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Published online by Cambridge University Press: 14 August 2015
The term accretion originally referred, in astronomical contexts, to the capture of mass by stars, and, later, to mass capture by other centers of gravitational force. As such, the process has not proved to be of general importance, in spite of early hopes. However, there are other aspects of the problem which may yet prove worthy of attention in interstellar gas dynamics. In particular, the effects of stars, galaxies, or even clusters of galaxies, on ambient matter streaming by them may be detectable, directly or indirectly, and it is on such possible effects that I shall concentrate here. These effects are related to the original accretion problem but may be thought of separately; nevertheless, I retain the use of the term accretion to refer to all aspects of the motions induced in an ambient medium by a gravitating object.