Astrophysical fluid dynamics

Astrophysical fluid dynamics is a modern branch of astronomy involving fluid mechanics which deals with the motion of fluids, like the gases which the stars are made up of or any fluid which is found in outer space. The subject covers the fundamentals of mechanics of fluids using various equations, ranging from continuity equation, Naiver Stokes to Euler's equations of collisional fluids and the like. It is an extensive study of the physical realms of the astral bodies and their movements in space. A thorough understanding of this subject requires detailed knowledge of the equations governing fluid mechanics. Most of the applications of astrophysical fluid dynamics include dynamics of stellar systems, accretion disks, Astrophysical jets,Newtonian fluids, and the fluid dynamics of galaxies.

Astrophysical fluid dynamics deals with the application of fluid dynamics and its equations in the movement of the fluids in space. The applications are entirely different from what we usually study as all of this happens in vacuum with zero or lesser gravity.

Most of the Interstellar Medium is not at rest, but is in supersonic motion under the action of supernova explosions, stellar winds and radiation fields and the time dependent gravitational field due to spiral density waves in the stellar disc of the galaxy. Since supersonic motions almost always involve shock waves, these play a crucial role. The galaxy also contains a dynamically significant magnetic field which means that the dynamics is governed by the equations of compressible magnetohydrodynamics.

In many cases the electrical conductivity is large enough for the ideal magnetohydrodynamics to be a good approximation, but this is not true in star forming regions where the gas density is high and the degree of ionization is low.

One of the most interesting problems is that of star formation. It is known that stars form out of the Interstellar Medium and that this mostly occurs in Giant Molecular Clouds such as the Rosette Nebula for example. It has been known for a long time that an interstellar cloud can collapse due to its self-gravity if it is large enough, but in the ordinary interstellar medium, this can only happen if the cloud has a mass of several thousand solar masses - much larger than that of any star. There must therefore be some process that fragments the cloud into smaller high density clouds whose masses are in the same range as that of stars. Self-gravity cannot do this, but it turns out that there are processes that do this if the magnetic pressure is much larger than the thermal pressure, as it is in Giant Molecular Clouds. These processes rely on the interaction of magnetohydrodynamic waves with a thermal instability. A magnetohydrodynamic wave in a medium in which the magnetic pressure is much larger than the thermal pressure can produce dense regions, but they cannot by themselves make the density high enough for self-gravity to act. However, the gas in star forming regions is heated by cosmic rays and is cooled by radiative processes. The net result is that gas in a thermal equilibrium state in which heating balances cooling can exist in three different phases at the same pressure: a warm phase with a low density, an unstable phase with intermediate density and a cold phase at low temperature. An increase in pressure, due to a supernova or a spiral density wave can flip the gas from the warm phase into the unstable phase and a Magnetohydrodynamic wave can then produce dense fragments in the cold phase whose self-gravity is strong enough for them to collapse to form stars .

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