The significance of He+ for astronomy and astrochemistry
Astrophysicist John Cox speaking at an International Astronomical Union meeting. Preceded by Sergei Zhevakin in the Soviet Union, in the 1950s and ‘60s Cox advanced the idea that helium ionization is a key driver of the pulsation of Cepheid variable stars.
Photograph by David DeVorkin, courtesy AIP Emilio Segrè Visual Archives.
In her 1925 PhD thesis, written at the Harvard College Observatory, Cecilia Payne provided this assessment of the significance of the test of Niels Bohr’s model afforded by the measurement of the Pickering series:1
The detection and resolution of the alternate components of [the Pickering] series, which fall very near to the Balmer lines of hydrogen in the spectra of the hottest stars, and the consequent derivation of the Rydberg constant for helium[+] … represents an astrophysical contribution to pure physics which is of the highest importance.
However, the spectral properties of He+ also proved to be of great consequence in astronomy itself, following the discovery of the enabling role of He+ in the pulsation of the Cepheids and of other variable stars populating chiefly the instability strip of the Herzsprung–Russell diagram: In the dimmest (and hottest) phase of the Cepheid cycle, the radiation emanating from the stellar interior is scattered and absorbed by He+ in the stellar envelope, leading to the envelope’s expansion and, thus, cooling. At the coolest point of the cycle, the passage of the thermal radiation from the stellar interior through the envelope is the least impeded and so the star shines the brightest. The workings of variable stars could thus be likened to a Carnot cycle with He+ playing the role of the intake/exhaust valve.2
Moreover, in the 1990s, the chemical properties of the He+ ion would be recognized as key to astrochemistry, through the ion’s role in the kinetically rather than thermodynamically controlled chemistry of the interstellar medium. In the 1970s, laboratory experiments conducted in conjunction with quantum-chemical calculations showed that while He+ does not react with the most abundant interstellar molecule, hydrogen (H2), it does react more than willingly with the second most abundant molecule, carbon monoxide (CO),
He+ + CO → He + C+ + O
yielding C+ in essentially every He+ + CO collision.3 This enhances the concentration of the C+ ion by the He/CO abundance ratio, i.e., by about a factor of a thousand. The abundant C+ ion reacts only reluctantly with the prevalent H2 molecules and so is spared for its avid reactions with methane and acetylene, giving rise to a reaction sequence responsible for the formation of many organic compounds. As Dudley Herschbach put it:4
The paradoxical irony is that the mutual distaste of the simplest inorganic species, He and H2 gives rise to the proliferation of complex organic molecules in the cold interstellar clouds.
Edward Pickering’s approach of collecting hosts of quality data in the face of ignorance resulted in many unexpected payoffs. Williamina Fleming’s sifting through the data uncovered precious nuggets that keep dazzling us to this day.
References
- Cecilia H. Payne, “Stellar atmospheres: A contribution to the observational study of high temperature in the reversing layers of stars,” Harvard Observatory Monographs, 1925
- Arthur S. Eddington, “The pulsation theory of Cepheid variables,” The Observatory, 40, 1917, 290–293; Erika Vitense, “Der Aufbau der Sternatmosphären IV. Teil, kontinuierliche Absorption und Streuung als Funktion von Druck und Temperatur,” Zeitschrift für Astrophysik, 28, 1951, 81–112; Sergei A. Zhevakin, “On the theory of the Cepheids,” Russian Astronomical Journal, 30, 1953, 161; John P. Cox, “On second helium ionization as a cause of pulsational instability in stars,” Astrophysical Journal, 138, 1963, 487–536, doi:10.1086/147661.
- Bruce H. Mahan, “Electronic structure and chemical dynamics,” Accounts of Chemical Research, 8, 1975, 55–61, doi:10.1021/ar50086a002. William A. Klemperer, “Some spectroscopic reminiscences,” Annual Review of Physical Chemistry, 46, no. 1, 1995, 1–28, doi:10.1146/annurev.pc.46.100195.000245; William A. Klemperer, “Interstellar chemistry,” Proceedings of the National Academy of Sciences, 103, no. 33, 2006, 12232–12234, doi:10.1073/pnas.0605352103.
- Dudley R. Herschbach, “Chemical physics: Molecular clouds, clusters, and corrals,” in More Things in Heaven and Earth: A Celebration of Physics at the Millennium, Volume II, edited by Benjamin Bederson (Springer, 2006), pp. 693–705, doi:10.1007/978-1-4612-1512-7_451999.
Cite this resource
Bretislav Friedrich and Maria McEachern, “Williamina Fleming,” Women in the History of Quantum Physics collection, American Institute of Physics, 2026, https://www.aip.org/history/williamina-fleming.