continually pursue.

  —Douglas Gough, “Impact of Observations on Prejudice and Input Physics,” 1993

  ONTO THE STAGE SET BY GEORGE ELLERY HALE strode another player attuned to historic possibilities: Edwin Powell Hubble. There was a dashing, almost cinematic, quality to Hubble when he arrived at Mount Wilson in September 1919. Six-foot-two and leading-man handsome, Hubble wore his mantle of authority comfortably, whether engaging with colleagues, hosting dignitaries in the observatory, or posing for the cover of a magazine. Colleague Milton Humason recalled his first vision of Hubble under the dome of the sixty-inch telescope: “His tall, vigorous figure, pipe in mouth, was clearly outlined against the sky. A brisk wind whipped his military trench coat around his body and occasionally blew sparks from his pipe into the darkness of the dome. . . . He was sure of himself—of what he wanted to do, and of how to do it.”

  If Edwin Hubble had thus far led a charmed life, it had been at his own insistence. His ascension among his peers had been a calculated campaign, starting with his Rhodes Scholar education at Oxford, through his graduate astronomical training at Yerkes and meteoric rise to major in the army during World War I, and presently to his appointment at Mount Wilson. In conversation, Hubble was apt to inject accounts of his exploits which, over time, accreted into a breathless exaltation that obscured the divide between man and myth. An amateur pugilist during his college days, he once dispatched a knife-wielding thug by knocking him senseless—after being stabbed in the back. He rescued a professor’s wife from drowning by hoisting her onto his shoulders and striding along the sandy bottom until he emerged Neptune-like from the waves. When the country slid into depression, the charismatic Hubble diverted attention toward America’s noble quest to comprehend the universe. Edwin Hubble became astronomy’s John Wayne, a living Rushmore of American mettle, who seemed to overstride obstacles through sheer resolve.

  Edwin Powell Hubble.

  At Oxford, and later on Mount Wilson, Hubble adopted the dress and manner of a highborn Englishman, complete with Norfolk jacket, plus-fours, high-top boots, and faux accent. It was a presentation designed to impress, if not intimidate. Colleagues tolerated his pretensions, while the press found this martial icon of science irresistible. Despite an abiding interest in astronomy—he worked at Yerkes Observatory while an undergraduate at the University of Chicago—Hubble acceded to his father’s wishes and studied jurisprudence at Oxford. From the start, it was an ill fit, the calcified subject matter stifling to one who sought wider horizons. While Hubble kept his father apprised of his studies and his many athletic prizes, he poured out his disillusionment to his mother: “I sometimes feel that there is within me, to do what the average man would not do, if only I find some principle, for whose sake I could leave everything else and devote my life.”

  After Oxford, Hubble idled for a year as a high school physics teacher before his outsize ambitions erupted into a full-bore drive into astronomy at the University of Chicago. In 1914, when Hubble began his graduate training, the Yerkes Observatory was a ghost of its former self. George Ellery Hale was a decade gone, living out his professional dream at Mount Wilson. Gone with him, the cream of the Yerkes staff: Walter Adams, George Ritchey, Ferdinand Ellerman, Francis Pease. Hale’s successor, the avuncular Edwin Frost, conducted stellar spectroscopy with the forty-inch refractor, but was going blind. Edward Barnard, the noted celestial photographer, lacked the academic training to direct a doctoral candidate, especially one with Hubble’s aspirations. Rounding out the Yerkes staff were several junior astronomers of no particular distinction.

  Rather than vie with his superiors for observing time on the great refractor, Hubble opted for George Ritchey’s sidelined twenty-four-inch reflector. Although diminutive, the instrument’s fast optics proved key to Hubble’s photographic foray into the realm of the nebulae. Among his first targets was a striking, fan-shaped nebulosity in Monoceros numbered 2261 in J. L. E. Dreyer’s 1888 New General Catalogue of Nebulae and Clusters of Stars. NGC 2261 beckons the viewer with its triangular form, intense star-like concentration at the southern apex, and streamers trending off to the north. Although motionless to the eye, there is an impression of activity, as though one had frozen the flutter of a candle flame.

  Hubble compared his images of NGC 2261 from 1915 and 1916 to those taken during the previous decade, including one by photographic pioneer Isaac Roberts in 1900. He was amazed to find that the nebula had indeed changed, both in its outline and in the form and placement of its internal features. Variable nebulae had been observed before, but never had such dramatic alterations been captured over so brief a time span. In an era when cosmic distance measurement was problematic at best, Hubble asserted that NGC 2261 must lie relatively close to the solar system for its transformation to be so manifest. Supporting his contention was the demonstrable movement of faint stars associated with the nebula, shifts otherwise imperceptible if the cloud were situated far away. A pair of well-turned articles about Hubble’s Variable Nebula, as it would come to be known, brought Yerkes’s rising star into professional view.

  In October 1916, with the one-hundred-inch mirror soon to be mounted, George Hale met with Edwin Hubble in Chicago and offered him a staff position at Mount Wilson, pending completion of his doctoral degree. Hubble accepted, no doubt keen to wield a telescope four times the aperture of his current one. Yet it would be three years before he reached California. On April 6, 1917, the United States declared war on Germany, and Hubble was itching to get into the fight.

  Hubble rushed an update on NGC 2261 to the Astrophysical Journal, featuring his latest exposure of the nebula. Enlarged twenty-four times from its half-millimeter extent on the plate, the bright, broad triangle of a year earlier had shriveled into a cometary wisp. Hubble could not say whether the changes in appearance stemmed from movement of gas and dust within the nebula or from variable lighting of the material by its starry nucleus. (NGC 2261 is indeed a cosmic shadow play: dust clouds circling the illuminating star sweep zones of darkness throughout the nebula.)

  Hubble typed up his dissertation—a minimalist seventeen pages—and submitted it to Edwin Frost at the beginning of May. He ignored Frost’s plea to fatten the skimpy production by incorporating his paper on NGC 2261. “[I]t does not add appreciably to the sum of human knowledge,” he avowed, then added prophe
tically, “Someday I hope to study the nature of these nebelflecken to some purpose.”

  Hubble’s dissertation outlines the scientific conundrums presented by the thousands of faint nebulae that crowd photographic plates, especially the spirals, and the need for a meaningful classification scheme for the various nebular forms. Significantly, he notes that the distribution of faint nebulae (excluding those which clearly belong to our own Milky Way galaxy) is nonuniform; instead, they show a tendency to cluster, in seeming analogy to gravitationally bound star clusters within the Milky Way. More than a summary of results, Hubble’s paper is a road map for the future of extragalactic studies. In an oblique attempt, perhaps, to justify the glaring insufficiency of his own work, Hubble writes, “These questions await their answers for instruments more powerful than those we now possess.” Hubble passed his hastily scheduled oral examination with high honors, then telegraphed George Hale, who assured him that his staff position at Mount Wilson would be held until after the war.

  Just three months into his military training, twenty-eight-year-old Private Hubble became Captain Hubble, and shortly thereafter, Major Hubble, in charge of six hundred officers and infantrymen. His unit did not arrive in France until September 1918 and, in the six weeks preceding the armistice, saw no direct combat. “I barely got under fire,” Hubble complained to Frost, “and altogether I am disappointed in the matter of war.” A frustrated Edwin Hubble, still in officer’s uniform, made an extended stopover in Cambridge, where he bathed in the attentions of England’s astronomical elite. A stern reminder from George Hale about his prior commitment dislodged Hubble from his adoring throng. In August 1919, he boarded a ship for America, seeking new realms to conquer.

  When Edwin Hubble arrived at Mount Wilson, one of the most incendiary problems in astronomy remained the nature of spiral nebulae: Are they assemblages of stars and gas situated within the borders of the Milky Way, or are they galaxies in their own right, external to our own? In other words, is the universe one in which the Milky Way is the central, dominating entity, or one in which it is a mere member of the horde? The arguments on both sides were compelling, often fundamentally incompatible. And while intuition drew an increasing number of astronomers toward the multigalaxy, “island-universe” scenario, resolution of the matter awaited some breakthrough piece of observational evidence.

  The stumbling block in studying the faint spirals that appeared on plates in such number is how to fix their distances. To ascertain, from the vantage point of your window, whether a tree is situated within your backyard or your neighbor’s, the tree’s location must be determined relative to the property line. Likewise, to ascertain whether a nebula is situated within our galactic system or in extragalactic space, the nebula’s location must be determined relative to the borders of the Milky Way. Thus, the nature of the spiral nebulae hinges on two parameters: the nebula’s distance and the extent of our Galaxy, in effect, the galactic property line. That the latter is a challenge to fix stems from the fact that Earth’s placement deprives astronomers of an external perspective on the Milky Way’s form and span. They are constrained to observe the Galaxy from within, yielding a depthless compression of celestial bodies onto the night sky.

  In the late eighteenth century, William Herschel attempted to locate the limits of our galactic system by an observational scheme he called “star-gaging.” Having assumed that the distribution of stars is, on average, homogeneous throughout the Milky Way, Herschel used his workhorse nineteen-inch reflector (the “twenty-feet telescope,” after its focal length) in a visual count of stars in twenty-four hundred directions in the sky. A geometry-based analysis of the count revealed the relative distances at which stars give over to empty space, that is, the location of the Galaxy’s edge in that particular direction. The resultant stellar map is roughly elliptical in cross-section, with a series of clefts along the galactic plane and Earth prominently situated near the center.

  Herschel came to realize that the presence of star clusters and the clear variation in star density along different sight-lines were at odds with his supposition of uniformity. Furthermore, each gain in telescope aperture brought to light a multitude of previously unseen stars. “By these observations,” he concluded, “it appears that the utmost stretch of the space-penetrating power of the 20 feet telescope could not fathom the Profundity of the milky way.” Even Herschel’s gaping forty-eight-inch reflector showed a background haze, which he took to be the gathered glow of yet more remote stars.

  In the early twentieth century, Dutch astronomer Jacobus C. Kapteyn, at the University of Groningen, carried out the photographic successor of Herschel’s star census using plates obtained from other observatories. Kapteyn found that the galaxy is disk-shaped, no more than thirty thousand light-years across and five thousand light-years thick, with Earth centrally situated. (A light-year is the distance light travels through space in a year, about six trillion miles.) Although Kapteyn’s Milky Way would ultimately prove to be a pipsqueak version of the real thing, a span of thirty thousand light-years seemed suitably immense for a galaxy at the time.

  Unknown to Herschel, Kapteyn, and other galactic surveyors, interstellar space is peppered with silicate and carbon dust that absorbs and scatters starlight in transit, often to the point of extinction. The dust renders invisible the more remote sectors of our galactic disk, as stellar photons are plucked from their paths before they reach our telescopes. Harvard astrophysicist Cecilia Payne-Gaposchkin likened the problem of surveying the galaxy to “drawing a map of New York City on the basis of observations made from the intersection of 125th Street and Park Avenue. Although it would be clear to an observer that the city is a big one, any statement as to its extent and layout would clearly be impossible. London would offer an even better analogy, for the neighborhood is not only congested but foggy.” Given the aggregate evidence of light-absorbing material in outer space, some astronomers feared that the census-based Kapteyn model understated the galaxy’s girth. What they needed was an alternative method to size up the galactic system—one that would circumvent the hypothetical obscuration of starlight by interstellar dust.

  Among the night sky’s grandest spectacles are the globular star clusters: magnificent, spherical swarms of tens of thousands to hundreds of thousands of stars, each held fast by the unseen hand of gravity. The nature of these systems was disputed well into the twentieth century. Cambridge astrophysicist James Jeans speculated that globulars are the condensed, stellar end-products of spiral nebulae. Fellow Cantabrigian Arthur Eddington mused, in the style of the times, about their spatial relationship to the Milky Way: “The question of whether they are to be regarded as coequal empires, or as dependent but nearly self-governing colonies, must await more precise evidence.”

  In the early 1800s, William Herschel’s son John noticed that globular clusters are distributed differently than the majority of isolated stars. While one hemisphere of the sky contains scores of clusters, the opposite contains almost none. Fully one-third of known globulars occupy a region in Sagittarius comprising only 2 percent of the celestial sphere. Many are also found far from the luminous band of the Milky Way, where the bulk of galactic stars lie. The upshot of this overtly skewed distribution is that, in the compact Kapteyn model, the system of globular clusters appears to hover in spatial exile at one end of the galaxy, an asymmetry offensive to reason.

  The globular star cluster Messier 15, in a two-hour exposure by Isaac Roberts from November 4, 1890.

  In 1914, five years before Edwin Hubble’s arrival at Mount Wilson, George Hale recruited an ambitious Princeton graduate, Harlow Shapley, to study variable stars in globular clusters with the sixty-inch reflector. Globular clusters are rich in Cepheids, named for the category prototype Delta Cephei, whose variability was discovered in 1784 by English astronomer John Goodricke. These highly luminous stars cycle in brightness over periods ranging from about a day to seventy days. The character of the light variation is easily identifiable: a rapid ri
se to peak brightness, followed by a gradual dimming. Citing recent evidence that Cepheids are stars of immense diameter, Shapley proved that the observed brightness changes cannot stem from binary-star eclipses, as had been postulated: the requisite orbits would be smaller than the stars themselves. Instead, the variation of a Cepheid’s light must arise from radial pulsations of a single star, acting as a classical heat engine. (The specifics of the energy-generating mechanism were not elucidated for decades.)

  Harlow Shapley.

  Cepheids are sufficiently luminous to be visible at great distances within the Milky Way, the Southern Hemisphere’s famed Magellanic Clouds, and globular clusters. In a study of variable stars in the Small Magellanic Cloud, Harvard astronomer Henrietta Leavitt found that the period of a Cepheid’s brightening-dimming cycle correlates with its average light output: Cepheids of similar brightness have similar periods; and more significantly, the longer the period, the more luminous the star. This latter dependence, called the period-luminosity law, is typically rendered in graphical form: the Cepheids’ periods arrayed along the x-axis and their corresponding luminosities along the y-axis. Once calibrated, the period-luminosity law provides astronomers with a stepwise method of computing a Cepheid’s distance—and, by extension, that of any host system in which the Cepheid happens to reside.

  First, the star’s period is determined through repeated visual or photographic observations. The period-luminosity law, in turn, reveals the star’s absolute light output. Because the intensity of a light source diminishes with the square of its distance from the viewer, a quantitative comparison of the Cepheid’s absolute brightness versus its perceived brightness yields its distance. Thus, Harlow Shapley had a means to gauge the remoteness of any globular star cluster that harbors even a single Cepheid.