  Recently, it has been realized that in some systems internal space rotation
can induce energy amplification for scattering waves, similar to rotation in
real space. Particularly, it has been shown that energy extraction is possible
for a Q-ball, a stationary non-topological soliton that is coherently rotating
in its field space. In this paper, we generalize the analysis to the case of
boson stars, and show that the same energy extraction mechanism still works for
boson stars.
