1. PANNEKOEK'S GALAXY.
- Author
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van der Kruit, Pieter C.
- Subjects
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DISTRIBUTION of stars , *ROTATION of galaxies , *MILKY Way , *STARS , *STELLAR atmospheres , *ZODIAC - Abstract
Antonie ('Anton') Pannekoek (1873 -1960), the founder of the Astronomical Institute of the University of Amsterdam, is remembered as one of the initiators of the field of stellar atmospheres. A second, maybe equally significant part of his research and legacy did not concern stellar, but Galactic astronomy. He was awarded an honorary doctorate by Harvard University and the Gold Medal of the Royal Astronomical Society (UK) for his total research effort. From his long-term interest in viewing and mapping the Milky Way he became convinced that the sidereal system was built up of clouds of stars, condensations of various sizes in a smooth but low-density stratum of stars. In addition, there were dark clouds together with streaks with little or no extinction in between them. So, he took the opposite view of Jacobus C. Kapteyn, concerning the stellar distribution, which the latter regarded in first approximation to be smooth and uniform in longitude so that star counts as a function of apparent magnitude depended primarily on latitude. Pannekoek's research into the structure of the stellar system consisted of various parts. He first looked at bright clouds of the Milky Way, such as in Cygnus, and assuming these were isolated structures he estimated their distance from their contribution to star counts by stars far out in the bright tail of the luminosity function. He found values of tens of kpc, which would mean their distribution was similar in extent to that of Shapley's globular cluster system. Later he had to reduce his distance by a factor over two, and later still to admit he had to retract the method altogether. He developed a rigorous method of estimating distances of dark clouds from modeling star counts off and on the cloud, preceding Wolf's quick and crude method that became very widely applied. He should have received more credit for this. During the 1920s it became clear that extinction was a major effect and that the Galaxy rotated around a distant center. Pannekoek unsuccessfully looked for the large central mass, that Oort found had to be in the center of rotation in the Galaxy, by estimating masses of some bright star clouds in the Milky Way. He also pioneered the mapping of the local structure in the Galaxy from spectroscopic parallaxes, using the fact--following Kapteyn's study of 'helium stars'--that for B- and A-stars and K-giants the intrinsic dispersion in absolute magnitude is small. The distributions of different spectral types are substantially different. He tried without success to explain Kapteyn's Star Streams from the gravitational pull of the irregular distribution of matter in the solar neighborhood. As a teenager he was fascinated by the appearance of the Milky Way and he started producing sketches of the Milky Way by drawing curves of equal (surface) brightness or in modern terms isophotes. In the course of his career this resulted in isophotal maps of both the northern and the southern Milky Way, first from visual observations, later from photographic surface photometry using out-of-focus exposures. The care and patience with which he visually produced his maps is admirable. For the photographic work he enlisted help from Wolf in Heidelberg and Vote at Lembang, who both provided extensive plate material. Unlike the care that has been employed for photographic surface photometry in more recent times, a wide range of photographic emulsions and developers was used and calibration of the characteristic curve was done on separate plates "... of the same type, if possible!" (my italics). In this paper I compare Pannekoek's resulting isophotal maps with detailed photographic surface photometry of the southern Milky Way in the 1980s and 1990s by the group in Bochum and to the almost all-sky mapping by the Pioneer 10 spacecraft, free of zodiacal light, from beyond the asteroid belt. This shows Pannekoek's visual and photographic maps to be surprisingly accurate, in spite of all the non-uniformity in observers for the first category and in exposure, emulsion, etc., with magnitude scale and zero point errors of a few tenths of a magnitude, except in the southern Coalsack, where he quoted a surface brightness that is much too faint. The legacy of Pannekoek in the area of Galactic research consists of his mapping of the structure of the nearby part of the Galaxy, the distances of dark clouds, and isophotal maps of the Milky Way. His other contributions turned out inconclusive or wrong as a result of his conviction, resulting from his many years of observing and mapping the Milky Way, that the nearby distribution is characterized primarily by more or less isolated clouds of stars and by dust restricted to isolated dark clouds and streaks. In Appendix 1 I show that the widespread notion that the center of the Galaxy is located in the constellation Sagittarius is not so compelling as usually thought and could with equal justification be placed in Scorpius. Appendix 2 provides short curriculae vitae of a few Dutch PhDs that have been referred to in this paper. [ABSTRACT FROM AUTHOR]
- Published
- 2024
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