Let $A$ be a C*-algebra. It seems that for given an *-representation $\pi$ of $A$, there is unique central projection $z_{\pi}$ in $A^{**}$ such that $\pi$ is just (unitary equivalent to) $\rho_z$ where $$\rho_z:A\to A^{**} : a\to az$$
It means that there is a one to one correspondence between Rep($A$), the equivalence class of *-representations of $A$, and $Z(A^{**})$, the set of all central projectionas in $A^{**}$