Diffuse material between the stars of the Galaxy first entered the astronomers’ awareness in the form of dark clouds whose absorption weakens and reddens the light from stars seen through them. However, in 1930 R. J. Trumpler was first able to show that, in addition to detectable dark clouds, interstellar absorption and reddening are by no means negligible anywhere in the Galaxy in regard to the photometric measurement of distances beyond a few hundred parsecs. In 1922 E. Hubble had already realized that (diffuse) galactic reflection-nebulae, like those around the Pleiades, are produced by the scattering of the light of relatively cool stars by cosmic dust-clouds, while in (diffuse) galactic emission-nebulae interstellar gas is excited by the radiation of hot stars so as to emit a line spectrum. The study of the interstellar gas followed quickly in 1926—1927. Indeed, as long ago as 1904 J. Hartmann had discovered the stationary lines of CaII which do not show any orbital displacement in the spectra of double stars. However, in 1926 A. S. Eddington from theory, and O. Struve, J. S. Plaskett and others from the observations, first developed the concept that the interstellar lines of CaII, NaI, etc., arise in a layer of gas which is partly ionized by stellar radiation, which fills the whole disk of the Galaxy, and which shares in the differential rotation. On another aspect, in 1927 I. S. Bowen succeeded in finding the long-sought identification of the “nebulium lines” in the spectra of the gas-nebulae, as the forbidden transitions in the spectra of [OII], [OIII], [NII], etc., and H. Zanstra worked out the theory of nebular radiation. It was some ten years later that astronomers recognized bydrogen as being by far the most abundant element in the interstellar gas, as it is in stellar atmospheres. O. Struve and his collaborators first remarked that the assumption of large hydrogen abundance considerably diminished the quantitative difficulties in regard to the ionization of the interstellar gas. With the help of nebular-spectrographs of great light-gathering power, they then discovered that many O and B stars, or groups of these, are surrounded by a fairly well-defined region which radiates in the red Balmer line Ha. So here interstellar hydrogen must be ionized. B. Strömgren then in 1938 developed the theory of these HII regions.
KeywordsGalactic Plane Planetary Nebula Neutral Hydrogen Dark Cloud Interstellar Matter
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