  We establish the time decay rates of the solution to the Cauchy problem for
the compressible Navier-Stokes-Poisson system via a refined pure energy method.
In particular, the optimal decay rates of the higher-order spatial derivatives
of the solution are obtained. The $\Dot{H}^{-s}$($0\le s<3/2$) negative Sobolev
norms are shown to be preserved along time evolution and enhance the decay
rates. As a corollary, we also obtain the usual $L^p$--$L^2$($1<p\le 2$) type
of the optimal decay rates. Compared to the compressible Navier-Stokes system
and the compressible irrotational Euler-Poisson system, our results imply that
both the dispersion effect of the electric field and the viscous dissipation
contribute to enhance the decay rate of the density. Our proof is based on a
family of scaled energy estimates with minimum derivative counts and
interpolations among them without linear decay analysis.
