Let $C_1$ and $C_2$ be circles defined by:
\[
(x - 10)^2 + y^2 = 36
\]
and
\[
(x + 15)^2 + y^2 = 81
\]
respectively. Find the length of the shortest line segment $\overline{PQ}$ that is tangent to $C_1$ at $P$ and to $C_2$ at $Q$.

Let's think step by step and output the final answer within \boxed{}.
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