Building on Newton's discovery, Bunsen and Kirchhoff quickly set up a prism, and within hours they discovered something extraordinary. Each of the elements they tested had its own spectral pattern—a light fingerprint. When they heated a small sample of sodium in the burner's flame and examined its light through the prism, they didn't see a full rainbow spectrum. Instead, where there would otherwise be a spectrum, it was mostly dark, except for two bright yellow lines. When they burned calcium, they saw at a glance that its spectral signature was characterized by an intense green line and a strong orange one. Strontium, on the other hand, has no green lines, but it has eight lines that are very prominent: six red ones, one orange, and one blue. To extend their experiment and see more detail, Bunsen and Kirchhoff built a small device—the world's first laboratory spectroscope—a viewing tool (the scope part) for studying a substance's spectral signature (the spectro part). This first spectroscope consisted of a box containing a prism and a small mirror. Light from a burning substance was funneled into the box via a small “telescope,” and the mirror onto which the spectrum fell was also observed with a small telescope.
Bunsen and Kirchhoff were stunned by what they saw. Each element's light fingerprint was as unique a marker of that element as is a person's fingerprint of each individual. Even this notion of clear identification of something, or someone, based on seemingly obscure physical characteristics was itself a surprise. (It's a wonderful confluence of events that the first use of a human fingerprint as a means of identification occurred just a year before.) In the scientific paper reporting their findings, the two savants effused that here was a foolproof way to identify an element no matter what compound it was part of, no matter what the temperature of the flame was, and no matter what type of chemical was used to heat it. Sodium's spectral fingerprint is the same, whether it is coming from sodium chloride or sodium fluoride, or whether it is heated using alcohol, hydrogen, or (this being Bunsen) something the authors called “detonating gas.” And it got better. Not only could an element be identified by its light fingerprint, but it required only a ridiculously small sample. A quantity of sodium utterly invisible to the naked eye, as little as one part per twenty million of air, revealed its presence, when heated, by its light signature.
Bunsen and Kirchhoff realized they'd hit on something of enormous scientific potential. “If there should be substances that are so sparingly distributed in nature that our present means of analysis fail for their recognition and separation,” they wrote, “then we might hope to recognize and to determine many such substances in quantities not reached by our usual means, by the simple observation of their flame spectra.” They were confident in saying this because, as is ever the case in the conservative world of science, they'd already done what they were describing. As they later reported, they analyzed the invisible contents of a bottle of mineral water from Durkheim and found a new element whose spectral fingerprint was dominated by a splendid blue line. They called the new element cesium, Latin for “sky blue.” It was the first element named not for its outward appearance, use, or place of origin but for its light fingerprint.
MYSTERY OF THE FRAUNHOFER LINES
As Bunsen and Kirchhoff worked to document the light fingerprint of one element after another, a different spectrum emerged into Kirchhoff's mind's eye: not a single element's spectrum but, incredibly, that of the Sun. In a flash of inspiration, Kirchhoff saw that spectroscopy was the key that might unlock one of the greatest mysteries of his day. With a simple step in the laboratory, he could make a great leap from the elements to the stars.
The mysterious solar spectrum that Kirchhoff was contemplating had stumped scientists for half a century. It was the nineteenth-century equivalent of a cosmic crossword puzzle for which no one could determine even the first letter of the first word. The spectrum had been produced by the deeply talented Austrian optician and telescope maker Joseph von Fraunhofer. Fraunhofer's own scientific journey was phoenixlike. The eleventh child of a struggling glassmaker, Fraunhofer was originally apprenticed to a strict master, with no hope of going to school. However, the master's house collapsed in a freak accident, killing several people but leaving the teenage Fraunhofer unhurt in the rubble. His amazing survival was witnessed by the future Bavarian king, who became Fraunhofer's sponsor. Within years, the young man's talent and drive made him a respected optician and entrepreneur.
As such, Fraunhofer tackled a major spectral problem of his time. Lens makers of the day struggled with small flaws in a lens's manufacture that resulted in unwanted spectra at the edges and sometimes gave eyeglass wearers a rainbow-colored view of the world. For eyeglass wearers, this was merely a bother, but for astronomers it was a fundamental problem, distorting their view of the heavens. Fraunhofer had the clever idea of finding a fixed spectrum by which he could calibrate and adjust lenses—and what better light source for this than sunlight itself? But the solar spectrum he produced with his telescope in 1814 was more of a head-scratcher than a solution. Strewn across the solar spectrum were thin, dark lines. Sitting in his darkened observatory, staring through his rudimentary spectroscope, Fraunhofer stared at this odd spectrum that recalled Newton's specter. It was as if some celestial jester had taken a pencil and ruler and drawn thin lines at irregular intervals across the Sun's spectrum, similar to what would appear today as a solar bar code.
Newton hadn't seen these lines because his spectrum was too diffuse—the equivalent of a low-resolution digital picture in which fine details are lost—and an earlier nineteenth-century astronomer, Francis Wollaston, who'd first detected these lines in 1802, had discounted them as the boundaries between spectral colors. Yet in his more detailed solar spectrum, Fraunhofer clearly saw that many of the dark lines occurred within individual colors. In the manner of the deeply obsessed and driven scientist throughout the ages, Fraunhofer sat at his telescope and painstakingly recorded the positions of the soon-to-be-called Fraunhofer lines. He eventually detailed almost six hundred of them, labeling the most prominent ones from A to H. He wondered what could cause these strange but consistently present lines that were polluting the Sun's pure spectrum. He even glimpsed them in the light from several stars. He might well have answered the question himself, but in 1826 he died of tuberculosis at the age of thirty-nine.
For more than thirty years, the origin of the Fraunhofer lines hung as the great cold case of nineteenth-century astronomy. Now, with his laboratory evidence, Kirchhoff thought he'd cracked the case. The key to solving the mystery was the fingerprint evidence; in this case, sodium's light fingerprint. When Kirchhoff saw sodium's spectrum with its dominant yellow lines, he was struck by the fact that he'd seen lines in exactly the same place in another spectrum—the Sun's. The difference was that in the Sun's spectrum, the lines were the dark “D” Fraunhofer lines—two lines, both in exactly the same place in the yellow part of the spectrum, but one bright, the other dark. Was it possible that both the dark and bright lines were produced by sodium?
Kirchhoff performed an ingenious series of experiments that culminated with this one: he modeled the Sun and its possible atmosphere in his lab. From his wheelchair, the physicist heated a piece of charcoal, as a stand-in for the Sun, and then shone its light through sodium gas before recording the spectrum. When he examined the spectrum, he became the first human to know part of what the Sun is really made of—there were the two dark Fraunhofer lines, corresponding to sodium's yellow lines. The Sun's atmosphere, Kirchhoff realized, is salty.
These experiments were the foundation for what is still taught as Kirchhoff's law of emission and absorption. According to Kirchhoff's law, the position of either dark or bright lines in a spectrum is an element's telltale signature. If the line is dark, the element is absorbing the light from another source; for example, elements in the Sun's atmosphere. If it's bright, the element is giving off light as a result of absorbing energy, for example, that from a flame. The relative strength of the lines depends on the density and abundance of the eleme
nts. But the kicker, concluded Kirchhoff, was that the chemical analysis of the Sun's atmosphere required only the search for those substances that produce the bright lines that coincide with the dark lines of the solar spectrum. Kirchhoff meticulously drew the solar spectrum with its Fraunhofer lines and placed this side by side with the spectra from thirty different elements. Like a detective identifying multiple fingerprints at a crime scene, Kirchhoff identified the elements present in the Sun's atmosphere. There were a remarkable sixty exact overlaps between the Fraunhofer lines and the laboratory spectrum of iron. The Sun's atmosphere was ablaze with iron, as well as being a chemist's cabinet of Earthly elements, including sodium, calcium, nickel, and magnesium, along with barium, copper, cobalt, and zinc.
Kirchhoff's observation of that first element, sodium, in the Sun's spectrum was as momentous as Galileo's observation of the moons of Jupiter. Galileo knew in a moment that the Sun was not the absolute center of creation—heavenly bodies orbited the planets as well. Kirchhoff realized that the power of the elements’ light fingerprints amazingly extended beyond the Earth to the stars. He and Bunsen had developed a tool, spectroscopy, that opened the heavens to a new kind of scientific exploration. “Spectrum analysis…opens to chemical research a hitherto completely closed region extending far beyond the limits of the earth and even of the solar system,” he wrote.
Auguste Comte was wrong. Kirchhoff knew that it would be possible to lift the Sun and the brighter stars out of a veil of mystery and to determine not just their movements but also exactly what they were made of. In 1861, after a demonstration of this revolutionary spectroscopic technology, Warren De la Rue, a British chemist and astronomer famous for his pioneering photographs of the Sun and Moon, exclaimed to an audience of scientists in London—as if rebuffing Comte—that stellar spectroscopy was akin to having a piece of a star in your hands. “If we were to go to the Sun, and to bring some portions of it and analyze them in our laboratories, we could not examine them more accurately than we can by this new mode of spectrum analysis.”
Kirchhoff and Bunsen thus created the most powerful, and most unsung, astronomical tool after the telescope. Stargazers previously had pitifully little to work with, compared with their terrestrial scientific colleagues in chemistry or biology. Astronomers could describe a star's position, comparative brightness, and observed color, but not much else beyond what can be seen by looking up into the night sky. Stellar spectroscopy presented a previously unimaginable way of knowing a star, making a point of light something physical and elementally related to the Earth and to us. Telescopes capture light, but it is spectroscopy that makes this light meaningful.
It's difficult to underestimate the impact of stellar spectroscopy on astronomy. The vast majority of what we know about stars—their composition, temperature, and movement—is due to spectroscopy. It enables us to tease out the incredible information starlight carries. This light message can travel from a distant star, across the known universe, without losing a single word of the story it carries. For astronomers, it was as if for decades they'd been receiving messages by mail, with the only information about the sender being a return address and the stamp on the envelope. Suddenly astronomers realized that they could open the envelope and find inside a letter containing an amazing story about the stars.
ORDER IN THE HEAVENS
Astronomers soon realized that stellar spectroscopy revealed more than just what stars were made of; it revealed how they were related. Just as the Victorian zeitgeist for categorizing inspired geologists to classify rock types, and natural historians to minutely describe and classify newly discovered species of animals and plants, astronomers now had a previously unimaginably powerful tool for bringing order to the heavens. Indeed, the discovery of stellar spectroscopy was greeted as a gift from the heavens by the Englishman William Huggins. In 1856, he'd sold the family's silk business—in the face of competition from the new phenomenon of department stores—and moved from London to Upper Tulse Hill on the city's southern outskirts where, under a dark night sky without London's smog or light pollution, he oversaw the building of a world-class observatory.
Although Huggins was nominally an amateur gentleman astronomer, he was ambitious and was looking for a challenge beyond the then run-of-the-mill observing of the positions and motions of stars. He found it in stellar spectroscopy. “The news reached me of Kirchhoff's great discovery of the true nature and chemical composition of the sun from his interpretation of the Fraunhofer lines,” Huggins later recounted. “This news was to me like the coming upon a spring of water in a dry and thirsty land. Here at last presented itself the very order of work for which in an indefinite way I was looking—namely, to extend his novel methods of research upon the Sun to the other heavenly bodies.”
Aided by his neighbor, William Miller, a professor of chemistry at King's College, Cambridge, and an early champion of laboratory spectroscopy, Huggins set to work, laboriously comparing stellar spectra with those of Earthly elements. In this way, Huggins and Miller studied fifty stars and compared their spectra with those of twenty-seven elements. To their amazement, no matter where they looked in the heavens, they saw evidence of the same elements. For Huggins, it was a profound realization: the stars are distant but are not fundamentally different from us. It was this connection, rather than the relationship between the stars, that fascinated Huggins when he and Miller published their findings with the Royal Society in 1864, marking the start of stellar astrophysics. They reached a stunning conclusion: not only was it possible to know the composition of the stars, but, based on this analysis, they concluded that there existed a common chemistry throughout the cosmos. A commonality others would soon see could be used to make cosmic comparisons.
For the comparison of stellar spectra to really take off, the intersection of astronomy with another transformational new way of seeing was required. On a clear night in 1872, American astronomer Henry Draper was in his observatory at Hastings-on-Hudson, about twenty-five miles north of Lower Manhattan. That evening, rather than simply observe the spectrum of the bright star Vega, Draper took a photograph of it. Although this might not seem like such a leap, it was the marriage of two world-changing technologies—spectroscopy and photography. Draper's first spectrogram—a visual record of a star's spectral fingerprint—meant that rather than sit in the dark and carefully record by hand a star's spectrum, astronomers could take a picture of it and then examine it at leisure in daylight. Imagine the delight of a police booking officer who had previously been required to hand-draw each suspect's fingerprint, but who was now able to use an ink pad to quickly and accurately record the print for later use. For astronomers, it was even better than this. Before spectrographs, they could see the spectra of only the very brightest stars—those bright enough to create a spectrum visible to their eyes. Now, with an exposure time of minutes or hours, astronomers could patiently collect photons on photographic plates from thousands, and soon hundreds of thousands, of faint and faraway stars not visible to the naked eye. Suddenly, a vast and literally endless bounty of stars and other celestial objects were made visible to spectroscopic fingerprinting.
Following Draper's death at the age of forty-five in 1882, his widow, Anna Palmer Draper, also an accomplished astronomer who'd observed alongside her husband, wanted to commemorate her late husband's stellar love. She found the perfect man to carry on his vision in Edward C. Pickering, the director of the Harvard College Observatory in Cambridge, Massachusetts. With seed money from Anna Palmer Draper, Pickering created the Henry Draper Memorial, one of the great star-cataloging missions of all time. Pickering saw in the combination of a well-staffed observatory and spectrographs the opportunity to collect thousands of stellar spectra that would allow him to discover stellar relationships and to organize them based on their common characteristics.
During a thirty-five-year period, Pickering led a series of spectroscopic star surveys and cataloging efforts that by 1924 listed the spectra of more than 225,000 North
ern and Southern Hemisphere stars—organized in a nine-volume series called the Henry Draper Catalogue—or HD Catalogue. Fueled by Pickering's vision and fundraising, his assistants did the vast bulk of the work: the observing, the spectral comparison, and the analysis that led to the HD Catalogue's historic conclusions. In a 1913 photograph, Pickering stands suited in the back row of his “harem”: thirteen middle-aged women wearing ankle-length dresses and blouses with high-neck collars and with their hair up in neat, controlled buns. These were the “computers”—the astronomy field's version of secretarial pools—women hired to do the detailed analysis of the astronomical plates, as well as related mathematical calculations. For example, Annie Jump Cannon, an astronomer educated at Wellesley and Radcliffe Colleges and hired as assistant astronomer, led the classifying of almost a quarter-million spectra. In the annals of astronomy, Cannon is the greatest star fingerprinter of all time: between 1896 and her death in 1941, she classified the spectra of an estimated 395,000 stars.
In 1922, this herculean effort led to the International Astronomical Union adopting a stellar classification system based on the dominant elements in a star's spectra. Thus was born the mnemonic device remembered by undergraduate astronomy students ever since: Oh Be A Fine Girl/Guy, Kiss Me! This was based on the seven main stellar types: O, B, A, F, G, K, and M, arranged in order of decreasing stellar mass and surface temperature, from the biggest, hottest blue O-stars on the left, to the smallest, coolest red M-type stars on the far right.
Less than a century after Auguste Comte's prediction that the stars would forever remain unknowable, astronomers had turned light's secret code into a way not just of knowing the composition of stars but also of seeing underlying order in the heavens. This new cosmic code was telling a deeper story. Comte's seemingly irrefutable logic had been transformed by the combination of spectroscopy, telescopes, and photography. A new breed of astrophysicists could indeed experiment by analogy with stars in their laboratories by observing the spectra of elements and then looking for them in the stars. And though astronomers still knew of only one solar system in the cosmos, it was now possible to compare the chemical structure of different corners of creation.
The Stardust Revolution Page 5