The Spectrum of He$^+$ as a Proving Ground for Bohr's Model of the Atom: A Legacy of Williamina Fleming's Astrophysical Discovery

Abstract

In 1896, Edward Charles Pickering (1846-1919), Director of the Harvard College Observatory (HCO), reported in a trio of publications the observation of "peculiar spectra" of the southern star $\zeta$ Puppis, which he attributed to an "element not yet found in other stars or on earth." Supported by laboratory spectra obtained by Alfred Fowler (1868-1940), Niels Bohr (1885-1962) showed in 1913 that this "element" was in fact ionized helium, He$^+$. Its spectrum has become known as the Pickering Series, even though Pickering credited Williamina Fleming (1857-1911) for the discovery. Fleming was one of HCO's "computers" and the future Curator of the Astronomical Photographic Glass Plate Collection. The series of spectral lines associated with Pickering's name played a unique role on the path to quantum mechanics by serving as a proving ground for Bohr's model of the atom. Our examination of the discovery of the Pickering series relied on the records held at the Center for Astrophysics $\vert$ Harvard \& Smithsonian (the successor institution to HCO), especially the Notebooks and Diaries of Williamina Fleming and others as well as on the Center's Glass Plate Collection. Glimpses of the "peculiar sociology" of a research institution, half of whose staff were women employed on grossly unequal terms with men, are given in the course of the narrative.