NOTES ON DIFFUSE GALACTIC NEBULAE

Abstract

This article represents a resumption of the work on diffuse galactic nebulosities (mainly of the reflection type) that was conducted by members of the Yerkes and McDonald Observatories about a quarter of a century ago. Since that time important advances have been made by a number of other astronomers (see, for example, Bok1). The magnificent Palomar photographs of the sky obtained with the 48-inch Schmidt telescope now make it possible to extend the earlier investigations. Since each field has been photographed separately on blueand red-sensitive emulsions, it is possible to distinguish between blue and red nebulae. But the red photographs do not, per se, allow the viewer to distinguish between reddish reflection nebulae and Ha-emitting gaseous nebulae. Of special interest are the nebulosities in and near Taurus and those forming the great complex in Scorpius-Ophiuchus. Both regions are rich in dark, obscuring clouds, and both are relatively close to the sun. Both are associated with Gould's belt, rather than with the average galactic equator. The Scorpius-Ophiuchus complex is located approximately in the direction of the galactic center and at an average distance of about 200 parsecs. The Taurus group of nebulae lies in the opposite part of the sky, at an average distance of 100 to 140 parsecs. However, A. Adolf sson (quoted by Bok) suspects, on the basis of spectral types and colors for the brighter stars, that there is a considerable range in distance for these same dark nebulae, with the nearest dense dark nebulae no farther than 50 parsecs away. Similar distance estimates by W. Gotz and W. Wenzel, at Sonneberg, seem to indicate that the nearest boundaries of the Taurus dark nebulae may, in fact, be near the sun. There is no such indication with regard to the ScorpiusOphiuchus complex. These very prominent dark clouds are apparently quite sharply bounded, at a minimum distance of perhaps 100 parsecs.