Summary

After having studied this lecture, you should be able to

• to derive an estimate for the stellar dynamical relaxation time in a spherical system of $N$ stars.
• explain why we can describe the dynamics of elliptical galaxies in terms of fluid equations.
• explain the difference in dynamics between an elliptical (hot stellar system) and a spiral (cold, rotating stellar system)
• explain what is a continuity equation, and derive the collisionless Boltzmann equation, Eq.(8.20), stating the underlying assumptions.
• explain that the ellipticity of ellipticals does not results from rotation, but from the anisotropy of the pressure tensor.