Hydrodynamics and the Morphology of Nebulae

Extraglactic nebulae, in the first approximation, may be regarded as gravitating continuous "viscous fluids," most of them endowed with total angular momenta different from zero. [1] In every nebula two coexisting partial systems must be considered: (1) a stellar systems and (2) a system formed by interstellar gases and dust particles. Hydrodynamically, the two partial systems have radically different characteristics inasmuch as the gaseous systems are characterized by much larger Reynolds numbers than the stellar systems. The Reynolds numbers of the stellar systems lie in the approximate range 0 < R < 10,000. The flow of stars in some nebulae therefore is "laminar", whereas in others it may be "turbulent." Nebulae in which the two types of flow coexist show interesting structural subdivisions.