Pannekoek's Galaxy

Abstract

Antonie (Anton) Pannekoek (1873-1960) is remembered as one of the initiators of the field of stellar atmospheres. A second part of his research concerned Galactic astronomy. He was convinced that the sidereal system was built up of clouds of stars in a smooth, low-density stratum. In addition there were dark clouds together with streaks with little or no extinction in between. Pannekoek looked at bright star clouds and estimated their distance from their contribution to star counts. He found values of tens of kpc, which would mean their distribution was similar in extent to that of Shapleys globular cluster system. Later he had to reduce his distance by a factor over two, and later still retract the method. 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 dirty method. He should have received more credit for this. He started isophotal maps of the northern and southern Milky Way, first from visual observations, later from photographic surface photometry using out-of-focus exposures. I compare Pannekoeks maps with detailed photographic surface photometry of the south 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 Panneloeks maps to be surprisingly accurate. 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.