•Jeans (1902) tackled gravitational instability in a static medium, dR/dt = 0 in eq. 11.24 we can get the dispersion relation for waves of the form ∆=∆0 exp i (k∙r – ωt): •This relation describes oscillations or instability depending on the sign of the right hand side.  If positive, oscillatory as pressure gradient supports the region for wavelengths less than the critical Jean’s wavelength: