The origin of the slow relaxation observed in dynamic laser light scattering (LLS) measurements of semidilute polymer solutions remains controversial. Even though the chain reptation is invisible in dynamic LLS, the slow relaxation was still attributed to the reptation induced density fluctuation by those who believe that there is no other slow relaxation except the reptation. To clarify such a point, we purposely studied dynamics of semidilute solutions of narrowly distributed 4-arm star polystyrene (M-w = 1.1 x 10(5) g/mol and M-w/M-n = 1.02) chains in cyclohexane, wherein the chains are not entangled but topologically constrained. Our results reveal that there still exists a slow relaxation mode besides a fast one that is related to the diffusion of short chain segments ("blobs") in the semidilute regime, clearly excluding its possible reptation origin. The average diffusion coefficient (< D-f >) and scattering intensity (< I-f >) related to the fast mode are scaled to the polymer concentration (C) as (D-f) similar to C-0.5 and < I-f > similar to C-0.77, different from those for linear chains. For the slow mode, our results show that as C increases, the relaxation slows down but the correlation length becomes shorter, apparently contradicting each other, presumably because individual "blobs" become smaller but more correlated with a correlation length of similar to 10(2) nm, reflected in an increase of its related scattering intensity. Since the correlation length of the scattering objects related to the slow mode is comparable to the observation length used in LLS, the average line width of the slow relaxation measured in dynamic LLS contains a mixture of internal motions and diffusion before the blobs become strongly correlated at higher concentrations.