Gas Dynamics

Abstract

Matter in the Universe is highly dispersed on average; yet we believe there is nowhere strict vacuum. The different components of gas and radiation try to fill the maximum available volume each, in obeyance of the second law of thermodynamics. They can thereby either penetrate each other, or — if immiscible — form separate domains in mutual pressure equilibrium. In quasi-static situations, pressure equilibrium is expected on length scales which can be traversed at sound speed since the last excitation: we speak of a typical pressure in the (outer) Milky Way, for example. This equilibrium pressure is permanently disturbed by the blowing of stellar winds — which generate reduced densities (near-vacua) at large radii — by the heating via stellar radiation (Strömgren spheres), by nova and supernova explosions as well as by bipolar flows. Supersonic motions lead to shock waves where they impact on slower substrata. The thus-created discontinuity surfaces can be smooth, when stable, or rough (rugged, filamentary), when unstable.