Department of History
University of California, Irvine
Instructor: Dr. Barbara J. Becker
Lecture 15. Redshift, Blueshift.
Stellar Motion in the Line of Sight
In 1718, Edmond Halley reported that some of the so-called "fixed" stars have a "particular Motion of their own" across the sky. Since then astronomers have measured the proper motion of many stars. But even the nearest of our Sun's stellar neighbors is too distant to exhibit any of the visual cues we normally rely on as evidence of motion in the line of sight.
The ability to detect, let alone measure, a star's radial velocity eluded earthbound observers until the late 1860s when William Huggins brought the new instruments and methods of celestial spectroscopy to bear on the matter. It proved to be the greatest, and certainly the most influential, of his contributions to modern day astronomical practice.
Thanks to automated precision instruments, measuring stellar radial velocity is a straightforward and routine procedure today. For Huggins, by contrast, who had conducted his pioneering efforts entirely and exclusively by visual means, it was an audacious project fraught with overwhelming mensurational and interpretive difficulties. His observatory notebook records reveal the steps he took to overcome these challenges and they expose the rhetorical skill with which he attempted to persuade his contemporaries that he had, in fact, accomplished what he claimed.
Huggins was attracted to this line of investigation during an exceptionally eclectic period in his career. It was just one of many tempting puzzles he could have chosen to unravel. What led him to undertake the challenging task of trying to measure the motion of stars in the line of sight?
The Colors of Stars
Huggins and chemist William Allen Miller (1817-1870) presented their first collaborative paper -- "On the Spectra of some of the Fixed Stars" -- at the Royal Society meeting on 26 May 1864. In their concluding remarks, they drew heavily from the latest report on star color by Admiral William Henry Smyth (1788-1865), a passionate amateur astronomer who had spent decades studying the colors of stars.
Frontispiece from Smyth's Sidereal Chromatics (1864)
No one knew if star color was an extrinsic or intrinsic quality. It was difficult to account for recorded differences in the apparent color of some stars. Smyth pointed to the enigmatic example of Sirius. Though the ancients described it as a red star, Sirius "is now decidedly white, and brilliantly white too".
Were such disparities the result of atmospheric or instrumental distortion? Were they produced by natural frailties of human visual perception and the ease with which even the most careful observer can fall prey to suggestion or illusion? Were they simply emblematic of language's limits to communicate personal sensations? Or do stars really change their color over time?
Despite, or perhaps because of, these knotty and unresolved questions, Smyth believed it was just as important to record each star's apparent color as it was to map its position on the sky or gauge its brightness if astronomers were going to come to any rational understanding of the heavens and its occupants. He recommended several fruitful avenues of study to frame the work that needed to be done.
Smyth's comparisons of the colors of double stars inspired Father Benedetto Sestini (1816-1890) at the Collegio Romano to pursue this challenging research agenda. Sestini had already created a catalogue of colored stars based on his observations in Rome.
In 1851, Smyth published a lengthy and detailed account of current theories, unresolved questions, and advice on how best to look for answers. He included a list of one hundred pairs of double stars to encourage others to join in the effort. He displayed the data for these stars in tabular form so readers could easily compare Sestini's descriptions of their colors alongside his own.
Seen through a small telescope, the star Albireo in the constellation Cygnus will be seen as pair of closely-spaced stars. The brighter star, Albireo A, has a yellow-orange color while its companion, Albireo B, appears pale blue.
Smyth still viewed the cause of star color as an open question. Until more evidence had been gathered and carefully analysed, he advocated waiting to choose among the many interesting explanations that had been proposed.
He was intrigued, though largely unconvinced, by one of these proposals. "In the present incertitude," he noted, "it is suggested that variations in colour may be owing to variations in stellar velocity". He dubbed this idea "Sestini's theory" apparently unaware it was originally proposed nearly a decade earlier by Austrian mathematics professor Christian Andreas Doppler (1803-1853).
In fact, few astronomers, or any other men of science, were then familiar with Doppler's name or his work. His lack of notoriety was hardly surprising. When he presented his theory before the Royal Bohemian Society of Sciences in May 1842, only a scant handful of members were present. Among them was mathematician and logician Father Bernard Bolzano (1781-1848), who became one of the young professor's early advocates. Bolzano penned a warm and generally enthusiastic article on Doppler's new theory which was published the following year in the widely read Annalen der Physik.
Doppler considered his theory as a logical extension to -- and conceptual generalization of -- stellar aberration, the small annual periodic shift in a star's apparent position on the sky, a natural consequence of both the finite speed of light and Earth's orbital motion in the plane perpendicular to that of the star's incoming light. Indeed, Doppler believed he had discovered something analogous that occurs as a natural consequence of motion parallel to a star's incoming light.
He assumed light propagates as a wave, like ripples in water or sound in air. An observer's perception of any wave's frequency will change, he argued, if the wave source and/or the observer begins to move toward or away from one another. The waves will break more frequently and with greater intensity if the two move toward one another, less so if they move apart. The difference between a wave's intrinsic frequency and that perceived by an observer should provide a way to calculate the source's relative speed of approach or recession.
Doppler was a mathematician, not an experimentalist. He deduced the elements of his theory from formulas he derived from fundamental principles of wave behavior. Applying the formulas first to sound and then to light waves, he calculated what an observer would hear or see under a range of conditions. The results convinced him that his theory could account for many previously unexplained astronomical phenomena such as the complementary colors so often observed in binary star pairs as well as changes in the color and brightness of periodic variable stars, novae and extinguishing stars.
In June 1845, Dutch meteorologist Christoph Hendrik Diedrik Buijs-Ballot (1817-1890) tested Doppler's theory in an elaborate experiment involving musicians and musically trained observers who alternated serving as passengers on, and bystanders alongside, a rapidly moving railway train. Although the musicians struggled to sustain a single tone and observers strained to hear over the noise of the locomotive, Buijs-Ballot felt certain the change in pitch observed in each trial conformed to Doppler's predictions.
Nevertheless, he questioned the propriety of generalizing the theory to stars. He doubted any stars could be found traveling with sufficient speed to exhibit the effect Doppler proposed. Besides, he argued, even if one were to be found, Doppler was wrong to expect an observer to note any hint of color due to its motion. Invisible rays had recently been discovered beyond the red and violet extremes of the visible spectrum. Buijs-Ballot pointed out that as the shortest wavelengths of an approaching star's visible light are shifted beyond the violet end of the visible spectrum, the invisible rays in what is now called the infrared region of the spectrum will be shifted up into the visible range. The star's overall light will be composed of the same full array of colors regardless of its motion.
In August 1848, Scottish engineer John Scott Russell (1808-1882), who was unaware of either Doppler's or Buijs-Ballot's work, reported on the unusual auditory sensations experienced by passengers and bystanders alike when they were exposed to the sounds emitted by a rapidly moving railway train.
In a lecture he gave in December of that year, French physicist Armand Fizeau (1819-1896) used Russell's report to support the results of his own laboratory studies on variations in the pitch of sounds produced by a source moving towards or away from a stationary observer. Like Russell, Fizeau was unfamiliar with Doppler's and Buijs-Ballot's work. It is interesting to compare his thoughts with theirs as he considered the practical challenges of extending what he had observed in his acoustic experiments to the case of light waves.
First, like Doppler and Buijs-Ballot, Fizeau figured that the light source and/or the observer would have to be moving at a considerable speed in the line of sight to produce any noticeable shift. Unlike Buijs-Ballot, however, Fizeau was sure there were stars that moved at such speeds. Second, in his acoustic experiments, Fizeau had designed his sound source to emit a narrow range of tones in order to make a shift in pitch easy to hear. But he knew most natural light sources emit a wide range of wavelengths including rays beyond the visible spectrum. Like Buijs-Ballot, he concluded that any apparent shift in the lengths of those waves would result in no perceptible change in the light's overall color.
Nevertheless, Fizeau happily pointed out, this did not mean the star's motion was undetectable. Fraunhofer's dark lines could serve as visual guides in optical experiments just like single-frequency tones in acoustic experiments. Fizeau suggested any shift detected in the positioning of these lines in a star's spectrum should be interpreted as evidence of the star's relative motion toward or away from the observer. He was genuinely optimistic that astronomers could and would develop instruments capable of measuring such small displacements with precision.
The full text of Fizeau's 1848 lecture was not published until 1870. However, in 1850, the prolific nineteenth century intelligentser Abbé Moigno (1804-1884) published a detailed summary of it as part of a critical review and analysis of Doppler's theory in the third of his four volume Répertoire d'Optique Moderne , an exhaustive compilation of current optical research.
One individual who knew of -- and frequently referred to -- Moigno's Répertoire was Scottish physicist James Clerk Maxwell (1831-1879). In 1857, while studying Fizeau's measurement of the speed of light in different media, Maxwell turned to Moigno's presentation of these important experiments in the Répertoire 's third volume. Coincidently, that presentation immediately precedes Moigno's review of Doppler's work in that same volume. It is therefore hard to imagine that Maxwell did not know about Fizeau's 1848 lecture.
James Clerk Maxwell
26 May 1864
Knowing that Maxwell was aware of Fizeau's lecture is important because he was among the Fellows present at the Royal Society meeting on 26 May 1864 when Huggins and Miller read their paper "On the Spectra of Some of the Fixed Stars".
It had been several years since the young physicist first studied Fizeau's comparison of the speed of light in water and air, but it is likely these experiments were very much in his mind that evening. Just one month earlier, he had submitted his own paper to the Royal Society in which he offered a plan for detecting Earth's motion through the luminiferous ether by looking for evidence of small changes in the index of refraction in prisms. Fizeau's work had served him as a stimulus and guide. Maxwell reported a null result and promised further trials. Unfortunately, the discovery of an error in the foundation of his experimental premise forced Maxwell to withdraw the paper.
Maxwell would have listened with interest as Huggins and Miller confirmed that star spectra are interrupted by an assortment of dark gaps like Fraunhofer had observed. A star's color could be accounted for by its arrangement of dark lines. The spectrum of "orange tinged" Betelgeuse, for example, is dark in its green and blue parts. That of white Sirius, by contrast, displays few notable dark lines.
The discussion that followed was reported in a popular weekly magazine by its science editor, Norman Lockyer. "We have pretty certain proof of the idea which has long been floating in many minds as to the cause of the colours of the stars", he wrote, "though their variability in colour,which has lately been so sternly insisted upon, is still yet to be explained." According to Huggins and Miller, "the differences of colour [depend] upon the differences of constitution of the investing atmosphere", which are "intimately connected with the chemical constitution of the stars."
Reports of a dramatic change in the color of one star in a binary pair in just a few years' time called into question the notion of a connection between the orbital motion of binary stars and their color. Lockyer suggested "it is evident that the colours of the stars must be better watched than they have been; and ... some other theory other than Doppler's must be found to account for their variability".
Maxwell apparently took Huggins's and Miller's observations as strong evidence that, contrary to Doppler's views, star color could not be explained solely as a consequence of motion in the line of sight. Lockyer tells us that Maxwell remarked, in words that seem to have come directly from Fizeau's 1848 lecture, that "if the colours were really tinged in consequence of the motion either of the star or our earth, the lines in the spectrum of the star would not be coincident with the bands of the metal observed on the earth, which gives rise to them."Lockyer felt sure "Messrs. Huggins and Miller ... will not let the matter rest...".
Stellar Motion in the Line of Sight
Nearly four years passed before Huggins submitted his paper on the relative motion of the Earth and stars along the line of sight in April 1868. He noted others' attempts to measure stellar motion in the line of sight, but he criticized their observational methods. In contrast, Huggins boasted a superior array of instrumentation which, he argued, permitted him to make the fine distinctions required in the positions of the spectral lines found in the light of celestial bodies compared to stationary terrestrial sources.
To put some British authoritative weight behind the theoretical foundation of his research, Huggins included an excerpt from a lengthy and technical letter he had received in June 1867 from Maxwell in reply to a request for information on the effect of Earth's motion on observers' perception of light from celestial bodies.
It had been three years since Maxwell's comments at the RS on using spectra to detect motion in light sources. Why Huggins chose to write to the young physicist at this particular time is unclear. Perhaps Huggins's recent acquisition of improved instruments encouraged him to think he could finally attempt the difficult research. Perhaps investigating stellar radial velocity had been a back burner project all along and he was just waiting for time and opportunity to carry it out.
There is one other thing worth mentioning which might have prompted Huggins to sit down and write Maxwell when he did. At the May 1867 RAS meeting, one Sidney Bolton Kincaid (1849-1898) described an apparatus which he called the "Metrochrome".
A staunch adherent to Smyth's plan to observe and record the colors of stars, Kincaid designed the instrument:
Advertised by its creator as easy to use and read, the Metrochrome created an artificial star by means of an incandescent platinum wire, an array of filters made of specially prepared chemical solutions and a rotating drum to blend the coloured light they produce. When the image of the artificial star was directed into the telescope for comparison alongside a real star and the colored filters were properly adjusted, Kincaid boasted:
Kincaid's presentation would have been of great interest to Huggins. For one thing Kincaid was one of his occasional collaborators in Miller's absence. For another, Huggins was fascinated by gizmos. The idea of the Metrochrome would have catalysed his creative energies. The presentation would also have focused his attention in new ways on the old star color question.
All these factors combined to give Huggins a broader sense of celestial spectroscopy's analytical and mensurational capabilities. When he did reconsider the problem of star color in the spring of 1867, he clearly had a new perspective on Doppler's theory. If he was planning to put it to the test, he was wise to seek advice from the one man who seemed to know how to go about designing a method to do so.
Huggins received Maxwell's now-famous reply to his query on 12 June 1867. In his reply to Huggins, Maxwell described two experiments based on the application of Doppler's principle to luminous bodies. The second experiment, which Maxwell had in fact performed in his failed attempt to determine Earth's motion through the luminiferous ether, does not seem to have interested Huggins.
But the first experiment, which Maxwell had not attempted, did intrigue Huggins and formed the basis of his research on the motion of stars in the line of sight. Maxwell, again seeming to take his words directly from Fizeau's 1848 lecture, suggested that if an individual line in a star's spectrum could be positively identified with one produced by a known terrestrial element, then observing the two spectra simultaneously should reveal any lack of coincidence in the lines' positions. This difference, he said, could then be used to ascertain the relative radial motion of the star and Earth.
Using a simple hypothetical example of a stationary star being observed from Earth as it orbited the Sun, Maxwell calculated it would be necessary to detect a shift on the order of one-tenth of the distance separating the sodium D lines, or about 0.06 nm in modern terms. Each division of Huggins's old wire micrometer only allowed him to register a displacement equivalent to about 0.2 nm.
Fortunately, he had just purchased a new double-image micrometer that was up to the task. Armed with this new instrument and with information about the specific position of emission lines in stationary terrestrial gases, Huggins attempted to observe the shift of stellar spectral lines.
On 25 June 1867, less than two weeks after receiving Maxwell's letter, he aimed his telescope and "two dense 60° prisms" on Arcturus. He recorded in his notebook:
In late November 1867, he obtained a new and more highly dispersive spectroscope. It was equipped with two dense compound prisms, two 60° prisms of Guinand's glass, and one 45° prism of Chance's flint glass.
William Huggins's notebook entry, 11 February 1868
Months later, on 11 February 1868, Huggins finally directed his new spectroscope with both compound prisms at the star Sirius. He compared the Fraunhofer F line in Sirius to that of a hydrogen spark. He wrote:
William Huggins's notebook entry, 24 February 1868
On 24 February, he made a second comparison, this time writing terms of visual comparisons suggested to him in Maxwell's June 1867 letter:
The proportional difference he claimed to have observed in the positions of the two lines was four times larger than the shift Maxwell suggested would be observed due simply to Earth's orbital motion.
It is important to note that Huggins seems unaware of this observation's direct conflict with the one he made on 11 February, in which he observed the F line of Sirius to be more refrangible than that of the comparison hydrogen spectrum.
At this point, Huggins was still comparing celestial and terrestrial spectra by the same means he and Miller had used in their early work on stellar spectra. This had been adequate for noting gross similarities between whole groups of spectral lines to spot the characteristic patterns associated with a particular element. Any displacement of them relative to a comparison spectrum was not of investigative interest. But now that difference had become the focus of attention and careful alignment of the telescopic spectrum with the comparison terrestrial spectrum became critical.
He designed a new comparison arrangement which included two small silvered pieces of glass positioned to produce two comparison spectra, one appearing above and the other below the celestial spectrum being observed. This new apparatus was complete on 5 March. Now, he complained, he could not be sure of the proper placement of the hydrogen spectrum:
A few days later, he observed again using the new apparatus, but this time with only one of the compound prisms. He recorded:
William Huggins's notebook entry, 12 March 1868
His confidence in these observations was soon shaken. On 12 March he wrote:
By 18 March, Huggins had become convinced that it was difficult to be sure of the exact calibration of his celestial and terrestrial spectra. In the past, he had contented himself that the two instruments were properly aligned by placing a luminous object directly in front of the object glass of the telescope to serve as a substitute star. He could then adjust the apparatus until the terrestrial spectrum appeared to coincide with that produced by the surrogate star. But slight changes in position of the light source serving as his stellar stand-in resulted in dramatic differences in the location of its spectrum, thus throwing into question the accuracy of his earlier calibrations.
On 30 March, he "strongly suspected" that the hydrogen line was indeed more refrangible than that of Sirius:
By 4 April, Sirius was too low in the sky and Huggins compiled his observations into a paper which he submitted to the Royal Society on 23 April. He was satisfied that he had successfully resolved the instrumental problems he had faced, and stressed the care he had taken to insure the reliability of his measures, as well as underscored the soundness of the theoretical foundation for his interpretation of those measures. The tone of the paper is therefore one of confidence and spirited adventure. All obstacles named are overcome either by clever manipulation of instrumentation or enviable patience. He believed he was justified in disregarding observations he deemed unworthy because sufficient numbers of observations were made in the first place to allow for judicious weeding.
The question of which observations "count" as supporting a particular interpretation is an important one. Huggins does not provide the reader with any clues to the total number observations he actually made and, of those, how many were discarded and why. Nevertheless, his method of presenting the data and his interpretation of it conveyed a feeling of extreme judiciousness and care.
Astronomer Royal George Airy (1801-1892) refereed the paper. He complimented Huggins's detailed descriptions of his apparatus and his exposition of the observational difficulties which had led to adjustments to this arrangement. However, Airy questioned the legitimacy of assuming that the line "at or near F observed in Sirius" was, in fact, due to hydrogen. It seemed "illogical" to Airy that one could argue on the one hand that the line was due to hydrogen because it coincided with the line in hydrogen seen in terrestrial comparison spectra, and on the other hand that the star was in motion because of its lack of coincidence. In addition, he argued that Sirius was a difficult star to use as a test case. It is always located close to the horizon where atmospheric distortion inevitably plagues the observer. Still, he deemed the paper a "very important one", and recommended it be published in the Transactions.
The physical theory supporting Huggins's line-of-sight measures was a challenge to the best of contemporary astronomers. A few years later, Airy wrote to George Stokes:
Stokes replied by citing Huggins's 1868 paper, the very paper Airy had himself refereed!
Airy had overseen a brief foray into the spectroscopic study of stars in 1863, about the same time Huggins launched his own research efforts in that area. But such observations did not fit into Airy's program of systematic and repetitive observations of celestial bodies to record changes in their positions. Celestial mechanics was not concerned with the chemical composition of stars. Nevertheless, spectroscopic study of the Sun found its way into the Greenwich routine beginning in 1872. And Airy eventually began to inquire about the utility of spectroscopy as a way of corroborating other kinds of observations being planned, such as the transit of Venus.
Greenwich published results of line of sight measurements for the first time in 1876. Thus it was that routine measurement of stellar radial velocities became an element in the research agenda of the mathematical astronomers, although spectroscopic study of any sort was given low priority until after the turn of the century.
In conceiving the method by which Doppler's principle could be applied to astronomical research, William Huggins gave astronomy an elegant and reliable research tool of broad utility. But his greatest contribution to the introduction of this new method into astronomical research lay in his successful persuasion of his contemporaries that he had, in fact, accomplished what he claimed, in spite of the overwhelming mensurational and interpretive difficulties the method entailed.
Huggins repeated his observations and revised his measures for Sirius's velocity in the line of sight in 1872. But the truth was, visual observations could never satisfy positional astronomers' need for precision. As Hermann Carl Vogel (1841-1907) pointed out years later, the slight displacement in a target star's spectrum was difficult to view due both to the faintness of the spectrum and the instability of the apparatus that produced them. Although the methods and practice of celestial photography had been improving over the years, particularly after the introduction of the dry gelatine plate in the 1870s, it was not until 1887 that Vogel, then working at Potsdam, captured the first photograph showing displacement of stellar spectral lines. Observatories began constructing apparatus solely for the purpose of detecting and measuring small displacements in star spectra to determine their radial velocity. They commissioned and purchased instruments designed to meet the exacting specifications that those measurements required, they improved their clock drives and introduced photographic equipment to capture images of star spectra reliable enough to make precise comparisons from them.
Such undertakings were beyond the capacity of private individuals. Huggins had long since moved on to other projects. As the world's major observatories became immersed in measuring the motion of many different types of celestial bodies it became increasingly clear that he had unwittingly introduced a method the ready implementation of which was beyond the ken, resources and ability of many of his fellow amateurs, thus effectively closing doors to them in favor of the mathematical astronomers.
Spectroscopy and the New Astronomy
"Astronomy, the oldest of the sciences, has more than renewed her youth. At no time in the past has she been so bright with unbounded aspirations and hopes."--William Huggins (1891)