Hydrodynamics of the solar wind expansion - Why is the solar wind supersonic?

A pedagogical introduction to the classical hydrodynamic steady state solut ions for flows in a spherically symmetric atmosphere is presented along wit h a detailed analysis of the stability of such flows and a discussion of th e Parker/Bondi phase diagram of solutions in the Mach number - stellar radi us plane. This leads naturally to the scenario presented by Velli (1994) fo r wind-accretion transitions, which helps to explain results from numerical simulations in many contexts and gives a more comprehensive answer to the question of why the solar wind and other steady state flows in spherical sy metry must quite generally become supersonic.