Abstract
We propose that many of the denser dust clouds represent transient stages between the initiation of gravitational collapse of ordinary interstellar clouds by passage through a spiral galactic shock and the appearance of a stellar association. The interstellar gas is not processed at too rapid a rate (1) because the time spent between galactic shocks is appreciably longer than the free-fall collapse time of a typical dust cloud, and (2) because only a small fraction of the total mass of gas of a given collapsing cloud is converted into stars – the rest is presumably redispersed by various dynamical processes which accompany the formation of massive stars. We note that the residue of atomic hydrogen observed to exist in dark dust clouds is compatible with the interpretation that they are transient objects whose lifetimes are not longer than ~ 107 yr.
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