If $\mathbf{r}=\langle x, y, z\rangle, \mathbf{a}=\left\langle a_{1}, a_{2}, a_{3}\right\rangle,$ and $\mathbf{b}=\left\langle b_{1}, b_{2}, b_{3}\right\rangle$
show that the vector equation $(\mathbf{r}-\mathbf{a}) \cdot(\mathbf{r}-\mathbf{b})=0$ represents a sphere, and find its center and radius.