Is pumping lemma for regular languages "closed" against Kleene star?
In order to show the pumping property of a string like $x_1x_2 \dots x_n$, where each $x_i\in L$ you can distinguish two cases. Either $x_1$ is "short", which means you can pump $x_1$ completely and stay within $L^*$, or $x_1$ is "long" and you can pump "inside" $x_1$.