About a century ago astronomy had a momentous year. Two discoveries in 1925 were to reshape cosmology and astrophysics. From Mt. Wilson Observatory, Edwin Hubble officially announced the discovery of Cepheid variables in the Andromeda nebula, triggering his all-out search for Cepheids in other galaxies that by 1929 provided evidence of an expanding universe. The other, equally paradigm-changing revelation in 1925 did not come from an established scientist at a well-known observatory, but from page 184 of a 200-page PhD thesis written by an unknown student at Harvard named Cecilia Payne: she found that stars were made mainly of hydrogen and helium. Although described as “the most brilliant PhD thesis ever written in astronomy,” her finding spelled trouble: her discovery flew in the face of renowned Princeton astronomer Henry Norris Russell’s firm opinion that the elements in stellar atmospheres mirrored terrestrial abundances – a fact she largely confirmed but for the glaring exceptions of hydrogen and helium. She found hydrogen in particular to be a million times more abundant in the stars than on the earth. The stage was thus set for conflict: an unknown female graduate student was prepared to publish a groundbreaking thesis contradicting a venerable giant in American astronomy whose approval she needed for her degree. To get a sense of the issues at stake, let’s look back to this time in astronomy when the science of astrophysics was a new and incredibly promising field [1].
Earth, Stars, and Atoms
In the late nineteenth century, Englishman William Huggins compared the dark spectral lines from the stars with the bright rays emitted by terrestrial metals and gases when put to the flame in the laboratory. In 1865 he identified seventy-eight metallic lines in the spectrum of Betelgeuse [2]. Nine elements, among them iron, sodium, calcium, and magnesium hydrogen made their telltale marks in the spectrum of Aldebaran. It was not long before astronomers were able to identify a host of chemical elements in the stars and even approximate their relative abundances from analysis of the widths and other characteristics of spectral lines. Henry Norris Russell in 1914 noted a resemblance between the chemical composition of the earth’s crust and the elements in the solar spectrum: “if the earth’s crust should be raised to the temperature of the sun’s atmosphere,” he wrote, “it would give a very similar absorption spectrum.” [3]. But identifying the actual abundances of elements of stellar atmospheres was a trickier business on the cutting edge of this new physics of atoms and stars. It turned out that the most powerful for investigating the immensity of stars and distances lay within the tiny innards of the atoms themselves.
A strange world uniting atoms and stars arose from the work of German physicist Max Planck, who in the year Cecelia Payne was born had a revolutionary insight. He considered that electromagnetic radiation had to be absorbed or emitted by matter only in discrete packets. Planck’s insight became the foundation of quantum theory, which in the early 20th century took physics by storm. Danish physicist Niels Bohr decided to try it out on the simple hydrogen atom. It was pictured as a miniature solar system with one tiny electron orbiting a proton. But this was no ordinary solar system. In Bohr’s model of atoms, electrons must obey two basic rules: only certain discrete orbits are allowed, and electrons cannot exist between orbits: they can only jump. It seemed very strange, and still does, but it worked to enable the exact prediction of hydrogen’s spectral lines. (The Bohr rules are also explained with examples in my May, 2022 article Astrophysics for People Not in a Hurry at https://www.douglasmacdougal.com/post/astrophysics-for-people-not-in-a-hurry)
If we follow a path inward to this miniature atomic realm, we’ll observe the mysterious game-like logic that follows from Bohr’s simple rules. If an electron absorbs a photon – a discrete packet of electromagnetic radiation from some external source – the electron will become “excited” and jump to a higher energy level (i.e., orbit), like a ball launched up in a pachinko machine. Higher energy photons absorbed will excite the electron to jump to still higher discrete orbits farther from the pull of the nucleus. An electron may after a time also “relax’ and return to a lower state, by single or multiple steps, banging pins in our metaphor as it bounces down, giving off its absorbed photons as it descends. When an electron jumps up or down to different energy orbits, the absorbed or emitted photon will have an energy (frequency) exactly equal to the difference of the energy levels in the two orbits: energy is thus neatly conserved. All this is why spectra have discrete lines. The photographic spectrum is a record of photons emitted (the bright “emission” lines) or absorbed (dark “absorption” lines) at specific wavelengths of energy – indicated by the spacing of the lines on the spectrum – which can be calculated. Collectively, each line of a spectral series corresponds to atoms in a discrete state of excitation. And different elements have uniquely different spectral fingerprints.
But what happens when we keep adding energy, such that the hydrogen electron, for example, gets excited to states of even higher energy? As one ventures farther out from the nucleus, the differences between energy levels within the atom become less and less (as in the computer-generated image below of hydrogen orbitals) until at the limit the electron is poised on the threshold of breaking loose from the proton’s nagging pull. Spectral lines appear jammed closer and closer together, so that with the slightest addition of more energy, at the atom’s “ionization potential,” the outermost electron will be evicted from its atomic home – the little ball ejected from the machine altogether – and the atom will be “ionized,” and left to wander alone with a positive charge [4]. Its spectrum too will be transformed. For an atom heavier than hydrogen, its lines will resemble the preceding element in the periodic system (so ionized helium, for example will resemble neutral hydrogen). This fact, unknown before, caused great confusion in the early interpretation of spectra.
Now these rules helped make sense of stellar spectra [5]. The originally puzzling ultraviolet spectrum of the white star Vega, for example, was astronomers’ first acquaintance with the ultraviolet spectrum of hydrogen in the stars and its harmonic character [6]. Remarkably, the pattern mirrored the hydrogen line sequence already discovered in the laboratory by Swiss schoolteacher of mathematics Johann Jakob Balmer (1825-1898). “Balmer’s Law,” as it was first known, would become a sure test of the presence or absence of hydrogen in the sun and distant stars. Cecilia Payne noted that (as of 1923) twenty-seven Balmer lines were identified in ζ Tauri, the most recorded in the spectrum of any star [7].
A determined scientist
Cecelia Payne came of age during this exciting time. Payne was born in 1900 in Wendover England, descended from a family of scholars and historians. From an early age she devoured books of all kinds in her family’s great library. Her autobiography, The Dyer’s Hand, is a warm and lively account of her life [8]. Payne had a voracious eye for everything in nature. As a child, she enjoyed collecting and classifying plants, a skill that would become useful later when analyzing spectra. Though she had vivid memories of seeing a brilliant meteor, Halley’s Comet, and (particularly) the Daylight Comet of 1910, “I had yet to realize that the heavenly bodies were within my reach. But the chemical elements were the “stuff of the world,” one “that was later to expand into a Universe.” [9]. Over time and with the help of a mentor who recognized her passion for science, she absorbed everything she could about botany, mathematics, physics, and astronomy. Eventually she enrolled in Newnham College at the University of Cambridge. There she encountered Arthur Stanley Eddington, fresh from his celebrated solar eclipse trip to the Island of Pŕincipe off the Coast of Africa.
A. S. Eddington, the foremost astronomer and physicist at Cambridge of the time, would have a decisive influence on the life of Cecilia Payne. Eddington had gone to Pŕincipe in May 1919 to test a prediction of Einstein’s General Theory of relativity that light from stars should be bent by the sun, a phenomenon, it was supposed, that should be visible during a total solar eclipse. Eddington was to present his results at a winter gathering in the Great Hall of Cambridge’s Trinity College. Eddington confirmed that the gravitational deflection of stars his expedition found agreed closely with Einstein’s calculations. The news made Einstein world-famous. For Cecilia Payne, the result “was a complete transformation of my world picture.” [10]. She would be “dedicated to physical science, forever.” According to science historian Marcia Bartusiak, Payne’s experience “amounted to a religious conversion.” [11]. The next day Payne told school authorities that she was going to read physics.
It was not easy for a woman to study physics at Cambridge; one needed a thick skin to survive. Payne was the only woman to attend Ernest Rutherford’s advanced course in physics. Regulations required that women sit by themselves in the front row. Gazing at her pointedly, Rutherford would begin each lecture in a booming voice, “Ladies and Gentlemen,” at which witticism the males in the room would applause thunderously and stamp their feet [12]. But she found a kindred and appreciative spirit in the soft-spoken Eddington. At an accidental encounter at the observatory, she impulsively confessed her ambition to become an astronomer.” Eddington replied, “I can see no insuperable objection,” words which she later said, “opened the doors of the heavens to me.” [13]. But Eddington and her other professors knew that no matter how gifted and passionate she might be, the opportunity for a woman to become an astronomer in England was nil. Yet here again, another spell-binding lecture by another astronomer, this one in London, rescued her for the next great journey of her life.
Payne and Shapley
The amiable Harlow Shapely was the newly appointed director of Harvard College Observatory. For her his talk was memorable: “He spoke with extraordinary directness, conveyed the reality of the cosmic picture in master strokes. Here was a man who walked with the stars and spoke of them as familiar friends. They were brought within reach; one could almost touch them.” [14]. When introduced to Shapely there, she came immediately to the point: “I should like to come and work under you.” [15]. He replied that he’d be delighted, and after graduation she departed for America in the Fall of 1923.
Only two years later, easily established as the brightest light at the Harvard College Observatory, Cecilia Payne completed her seminal 1925 Ph.D. thesis: Stellar Atmospheres ~A Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars [16]. Her thesis goal was to determine the abundances of elements in the stars using the most advanced techniques in physics. This meant mining data, and Harvard College Observatory had the greatest collection of photographic spectra in the world. E.C. Pickering and A. J. Cannon had meticulously examined and classified more than 100,000 stars, publishing their data in the Henry Draper Memorial Catalog [17]. The survey showed that among all the swarms of stars, there are only a handful of temperature-determined spectral types (originally, about seven: O, B, A, F, G, K, M) forming a more or less continuous series [18]. The apparently ordered gradation of stellar spectra arrayed across types, from the hot O stars to the cool M stars, was itself a beautiful discovery. Yet there were still mysteries in the lines: Helium lines are strong in O and B stars and nowhere else; hydrogen lines are strong in A stars then slope steadily downward toward cool stars; calcium lines peak in K stars [19]. Cecilia Payne had to solve the mystery of these overlapping rises and falls in spectral line intensities. In its solution lay the key to deciphering the secret of stellar compositions. Wonderful as it was, this grand vault of empirical data still lacked theory explaining exactly why things appeared as they did.
A breakthrough had occurred in 1920 with the work of the brilliant Indian astrophysicist Meghnad Saha, whom Cecilia Payne described as “a great and simple personality.” Saha emphasized the role of ionization in the appearance of spectral lines and how lines can appear strong or hidden depending on the condition of what is known as the reversing layer of the star’s outer atmosphere.
In her thesis Payne explained how the continuous radiation from a star’s photosphere (its visible layer) – which forms the background light of a star’s spectrum – is intercepted by this blanket of gases existing at perhaps a ten-thousandth of an atmosphere of pressure [22]. Photons rising up from the photosphere are selectively caught by atoms in the reversing layer (in the exact process of absorption and excitation envisioned by Bohr), which subtract those slices of radiation from the background light. The taken-out parts show up as the dark lines in the bright spectrum (“reversing” the bright to dark). The elements we detect in the spectra are thus the elements present in the reversing layer. Payne adeptly showed that Saha’s equations, along with the studies of Ralph Fowler and Edward Milne, quantified how temperature, pressure, and also density in a star’s atmosphere will tell which atoms in its reversing layer will be ionized and where they’ll appear and disappear in the spectrum. For example, the so-called H and K lines of calcium are due not to neutral calcium but to the singly ionized atom of calcium [23]. Of course, our own atmosphere absorbs much of the starlight too, and “this fact alone,” Saha reminds us, “tends to give decided preference to certain elements to the exclusion of others.” [24].
“The physical meaning of the appearance and disappearance of lines,” Saha had proclaimed, “now becomes apparent.” [20]. With Saha’s tools, Payne could now predict the relative abundances of the elements in the atmosphere of a star – that is, not just what elements were in stars but how much of each of them there were. It was through her mastery of every aspect of this complex astrophysical problem that Payne had been convinced that stellar abundances quite closely paralleled terrestrial abundances except for hydrogen and helium (as noted in the third column of the table below, from Payne, showing logarithmic values (powers of ten) the extraordiary abundances of those elements in relation to the others on the table).
Trouble with Russell
Having already published some of her research piecewise in journals, Payne sent her draft thesis to Henry Norris Russell in December 1924. Russell was Shapley’s former teacher and mentor at Princeton, and an acknowledged expert in incorporating cutting-edge physics into astronomy. On January 14, Russell sent a handwritten letter to Payne, that she had some “very striking results which appear to me, in general, to be remarkably consistent [with my own findings].” [25]. But then he went on in words that must have been a crushing disappointment to Payne: “There remains one very much more serious discrepancy, namely, that for hydrogen, helium and oxygen. Here I am convinced that there is something seriously wrong with the present theory. It is clearly impossible that hydrogen should be a million times more abundant than the metals.” After that, Payne changed her thesis. She would later be proven correct in a grand way, but given Russell’s formidable influence, she was induced to recant her heresy: her dramatic conclusion about hydrogen in stars was now sadly “spurious” and “almost certainly not real.” [26]. It is painful to read the words of her revised thesis (as in the image below) which bear the stamp of Russell’s censorship. Even though she had used the same methods for all elements studied, she declared the abundance of hydrogen in stars to have an “impossibly high value.” [27]. Thus revised, Payne’s thesis was accepted May 1, 1925, with Shapley’s rather downbeat forward cautioning that “the interpretation of stellar spectra from the standpoint of thermal ionization is new and the methods employed are as yet relatively primitive.” [28].
Afterword
After Russell had a chance to study Cecilia Payne’s published thesis with care, he wrote to Shapely in August of that year calling it, “the best doctoral thesis I ever read.” His tone was enthusiastic: “As I read it over – I have eaten it up since I got it yesterday – I am especially impressed with the wide grasp of the subject, the clarity of the style, and the value of Miss Payne’s own results.” Overall, he declared her findings “very nice indeed.” [29].
What Cecilia Payne had in fact had done masterfully was to confirm the overall chemical homogeneity of the universe. She showed how spectral differences in the stars resulted mainly from physical conditions rather than from abundance variations, and for many heavier elements, the compositions in the outer layer of the stars do parallel those of the earth [30]. With respect to hydrogen, however, Russell himself soon did a startling about-face. In his own paper written in 1929, Russell found an “incredibly great” abundance of hydrogen in the sun and concluded that the outer portions of stellar atmospheres “must be almost pure hydrogen.” [31]. Yet he made no mention of Payne’s earlier discovery of the hydrogen anomaly or his role in suppressing it. He did acknowledge toward the end of his 71-page paper how his own work, using different methods, agreed with hers, which he found “very gratifying.” [32]. A giant star, he proclaimed, evidently had an outer atmosphere of nearly pure hydrogen, “with hardly more than a smell of metallic vapors in it.” [33].
For a long while after, Russell was credited with the discovery of the astonishing abundance of hydrogen in stars. For example, his former student Donald Menzel in 1933 wrote, “It is difficult to escape the conclusion that hydrogen is the predominant element throughout the universe,” citing only Russell’s 1929 paper, with no mention at all of Cecelia Payne [34].
As most now realize, progress for women in science has been hard to come by, painfully slow. In material terms Payne described her career as “a tale of low salary, lack of status, slow advancement … a case of survival, not of the fittest, but of the most doggedly persistent.” [35]. Yet she went on, “rewarded by the beauty of the scenery, towards an unexpected goal.” Her advice to young people reflected both her struggles and rewards:
Do not undertake a scientific career in quest of fame or money. There are easier and better ways to reach them. Undertake it only if nothing else will satisfy you; for nothing else is probably what you will receive. Your reward will be the widening of the horizon as you climb. And if you achieve that reward you will ask no other. [36]
NOTES
I am grateful to author and science historian Professor Marcia Bartusiak of MIT for her thoughtful review of this article and helpful corrections and comments.
[1] In his classic 1926 work, The Internal Constitution of the Stars, English scientist A.S. Eddington described astrophysics as “the study of the mechanical and physical conditions in the deep interior of the stars,” in order to “throw light on external phenomena accessible to observation.” Agnes Clerke called it the “new astronomy,” distinguishing it from the art of calculating the motions of celestial bodies governed by the law of gravity. The latter, Newton’s triumph, is what she called “practical astronomy.” Wonderful as it is – “the most perfect of the sciences” – it tells us close to nothing about the constituent nature of the things it studies. Clerke (1903), 2-3. Payne described astrophysics broadly as the “application of physics to the Universe on the widest possible scale.” Haramundanis, 159. She said, “Nothing seemed impossible in those early days; we were going to understand everything tomorrow.”
[2] Clerke (1902), 382-383 for the spectral history in this paragraph.
[3] Russell (1914), 794.
[4] Two electrons stripped off means the atom is “doubly ionized.” Three off means “trebly ionized.”
[5] With these and other telltale chemical fingerprints, stars could now be classified according to their spectra. Most stars seemed to have dark spectral lines (called absorption lines) at particular places in an otherwise continuous rainbow-like spectrum. Stars seemed to display various types of individual spectra, revealing different patterns of lines, yet maintained strong similarities among a handful of types – just as the large population of diverse people on Earth have relatively few discrete combinations of eye and hair colors. A few stars (including the Wolf-Rayet type) and many nebulae and planetary nebulae seemed to be dominated by bright spectral lines, called emission lines, evidence of energized atoms in gaseous environments. To make sense of all these differences in stars, Annie J. Cannon and her team at Harvard led by Professor Pickering spent years poring over the spectra of thousands of stars on photographic plates. The parallel early 20th Century development of quantum physics and deepening understanding of the interaction between matter and radiation began to fit together with what the stars were saying to Ms. Cannon in the subtle bar-coded language of the spectra.
[6] As described by historian of 19th century science Agnes M. Clerke: “A group of broad dark lines intersected it [Vega’s spectrum], arranged at intervals diminishing regularly upward and falling into a rhythmical succession with the visible hydrogen lines.” (1902), 383.
[7] Payne Stellar Atmospheres, 58.
[8] Payne’s autobiography, The Dyer’s Hand is printed in full in Haramundanis, 70-234.
[9] Haramundanis, 99.
[10] Ibid, 117.
[11] Bartusiak (1993), 36. For an excellent and succinct account see also Bartusiak (2004), 250-256.
[12] Haramundanis, 118.
[13] Ibid, 120.
[14] Ibid, 124.
[15] Ibid.
[16] Payne Stellar Atmospheres.
[17] For Payne’s lively account of the massive labor entailed in assembling the catalog, see Haramundanis, 148-150.
[18] A description of the early realization of this remarkable continuity is in Saha, 203.
[19] An excellent introduction to this aspect of theory is Kahler, Chapter 6, and particularly the chart on the dependency of ionic spectra and temperature in Fig. 6.10.
[22] Payne Stellar Atmospheres, Chapter IV.
[23] Eddington cited this fact of calcium as an example of how Saha’s theory helped disentangle the confusion between neutral and ionized atoms in stellar spectra. Eddington, The Internal Constitution of Stars, 345.
[24] Saha, 231.
[20] Saha, 229. Added Eddington: “By Saha’s theory all the details of stellar spectra become quantitatively connected with the temperature and pressure in the reversing layer. [21]” Eddington, The Internal Constitution of Stars, 345.
[25] The quotes from H. N. Russell are from Gingrich.
[26] Payne Stellar Atmospheres, 186-188.
[27] Payne Stellar Atmospheres, 56.
[28] Payne Stellar Atmospheres, Editor’s Forward.
[29] Gingrich.
[30] Relying on Payne’s work, English astronomer A. S. Eddington wrote in 1926, "It is rather probable that the chemical elements have much the same relative abundance in the stars that they have on earth. All the evidence is consistent with this view…" Eddington, Atoms and Stars, 59. His popular book summarized the key points of his principal work, The Internal Constitution of Stars. There (pp. 369-370), he incorporated Payne’s table of abundances from her thesis, saying, “Miss Payne considers that there is a fairly close parallelism shown between stellar abundance and terrestrial abundance.” Eddington did not mention Payne’s findings about the spectacularly great abundances of hydrogen or helium, likely because Payne herself had already removed those elements from the table in response to recent criticisms from Russell. See Payne Astrophysical Data, where her edited table was published in March 1925 before her thesis was accepted. She said (p.197): “Hydrogen and helium are omitted from the table. The stellar abundance deduced for these elements is improbably high, and is almost certainly not real.”
[31] Russell (1929), 60 & 69.
[32] Ibid, 55.
[33] Ibid, 69.
[34] Menzel, 228.
[35] Haramundanis, 227. Payne’s slow progress in institutional recognition was measured by the decade. Her first official Harvard appointment came in 1938. Her courses were finally listed in the university’s catalog in 1945. She became a full professor in 1956, “the first woman at Harvard to attain that rank – perhaps twenty years after man of her achievements would have earned the position.” Bartusiak (1993), 38.
[36] Haramundanis, 227.
REFERENCES
Bartusiak, Marcia “The Stuff of Stars.” The Sciences 33, no. 5 (1993): 34-40.
_________. 2004. Archives of the Universe. New York: Pantheon Books.
Clerke, Agnes M. 2003 (1902). A Popular History of Astronomy During the Nineteenth Century. Decorah, IA: Sattre Press.
_________. (1903). Problems in Astrophysics. London: Adam & Charles Black.
Eddington, A.S. 1926. The Internal Constitution of Stars. Cambridge: Cambridge University Press.
_________. 1926. Atoms and Stars. New Haven: Yale University Press.
Gingrich, Owen. “Payne-Gaposchkin, Cecilia (1900-1980).” Harvard Square Library. https://www.harvardsquarelibrary.org/biographies/cecilia-payne-gaposchkin-3/
Haramundanis, Katherine. 1996 (1984) Cecelia Payne-Gaposchkin: An Autobiography and Other Recollections. Cambridge: Cambridge University Press.
Kaler, James K. 2006. The Cambridge Encyclopedia of Stars. Cambridge: Cambridge University Press.
Menzel, Donald H. “Hydrogen Abundance and the Constitution of the Giant Planets.” Publications of the Astronomical Society of the Pacific 42, no. 248 (August 1930): 228-232. https://www.jstor.org/stable/40668745.
Payne, Cecilia. 1926. Stellar Atmospheres. Stellar Atmospheres ~A Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars. Cambridge: Harvard Observatory Monograph 1.https://articles.adsabs.harvard.edu//full/1925PhDT.........1P/0000001,006.html.
_________. “Astrophysical Data Bearing on the Relative Abundance of the Elements.” Proceedings of the National Academy of Sciences of the United States of America 11, no. 3 (March 15, 1925): 192-198. https://www.jstor.org/stable/84871 [cited as Astrophysical Data].
Russell, H.N. “The Solar Spectrum in the Earth's Crust.” Science 39, no. 1013 (May 1914): 791-794.
_________. “On the Composition of the Sun’s Atmosphere.” Astrophysical Journal 70 (1929): 11-82. Reprinted in Contributions from the Mount Wilson Observatory, No. 383.
Saha, M.N. “On a Physical Theory of Stellar Spectra.” Journal of Astrophysical Astronomy 15 (1994): 203-239. Reproduced from the Proceedings of the Royal Society (Series A) 99 (1921): 135-153.
© 2024 Douglas MacDougal
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