“Humanity,” writes Alan Hirshfeld towards the end of Starlight Detectives, “inhabits a spinning globe, coursing round a central star” which is itself “moving through space among a multitude of stars, that themselves bob and weave as they circle a disk-shaped galaxy” (p. 320). This galaxy itself is only one among many, each receding from the others at breakneck speed.
This, needless to say, is not news. Any high school student today knows at least the general outlines of this picture, which is so familiar as to seem self-evident. But as Hirshfeld’s shows in this fascinating book, the picture is, in fact, remarkably recent. Up until the 1920’s astronomers didn’t even know what galaxies were, and it was not until the 1930’s that the notion of an expanding universe became widely accepted. Starlight Detectives is the story of how our image of the universe came to be.
Ever since the Scientific Revolution of the 16th and 17th centuries scientists had known that the planets revolve around the Sun in elliptical orbits governed by gravity and Newton’s laws of motion. But as late as the middle of the 19th century the stars in the sky remained what they had always been: mysterious pinpricks of light that revolve around the Earth once every 24 hours. All that remained for astronomers to do was to measure their position in the sky with ever increasing precision.
This was not as easy as it sounds. In order to increase precision 19th century astronomers developed what can only be considered a fanatical obsession with detail. They quantified every flaw and aberration in their telescopes and assigned a “personal equation” to each observer in order to compensate for his (or – more rarely – her) habitual error. Consequently they were able to draw astonishingly accurate star maps that were essential for navigation and cartography. But as one of the greatest of 19th century observers, Friedrich Bessel, aptly put it, that is where the astronomer’s role ends: “Everything else one might learn about the stars – the appearance and constitution of their surfaces for example – is of no real concern of Astronomy” (p. 24).
Fortunately for the future of astronomy, however, Bessel and his colleagues in the great European observatories were not the only ones pointing their telescopes at the skies in the 19th century. Amateurs, such as the Americans Lewis Morris Rutherford and Henry Draper and the Englishmen Warren de la Rue and William Huggins, were also doing so, in ever growing numbers and increasing levels of skill. What they lacked in formal training they made up in boundless enthusiasm, a craftsman’s understanding of instruments and their workings, and more than anything – an open mind. That the science of astronomy ever moved beyond Bessel’s narrow strictures, Hirshfeld shows, is due almost entirely to them.
Nature’s Own Imprint
The first innovation foisted on the professionals by these amateurs was the application of photography to astronomy. Since its beginnings in ancient Mesopotamia astronomy had relied on the human eye as its primary instrument of observation. Even the invention of the telescope did not change this, since astronomers still relied on their sharp and meticulously trained eyesight to capture a clear view of a celestial object. They then relied on their artistic skills to record what they had seen. Backed by thousands of years of experience, professional astronomers were confident that nothing could compare with the human eye for flexibility and accuracy. But it did not take long after the invention of photography before amateurs, not bound by tradition, started to attach cameras to their long telescopes.
The promise of photography was clear: instead of a single observer having to rely on fleeting sighting made in the harsh condition of a dark and cold observatory in the middle of the night, a photograph is available for all to see, and can be studied in the comfort of a home or a laboratory. And instead of an astronomer sketching the observed object from memory, the photograph is nature’s own imprint, unadorned and unfiltered by human memory or preconceptions. Finally, since photographs could be exposed for long periods of time, they held the promise of detecting faint objects invisible to the eye.
As it happened, astronomical photography turned out to be much harder than the early pioneers had hoped. Whereas human observers could wait for hours to capture a sharp image that might last only an instant, a photographic plate would record the image continuously during its entire exposure, leading to a fuzzy photographic picture. In order to acquire a sharp image it was also necessary to precisely and smoothly follow the motions of the object in the sky during the photographic exposure, a requirement that was often beyond the capabilities of the clunky mechanisms that controlled the movement of 19th century telescopes. And because they were sensitive to different wavelengths than the eye, photographic plates had to be placed at a different distance from the lens, which made focusing the image a constant challenge. Finally, and most critically, early photographic technology simply was not sensitive enough to record the faint light emanating from stars light-years away.
Faced with these challenges, it took a nearly half century after the introduction of the first daguerreotypes before photography could challenge traditional observations with the human eye. But when in the 1889 British amateur astronomer Isaac Roberts presented to the Royal Astronomical Society his photographs of the Orion, Andromeda, and Dumbbell nebulas, it was clear that the day had come. No astronomer – however proficient – working with eye and hand, could ever hope to match the detail and accuracy of Roberts’ photographs. As even the most conservative professionals were bound to concede, astronomical photography had finally arrived.
Signatures of Light
Powerful as astronomical photography proved to be, by itself it merely enhanced what traditional astronomers were doing: recording the positions of ever more stars with ever greater accuracy, and recording the images of objects such as star clusters and nebulas with increasing detail. It was only in combination with another novel technology that the revolutionary potential of photography was fully unleashed: spectroscopy.
The modern science of spectroscopy is the invention of two German scientists, Robert Bunsen and Gustav Kirchhoff of the ancient university of Heidelberg. Different elements, they discovered, when heated, emit unique light spectrums, which can be used to identify the presence of that specific element. The also noted that the same elements would absorb light at precisely the same wavelengths when lit by an external source. Thanks to the work of Joseph Fraunhofer earlier in the century, Bunsen and Kirchhoff knew that that the spectrum of the Sun was not continuous but riddled with dark lines at specific wavelengths. They concluded that elements in the Sun’s outer layer are absorbing light emanating from the Sun’s core, and that comparing the dark lines with the spectrum of known elements would reveal their presence in the Sun.
Solar spectroscopy was a breakthrough in the study of the Sun, revealing the presence of many known elements and at least one – Helium – that had yet to be detected on Earth. It was only natural for astronomers to then turn their sites to deep space, trying to detect the spectrums of stars and nebulas. The first to succeed was Henry Huggins, who in the 1860’s managed to record the spectrums of four bright stars using his eyesight alone. It was another decade before Huggins and his American rival Henry Draper managed to record the faint light spectrums of distant stars on photographic plates, opening the door to the systematic study of stars and their composition. By the late 1880’s Harvard’s Edward Pickering was recording the spectral signatures of thousands of stars and many nebulas.
The powerful combination of photography and spectroscopy finally overturned the reigning paradigm of observational astronomy. No longer mere pinpricks, the stars were revealed to be distant suns, some similar in composition to our own, some very different. Nebulas, formerly just indistinct hazes in the night sky, also acquired substance: some, as their spectrums indicated, were vast collections of stars, too distant to be distinguished from one another; others were enormous clouds of gas, floating through interstellar space. Deep space, which from time immemorial was just impenetrable blackness interspersed with points of light, now became a physical place to be studied, just like the Earth. The science of astrophysics was born.
Yet dramatic as those developments were, they told astronomers little about the structure of the universe as a whole. That the stars were distant suns was now established, but were they equally distributed throughout the universe? Were they all part of a single massive cluster, the Milky Way, with nothing but emptiness beyond? Or was the Milky Way just one of many “island universes,” floating through space? Photography and spectroscopy, powerful as they were, provided no answers.
Mirroring the Heavens
So things stood at the turn of the 20th century, when American astronomers launched a building spree of giant new telescopes, many times larger and more powerful than their predecessors. Unlike the refractors, favored by astronomers since the invention of the telescope, the new telescopes were mostly reflectors, gathering light not by a massive lens at the end of a long tube but by a massive mirror at the telescope’s base. Whereas a giant lens held in place only around its circumference tended to warp under its own weight, a mirror resting close to the ground could be effectively supported to maintain its precise parabolic shape.
The moving spirit behind the American telescope boom was Chicago native George Ellery Hale, who in the space of two decades managed to construct the largest telescope in the world three times in succession. His greatest triumph was the 100-inch reflector at the Mount Wilson Observatory overlooking Pasadena in California, which saw first light in 1917, and a fourth giant telescope that he designed and planned, the 200-inch reflector at the Palomar Observatory, opened in 1948, a decade after his death. For an idea of the scale of these instruments, consider that only a few decades before William Huggins made most of his discoveries with an 8-inch refractor.
With such massive telescopes setting the bar, amateurs could no longer keep up. They simply did not have the resources to compete with the professionals in their institutional observatories, and gradually retreated from the forefront of astronomical research. Fortunately, however, the professionals had by this time absorbed a great deal from the amateurs: finally free from the traditions of positional astronomy, they made spectacular use of the new instruments at their disposal.
Candles of the Sky
The key to deciphering the structure of the universe turned out to be variable stars known as Cepheids, which grow brighter and then dimmer in precisely regular cycles. Because the period of these stars is dependent on their inherent brightness, astronomers can deduce just how bright they “truly” are, and use their observed luminosity to calculate their distance. Consequently Cepheids serve as milestones in the sky: if a star cluster or nebula contains a Cepheid, then its distance can be known.
Astronomer Harlow Shapely of the Mount Wilson Observatory (later of Harvard) was among the first to make systematic use of Cepheids to construct a model of the universe. He correctly inferred that the solar system was at the edge of the Milky Way, and that a halo of star-clusters surrounds the bulgy center of the galaxy. But he also vastly expanded the estimated size of the Milky Way to 300 thousand light-years, and claimed that everything in the observable universe lies within its boundaries. Beyond that, he argued in a 1919 paper, all was emptiness.
It was left to Edwin Hubble, Shapely’s junior colleague at Mount Wilson, to challenge and ultimately demolish this model. On the night of October 3, 1923, Hubble pointed the 100-inch telescope at the Andromeda nebula and repeatedly exposed his photographic plates 60 minutes at a time. To his surprise he detected 3 points of light he had not seen before, and which he suspected were supernovas – brilliant exploding stars that can be seen at enormous distances for a short period of time. Subsequent observations revealed otherwise. Two of the stars were indeed supernovas, but the third was a pulsating Cepheid. Using the well tried-formula for gauging the distance of Cepheids, Hubble came to a startling conclusion: The Andromeda nebula was approximately 1 million light-years away, well outside even Shapely’s inflated Milky Way.
Hubble did not stop there. Over the following years he added new observations of Cepheids in other nebulas, and combined them with careful inspections of older photographic plates. Nearly all the nebulas turned out to be extraordinarily distant, vast island-universes floating freely in space. The Milky Way, it turned out, was but one of many, many “galaxies.”
Hubble was not yet done. In the late 1920’s astronomer Vesto Slipher of the Lowell Observatory in Arizona recorded minute shifts in the spectrum of distant galaxies, which in accordance with the Doppler effect, signify a galaxy’s motion towards or away from the Earth. Much to his surprise, Slipher found that the spectrum of nearly all the galaxies was shifted towards the red, meaning that they were moving ever further away from us, each at a different rate of speed. Hubble now took Slipher’s measurements, added a few of his own, and combined them with his measurements of the distance of the different galaxies. The results were striking: The further a galaxy was from us, the faster it was receding. The universe, it seemed, was expanding at an accelerated pace.
A New Universe
Starlight Detectives is the story of an astronomical upheaval as profound as the Copernican revolution that displaced the Earth from the center of the cosmos and set it orbiting the Sun. As late as the middle of the 19th century the Earth was still part of a single solar system nestled among unreachable and indecipherable points of light in the night sky. By the 1930’s the Earth was orbiting one undistinguished star out of billions, on the edge of vast rotating galaxy, which itself is one of billions of such islands each receding from the others in an ever-expanding space.
It is a story of the technologies that made this revolution possible – photography, spectroscopy, and telescopy. It is the story of astronomy, its entrenched traditions, and the indispensable role of outsiders in pushing its boundaries. It is the story of the scientific rise of the United States which, free of the constraints of tradition and possessed of vast resources, finally surpassed its more established European rivals. But more than anything it is the story of the men and women, who with fanatical dedication braved cold and wind to observe the heavens night after night after night. From William Bond, the “ingenious mechanic,” to Edwin Hubble, the suave professional, and from the burly laborer William Common to the high-strung George Hale, Hirshfeld bring them all to life.