Isolated Stroemgren spheres as a source of galactic H alpha emission The isolated H alpha emission features reported by Reynolds et al. areidentified with Stroemgren spheres surrounding observed O stars; theroot mean square electron densities and Stroemgren radii are calculatedfrom the observed emission measure and stellar type. The density has anaverage value of 2 per cu cm but increases to 3.3 per cu cm for starswithin 200 pc of the galactic plane. An analysis of the Catalogue ofGalactic O Stars shows that enhanced H alpha emission is observed aroundall known O stars. The diffuse component of H alpha emission is what onewould expect from the line-of-sight overlapping of isolated Stroemgrenspheres surrounding B stars and appears to correlate with features onextinction maps, as derived. There is no compelling reason to suggestany large-scale intercloud ionization from H alpha data alone.
