(b. Vaassen, Netherlands, 2 January 1873; d. Wageningen, Netherlands, 28 April 1960)
Pannekoek was the son of Johannes Pannekoek and Wilhelmina Dorothea Beins. In 1903 he married Johanna Maria Nassu Noordewier, a teacher of Dutch literature. His family belonged to the rural middle class, and through his wife Pannekoek entered literary and musical circles. He was a member of the Royal Netherlands Academy of Sciences and an honorary member of the American Astronomical Society. In addition he received an honorary doctorate from Harvard University and the gold medal of the Royal Astronomical Society.
An amateur astronomer since his youth, Pannekoek studied astronomy at Leiden University. He began his career in 1895 as a geodesist and became observer at the Leiden observatory in 1898, but he grew disenchanted with the old-fashioned meridian work, which he considered of little scientific use. A teacher of Marxist theory at the Socialist party school in Berlin from 1905, and later in Bremen, he came to oppose the increasing opportunism in the German Socialist party.
At the outbreak of World War I, Pannekoek returned to the Netherlands, where he became a highschool teacher. Since leaving the observatory he had followed the progress of astronomy and had written several scientific papers. He now finished his book De wonderbouw der wereld (1920), an excellent and original historical introduction to astronomy. Long interested in Babylonian astronomy, he published several papers on this subject, while continuing his lectures on Marxism at Leiden. His nomination as a vice-director of the Leiden observatory was rejected by the minister of education; but the city of Amsterdam, not dependent on the state, appointed him lecturer in mathematics and astronomy at its municipal university, where he founded a modest but very active astronomical institute. Named professor in 1925, he was dismissed by the German occupation government in 1941.
A chance readings of Saha’s paper on ionization in stellar atmospheres prompted Pannekoek to begin work in astrophysics, and he became the founder of modern astrophysics in the Netherlands. His investigations of the structure of the galaxy that includes our solar system extended over sixty years. He made careful and detailed drawings, with isophotes, of the northern and southern Milky Way, and later repeated this work on extrafocal photographs. Improving on the work of Kapteyn, he studied our galaxy as a function of galactic longitude as well as latitude. Dissatisfied by smoothed mean values, he gave full attention to star clouds and dark nebulae. He discovered the typical groups of early stars that were later called associations.
In the area of ionization theory and the composition of stellar atmospheres, Pannekoek was the first to modify Henry Norris Russell’s work and to assume a huge preponderance of hydrogen, a view subsequently confirmed. He was also the first to apply “detailed analysis” to stellar atmospheres, taking into account the change in physical properties of the successive layers. With M. Minnaert he published the first quantitative analysis of the flash spectrum during a solar eclipse. Calling attention to the surprisingly low value of gravitation that may be deduced from the spectra of giant stars, he interpreted the brightness maxima of Cepheids as the ejection of gaseous shells.
Pannekoek’s work on the history of astronomy, culminating in his History of Astronomy, emphasized the broad lines of the evolution of the disciplines and the relations between astronomy and society. Of still wider scope is his Anthropogenesis, in which he traced the origin of man and the development into Homo sapiens.
I. Original Works. Among Pannekoek’s earlier writings are Untersuchungen über den Lichtwehsel Algols (Leiden, 1902), his diss.; De astrologie en hare betekenis voor de ontwikkeling der sterrekunde (Leiden, 1916), his insugural lecture; “Die nördliche Milchstrasse,” in Annalen van de Sterrewacht te Leiden,11 no. 3 (1920); De wonderbouw der wereld (Amsterdam, 1920); Researches on the Structure of the Universe, Publications of the astronomical Institute of the University of Amsterdam, nos. 1–2 (1924–1929); “The Ionization Formula for Atmospheres Not in thermodynamic Equilibrium,” in Bulletin of the astronomical Institutes of the Netherlands,3 (1926), 207–209; Results of Observations of the Solar Eclipse of June 29, 1927. Photometry of the Flash Spectrum (Amsterdam, 1928), written with M. Minnaert; “Die südliche Milchstrasse,” in Annalen van der Bosscha-Sterrewacht (Lembang), 2 , no. 1 (1929), 1–73; “Die Ionisation in den Atmosphären der Himmelskörper,” in Handbuch der Astrophysik, III, pt. 1 (Berlin, 1930), 256–350; “The Theoretical Contours of Absorption Lines,” in Monthly Notices of the Royal Astronomical Society,91 (1930), 139–169, 519–531; Photographische Photometric der nöordlichen Milchstrasse, Publications of the Astronomical Institute of the University of Amsterdam, no. 3 (1933); “Theoretical Colour Temperatures,” in Monthly Notices of the Royal Astronomical Society,95 (1935), 529–535; The Theoretical Intensities of Absorption Lines in Stellar Spectra, Publications of the Astronomical Institute of the University of Amsterdam, no. 4 (1935); and “Ionization and Excitation in the Upper Layers of an Atmosphere,” in Monthly Notices of the Royal Astromical Society,96 (1936), 785–793.
Later works include “The Hydrogen Lines Near the Balmer Limit,” Ibid.,98 (1938), 694–709; A Photometric study of Some Stellar Spectra, Publications of the Astronomical Institute of the University of Amsterdam, no.6, 2 pts. (1939–1946), written with G. B. van Albada; Investigations on Dark Nebulae, ibid., no. 7 (1942); “Anthropogenese, een studie over het ontstaan van den mens,” in Verhandelingen der K. Akademie van weternschapen,42, no.1 (1945), translated as Anthropogenesis, a study of the Origin of Man (Amsterdam, 1953); “The Line Spectra of Delta Cephei,” in Physica,12 (1946), 761–767; “Planetary Theories,” in Popular Astronomy,55 (1947), 422–438, and 56 (1948), 2–13 (Copernicus), 63–75 (Kepler), 177–192 (Newton), 300–312 (Laplace); Photographic photometry of the Southern Milky Way, Publications of the Astronomical Institute of the University of Amsterdam, no.9 (1949), written with D. Koelbloed; “Line Intensities in Spectra of Advanced Types,” in Publications of the Dominion Astrophysical Observatory, Victoria, B.C., 8 (1950) 141–223; De groei van ons wereldbeeld (Amsterdam–Antwerp, 1951), translated as A History of Astronomy (London, 1961); and “The Origin of Astronomy,” in Monthly Notices of the Royal Astronomical Society, 111 (1951), 347–356.
II. Secondary Literature. Two short biographies are G. B. van Albada, “Ter nagedachtenis van Prof. Pannekoek,” in Hemel en dampkring, 58 (1960), 105; and B. J. Bok, “Two Famous Dutch Astronomers,” in Sky and Telescope,20 (1960), 74–76.