  We study the nonlinear Schr\"odinger equation with an arbitrary real
potential $V(x)\in (L^1+L^\infty)(\Gamma)$ on a star graph $\Gamma$. At the
vertex an interaction occurs described by the generalized Kirchhoff condition
with strength $-\gamma<0$. We show the existence of ground states
$\varphi_{\omega}(x)$ as minimizers of the action functional on the Nehari
manifold under additional negativity and decay conditions on $V(x)$. Moreover,
for $V(x)=-\dfrac{\beta}{x^\alpha}$, in the supercritical case, we prove that
the standing waves $e^{i\omega t}\varphi_{\omega}(x)$ are orbitally unstable in
$H^{1}(\Gamma)$ when $\omega$ is large enough. Analogous result holds for an
arbitrary $\gamma\in\mathbb{R}$ when the standing waves have symmetric profile.
