What does $r_*$ represent in Poisson's formula for the Laplace equation on a disk?

Question

I am currently taking a course in partial differential equations, we have recently covered solving the Laplace equation on a disk. We derived Poisson's formula: $$U(r, \theta) = \frac{r_*^2-r^2}{2\pi} \int_0^{2\pi}\frac{f(\theta')}{r_*^2-2rr_* cos(\theta-\theta') + r^2} d\theta' $$
I am confused as to what the physical interpreation of $r_*$ is, can anyone explain this?