6. For the hyperbola $\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$, one of its foci is $F_{1}$, the vertices are $A_{1}$ and $A_{2}$, and $P$ is any point on the hyperbola. Then the two circles with diameters $P F_{1}$ and $A_{1} A_{2}$, respectively, must ( ).
(A) intersect
(B) be tangent
(C) be separate
(D) any of the above situations is possible
6. (B).

Let $P$ be a point on the hyperbola, $F_{1}$ be the left focus, and take the midpoint $M$ of $P F_{1}$. Then $|O M|$ is the distance between the centers of the circles, $\frac{\left|P F_{1}\right|}{2}$ and $a$ are the radii of the two circles. In $\triangle P F_{1} F_{2}$, the midline $|O M|=\frac{1}{2}\left|P F_{2}\right|$.

When $P$ is on the right branch of the hyperbola, $\left|P F_{1}\right|=\left|P F_{2}\right|+2 a$, then $|O M|=\frac{\left|P F_{1}\right|}{2}-a$. It can be seen that the circle with $P F_{1}$ as its diameter is internally tangent to the circle with $A_{1} A_{2}$ as its diameter;

When $P$ is on the left branch of the hyperbola, by $\left|P F_{2}\right|-\left|P F_{1}\right|=2 a$ we know that the two circles are externally tangent.