Huggins’s surprise stemmed not so much from the presence of nebular emission lines—low-density, incandescent gases exhibit such lines—but from their paucity. Planetary nebulae emitted almost all of their luminous energy in the blue-green portion of the color spectrum. The spectra of seven other planetary nebulae possessed at least one of the Cat’s-Eye trio of emission lines. Huggins provided no explanation for this remarkable trait, but he did conclude that the spectra were not of stellar origin.
“It is obvious,” Huggins reported to the Royal Society in 1864, “that [at least some nebulae] can no longer be regarded as aggregations of suns after the order to which our own sun and the fixed stars belong. . . . In place of an incandescent solid or liquid body transmitting light of all [colors] through an atmosphere which intercepts by absorption a certain number of them, such as our sun appears to be, we must probably regard these objects, or at least their photo-surfaces, as enormous masses of luminous gas or vapour. For it is alone from matter in the gaseous state that light consisting of certain definite [colors] only, as is the case with the light of these nebula, is known to be emitted.”
To check his hypothesis, Huggins observed the spectra of several star clusters, as well as the great lenticular cloud in Andromeda. (Its spiral form had not yet been disclosed by the camera.) These objects gave off a continuous spectrum, indicative of diffused starlight, albeit too faint to manifest the characteristic absorption lines. The spectrum of the Orion Nebula, on the other hand, displayed the telltale emission lines of a gaseous body. But when Huggins aimed the spectroscope at stars embedded within the nebula, the gaseous and stellar spectra appeared together. The spectroscopic evidence was clear: nebulae are not a unitary class. Planetary nebulae are incandescent, gaseous objects, whereas at least some fraction of nonplanetaries are, in Agnes Clerke’s lyrical phrase, “star clusters grown misty through excessive distance.”
Huggins’s theoretical assertions were more problematic, especially in hindsight. He took the extreme simplicity of nebular spectra to reflect their chemical makeup rather than physical conditions within the gas: The familiar stellar line patterns are missing because nebulae lack these elements. Thus, the accepted theory that element-rich stars condense out of more basic nebulous material is unsustainable. Where do all the stellar elements come from if not already present in the accreting gas? The spectra of nebulae, Huggins maintained, should be as crowded with bright lines as the stellar spectra are with dark lines. At the same time, he suggested that the Andromeda Nebula, despite its continuous spectrum, might be gaseous, only in a denser state than other nebulae. It would be many decades before the physics of nebulae, as well as the cosmic distance scale, were sufficiently refined to settle these issues. Evidently, William Huggins understood the risk of drawing conclusions from incomplete evidence. In his 1865 report to the Royal Society, he admitted that “science will be more advanced by the slow and laborious accumulation of facts, than by the easier feat of throwing off brilliant speculations.”
By the mid-1860s, Rosse’s six-foot Leviathan reflector had managed to resolve a number of nebulae into clusters of stars, or at least it appeared so to the observers. (Some of these “resolved” objects were subsequently proven to be gaseous, not stellar.) The nature of the other nebulae remained shrouded in mystery, with astronomers unsure about what their eyes were perceiving in the telescope’s dim view-field. That shroud was partially lifted by a modest eight-inch refractor equipped with a spectroscope. Centuries after the power of the human eye had been compounded by the telescope, the act of seeing was again being transformed. Another optical breakthrough, as revolutionary as the first, endowed astronomers with a new, if rangebound, ability to analyze light. Huggins’s spectroscopic study of the nebulae was the harbinger of the “New Astronomy,” whose adherents would come to dominate the field.
Huggins’s stellar and nebular work was immediately hailed by the scientific community. The Royal Society elected him a Fellow; he received awards from both it and the Royal Astronomical Society. The Astronomical Society’s official history acknowledges that Huggins became the go-to guy for all things spectroscopic: “[H]e was frequently called upon to speak of the nature and real significance of the harvest of data that were being gathered in the new branch of astronomy to which he devoted his pioneering activities. His clear understanding, both of the power and also of the limitations of the new methods, did much to keep men’s minds from jumping to hasty conclusions.”
In the 1860s, spectroscopy was reckoned to be the critical breach in the once impenetrable barrier between terrestrial and celestial physics. The advance launched by Robert Bunsen and Gustav Kirchhoff and spurred on by William Huggins and others had penetrated with stunning rapidity into the unknown—indeed, what some had adjudged the unknowable. “Forward!” was Warren De La Rue’s rallying cry to the Royal Astronomical Society in 1864, urging his colleagues to embrace the new technology so ably implemented by one of their own. The charge so ordered, but the battle lines unclear, spectroscopic astronomers wondered, “Which way is forward?”
Chapter 18
TRUMPETS AND TELESCOPES
This time was, indeed, one of strained expectation and of scientific exaltation for the astronomer, almost without parallel; for nearly every observation revealed a new fact, and almost every night’s work was red-lettered by some discovery.
—William Huggins, The New Astronomy, 1897
ON TUESDAY, JUNE 3, 1845, IN A SCENE of Pythonesque strangeness, a locomotive sped through the Dutch countryside with a single flatcar, on which sat a trumpet player. The red-faced musician huffed desperately into his horn, trying to pierce the combined din of the steaming behemoth and the whipping wind. His prearranged tune was unadorned with tonal, rhythmic, or dynamic complexities; in fact, it was a single note, sustained as long as human lungs allow. Two other riders were intent on their colleague’s performance despite the railcar’s breakneck hurtle across the landscape. The trackside audience was similarly spare: at three places, a trio of listeners, heads cocked in unison toward the approaching thrum of music and machine. The men stood as close to the rails as they dared, having been told that a minimal distance from the travel path was mandatory. Meanwhile, the orchestrator of the surreal concert, Christoph Hendrik Diederik Buys-Ballot, a young meteorologist at Utrecht University, rode in the locomotive with the engineer, scribbling down the train’s speed.
As the train approached at nearly forty miles an hour, the spectators along the track heard the crescendoing pulse of the steam engine. Then, from deep within the mechanical tempest, arose a faint whine: the bray of a trumpet. The instrument was audible for only a second or two before the locomotive whooshed by. But to the trained ears of the listeners, the preagreed note sounded perceptibly sharp. Now, in the train’s recession down the track, again the trumpet’s sound—only this time, unmistakably flat. In surreal splendor, the musical cavalcade swept past the other listeners’ posts, where similar alterations of pitch were heard.
The train reversed in Maarssen, five miles from its embarkation point in Utrecht, and accelerated home. This time, a trumpeter at each of the ground stations sounded the standard note, while the flatcar riders strained to hear above the clatter. The result was the same as before: the pitch of the horn was sharp upon the train’s approach and flat upon its recession.
Back in Utrecht, Buys-Ballot compiled the data from the day’s experiment. He compared the attested degrees of sharpness or flatness to those derived from a mathematical formula based on the train’s speed and the note’s nominal frequency. The agreement between the mathematical predictions and the experimental outcomes was marginal at best. The designated listeners, a mix of concert-hall musicians and Buys-Ballot’s friends, had complained of the background noise: they could barely hear the trumpet amid the clatter of the engine. The musicians responded that they could not play any louder without having to take more frequent breaths.
Holland’s Interior Minister gave the go-ahead for a secon
d trial, and Buys-Ballot reconvened his team later that week. This time, he had enlisted two players for the flatcar; one would continue to blow while the other took a breath. And he replaced the mellow trumpets with ear-piercing bugles. The results of this follow-up run were little better than they had been two days before. Yet Buys-Ballot was satisfied. He submitted his aptly titled report, “Acoustical Researches on the Dutch Railway,” to the venerable scientific journal Annalen der Physik und Chemie. Buys-Ballot had pursued—and captured—the most alluring quarry in the intellectual hunt that is science: experimental validation of an idea.
Buys-Ballot’s novel experiment had been driven by an equally novel treatise by Austrian mathematician Christian Doppler, who claimed in 1842 that the perceived frequency of a wave is altered by one’s state of motion. The so-called Doppler effect was said to hold for any form of wave: ripples in a pond, ocean swells, sound, even light. (By the mid-1800s, there was ample evidence that light exhibits wave properties.) The key factor in the perceived frequency alteration is the relative motion between the wave source and the observer. Doppler provides a mathematical formula that quantifies the frequency changes under various circumstances: observer stationary and source moving, observer moving and source stationary, or both observer and source moving with respect to one another.
Christian Doppler.
In Doppler’s schema, waves from a steadily approaching source are compressed: as their frequency is increased, their wavelength is shortened. Waves from a steadily receding source are stretched: as their frequency is reduced, their wavelength is elongated. But if the source is stationary relative to the observer, no alteration of the waves is perceived.
Doppler next made a bold but ill-informed trespass into astronomy. At the time, no one knew the origin of stars’ colors. Most astronomers presumed that color is an intrinsic property of a star, perhaps related to its temperature or some other physical condition of its matter. Doppler proposed instead that the perceived color of a star arises from its movement, specifically, that the line-of-sight, or radial component of the star’s velocity through space, dictates its coloration in the telescope. Light waves from a star heading in Earth’s general direction will be compressed; effectively, the star is chasing its own Earth-directed waves as it emits them. If, as Doppler believed, stars are intrinsically white, an approaching star will instead appear blue; its luminous energy is skewed toward shorter wavelengths of the spectrum. By the same token, a receding star will appear orange or red, its light shifted toward colors of longer wavelength.
The magnitude of the color shift, Doppler explained, depends on the velocity of the star relative to Earth. A recession velocity of around six hundred miles per second, for instance, would noticeably redden a white star to the eye. A line-of-sight velocity of sixty thousand miles per second would shift all of a star’s luminous energy out of the optical range and render the star invisible. Doppler maintained, without evidence, that stars typically move at such breakneck speeds through space. (Modern studies indicate that stellar speeds rarely exceed one hundred miles per second relative to Earth.)
Doppler also ascribed the contrasting colors of many double-star systems to their orbital motion. Locked in a mutual gravitational embrace, the double-star member that is headed toward Earth might appear bluish, while its partner, headed away, might have an orange or red cast. Given that the orbital motion is periodic, Doppler predicted that double-star colors evolve over time. Indeed, because visual color estimation is so subjective and the 1840s stellar database was so sparse, Doppler did find spurious observational support for his assertions.
Only a handful of people were present when Christian Doppler presented his ideas to Prague’s Royal Bohemian Society of Sciences in 1842. A year later, a summary appeared in Annalen der Physik und Chemie, and two years after that, the unabridged work, in pamphlet form, fell into the lap of Christoph Buys-Ballot. Like many who encountered the paper, Buys-Ballot was intrigued by the logical link between relative motion and wave frequency but suspicious of Doppler’s astronomical inferences. As yet, there was no terrestrial means to test the Doppler principle using light waves. Not even a cannon could propel a light source rapidly enough to generate a measurable alteration of color and there was no astronomical route to verification. The radial velocity of an isolated star could not, as yet, be determined by any independent method. And although the laws of Newtonian mechanics do govern the orbital movements of binary stars, without prior knowledge of their distance, the velocities of the stars were incalculable.
Sound travels much slower than light; therefore it presents a more viable path toward a demonstration of the Doppler effect. Accelerating a sound source up to forty miles an hour, Doppler calculated, was sufficient to sharpen or flatten a musical note by a sensible degree. Christoph Buys-Ballot had only to look at Utrecht’s new high-speed train to realize that he had the means to test Doppler’s principle for acoustic waves. In no time, he convinced the railway’s director and the Dutch Minister of the Interior to clear the tracks for his unusual musico-scientific project.
The first half of Buys-Ballot’s 1845 report details his confirmation of the Doppler effect for sound waves, while the second half identifies a fatal flaw in Doppler’s theory of star colors. The Sun had long been known to emit a significant amount of energy outside the bounds of the visible spectrum, in the form of infrared and ultraviolet light. Stars, too, are presumed to radiate such emissions. Therefore, any luminous energy Doppler-shifted out of a given region of the visible spectrum will be replaced by a comparable amount of infrared or ultraviolet energy shifted into that region. As a result, the perceived color of a star, regardless of its velocity, will be the same as if it were stationary relative to Earth.
Starting in 1846, Doppler published the first in a series of rejoinders to Buys-Ballot’s argument, asserting, again without evidence, that the infrared and ultraviolet output of stars is negligible; hence, there is no replacement energy for emissions motion-shifted out of the visible spectrum. While the scientific community accepted the reality of the Doppler effect (train whistles were everywhere!), volleys over Doppler’s star-color theory were exchanged in meeting halls and journal pages well into the 1850s. It didn’t help that Doppler was hopelessly outdated in the experimental underpinnings of his own theory. Not only had he neglected the existence of solar ultraviolet and infrared radiation, he had adopted a long-discredited value of the speed of light from 1676.
When Doppler’s collected works were published in 1907, Nobel Prize–winning physicist Hendrik A. Lorentz pronounced judgment on his controversial predecessor: “[I]n considering the importance of his principle and the productive use to which it has been put, we must include Doppler as one of the great men of science, although it seems to me, that neither his other work nor the manner in which he defended his theory against various objections and applied it to the colours of the stars, confer on him any claim to such an honorific title.”
While astronomers were engaged in the Dopplerian debate over star colors, a little-known conjecture wafted in the background. In December 1848, French physicist Armand-Hippolyte-Louis Fizeau delivered a lecture at the Société Philomatique in Paris. (Fizeau and his collaborator Léon Foucault had taken the first successful daguerreotype of the Sun in 1845.) Unaware of Christian Doppler’s prior work, Fizeau laid out the same mathematical formulations now identified with the name of his colleague in Prague. But Fizeau’s astronomical assertions were rooted in hard observational facts. Instead of addressing the influence of motion on the overall colors of stars, Fizeau focused on the spectroscopic ramifications. Even at a small fraction of Doppler’s breakneck stellar velocities, he found the Fraunhofer lines would be measurably shifted either redward or blueward, depending on the star’s direction of motion. Significantly, the lines would retain intact, their diminishment of light merely displaced to a different wavelength. Measurement of the shift of a spectral line from its laboratory-standard wavelength allowed the computation of
a star’s radial velocity in space.
Since antiquity, astronomers have kept track of the positions of stars, both to provide a reference grid against which to measure the movements of planets and comets, and to track the movements of the stars themselves. These minuscule translations are seen in two dimensions, projected onto the plane of the night sky. If the distance to a star is known, its gradual creep through a constellation can be mathematically converted into actual velocity units, such as miles or kilometers per second. With Fizeau’s proposal, the spectroscope promised to reveal a star’s velocity in the third dimension, radially toward or away from Earth. Combining a star’s planar and radial movements renders its full, three-dimensional motion through outer space. In sufficient number, stellar velocities are key to probing the dynamics of our galaxy. The measurement of radial velocities was one part of the new astrophysics with the potential to arouse the interest of classical astronomers.
Given their eventual impact on astronomy, it might seem incredible that Doppler’s and Fizeau’s findings were not more widely disseminated upon their release; technical journals did exist, but scientific exchange across disciplines and across national borders still relied heavily upon personal correspondence and word of mouth. (Fizeau’s paper was not published until 1870.) It was not until the late 1850s that the famed Scottish mathematician James Clerk Maxwell encountered the Doppler–Fizeau effect in a retrospective volume on optical research. Maxwell had an abiding interest in the phenomenon of color, publishing a series of papers on the subject and conducting the first public demonstration of color photography at London’s Royal Institution in 1861. (Maxwell obtained three black-and-white photographs of a tartan ribbon taken, respectively, through red, green, and blue filters, then projected the images through these filters onto a screen, where the magnified ribbon appeared in its true coloration.)
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