So... I'm trying my hand at programming an n-body simulation. For those of you unfamiliar, that is a simulation where every body attracts every other body. I have something effectively like this: http://www.youtube.com/watch?v=byI9yhITDsM" But now... these straightforward n-body simulations seem to do a poor job of simulating galaxies. There's no "gas" behavior, and so most of the time whatever I simulate tends to form a globular cluster, and it tends to stay that way. Now, from what I've gathered, milky-way style systems (that tend to have most of their mass on a plane) tend to form because of gas<->gas interactions during the formation of the system. So I'm thinking about adding in a hydrodynamics simulation to more accurately describe the system. My question is: What is the pressure inside a nebula, and over what time scales would the force of this pressure be significant? I'm planning on using an SPH/compressible flow simulation.