Using the tube model we calculate the relaxation dynamics of density fluctuations in a concentrated solution of star polymers, in which the stars are assumed to relax by arm retraction. We find the surprising result that, in the long wavelength limit, the dynamics have a simple power law dependence on the arm length unlike the exponential dependence that one usually associates with star polymers. Consequently, the contribution of branch point diffusion to relaxation dynamics is negligible compared to the effect of arm retraction. We then apply our results to early time phase separation kinetics in star/linear and star/star blends, using the Cahn-Hilliard-Cook theory. This calculation requires a rederivation of Brochard’s theory for the mutual diffusion coefficient, in terms of polymer fluxes instead of polymer velocities. Following Brochard we assume incompressibility and use the concept of a tube velocity which is common to all polymers. We pay particular attention to the dependence of the growth rate of the fastest growing wavelength fluctuation on the molecular weight of the polymers.