Note on the extragalactic distance scale.

Abstract

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Note on the extragalactic distance scale. Shapley, Harlow Abstract Once we have granted that the concentrated star clusters in the Magellanic Clouds are globular clusters comparable with those of the galactic system, we can use their integrated apparent magnitudes, in comparison with the integrated absolute magnitudes of our own globular clusters, to find the distances without using variable stars. This operation has been carried out and published in Harvard Reprint No. 372, 1953, where it is shown, however, that the considerable spread in the total magnitudes of globular clusters, the difficulty of measuring diffuse images satisfactorily, and the probably unequally incomplete census of clusters in the two systems, lead only to provisional results. A correction factor of approximately 2.0 was obtained. A more dependable evaluation of the distances, and of the zero point of the period-luminosity relation for classical Cepheids can be obtained by a method employed in my first survey of the distances of globular clusters, namely, by using the apparent luminosities of the brighter non- variable stars in the clusters. The absolute magnitudes of the stars of top luminosity were first determined by comparison with the variables in those systems that have cluster-type variable stars. The reduction factors have been tabulated for the twelve classes of globular clusters.1 The bright stars can be used when cluster-type variables are not available. A further advantage of their use over the use of the total magnitudes of clusters is that the measures involve only stellar images. In seven globular clusters of the Large Cloud and three in the Small Cloud the brightest thirty or forty stars have been measured, referred to sequences established photoelectrically to the fifteenth magnitude and photographically to the eighteenth. From the means, we derive for the Large Cloud a distance modulus of 19.05, with an internal mean error of less than 0.1 mag. The systematic errors, due to uncertainties in the faint magnitude sequences and in the zero point for cluster-type variables, are of course larger. The Small Cloud's clusters require further study, but a provisional modulus is 19.2. With a correction for space absorption estimated at 0.4 mag., the distance of the Large Cloud is 175,000 light years, and the correction factor for the extra- galactic distance scale is 2.2. The foregoing correction of the zero point for the classical Cepheids requires the abandonment of the zero point calculated from the space motions, such as that obtained at Mount Wilson fifteen years ago by R. E. Wilson; but H. Mineur's subsequent analysis of proper motions and radial velocities has produced a zero point for classical Cepheids which approaches that obtained from the integrated magnitudes and from the bright stars in the globular clusters of the Magellanic Clouds. I.Harv. Obs. Monograph No. 2, 160, 1930. Harvard College Observatory, Cambridge, Mass. Publication: The Astronomical Journal Pub Date: 1953 DOI: 10.1086/106930 Bibcode: 1953AJ.....58R.227S full text sources ADS |