Interstellar Matter and Star Formation

Abstract

The matter which is finely distributed between the stars of the Milky Way at first came to the attention of astronomers in the form of dark clouds, which weaken and redden the light of those stars which are behind them, due to absorption and scattering. But it was only in 1930 that R. J. Trumpler was able to show that even outside the recognizable dark clouds, interstellar extinction and reddening are by no means negligible in the photometric determination of distances of a few hundred parsec throughout the Milky Way Galaxy. Already in 1922, E. Hubble had recognized that the galactic (diffuse) reflection nebulae (like the one which surrounds the Pleiades, for example) are due to scattering of the light from relatively cool stars in cosmic dust clouds, while in the galactic (diffuse) emission nebulae, interstellar gas is excited bythe radiation from hot stars and therefore emits line spectra. Following his observations, the investigation of the interstellar gas quickly gained momentum in the years 1926/27. The “stationary” Call lines had already been discovered in 1904 by J. Hartmann; they occur in the spectra of binary stars but do not show Doppler shifts corresponding to the orbital motion. Only in 1926 was an explanation developed, theoretically by A.S. Eddington, and based on observations by O. Struve, J. S. Plaskett, and others: the interstellar Ca II, Na I,... lines are produced in a gas layer which is partially ionized by the stellar radiation. This gas layer fills the entire disk of the Milky Way and participates in its rotation. About the same time, in 1927, I. S. Bowen succeeded in making the long-sought identification of the “nebulium lines” in the spectra of gaseous nebulae, finding that they are due to forbidden transitions in the spectra of [O II], [0 III], [N II],...; and H. Zanstra developed the theory of nebular luminescence. Only about ten years later was it recognized that in the interstellar gas, as in stellar atmospheres, hydrogen is the strongly predominant constituent. O. Struve and his coworkers discovered with the aid of their nebula spectrograph, which had great light-gathering power, that many O and B stars are surrounded by well-defined regions which fluoresce in the red hydrogen recombination line, Hα. Here, the interstellar hydrogen must thus be ionized. The theory of these H II regions was formulated in 1938 by B. Strömgren.