We consider a nonlinear Schrödinger (NLS) equation with any positive power nonlinearity on a star graph Γ (N half-lines glued at the common vertex) with a δ interaction at the vertex. The strength of the interaction is defined by a fixed value α ∈ R. In the recent works of Adami et al., it was shown that for α = 0 the NLS equation on Γ admits the unique symmetric (with respect to permutation of edges) standing wave and that all other possible standing waves are nonsymmetric. Also, it was proved for α < 0 that, in the NLS equation with a subcritical power-type nonlinearity, the unique symmetric standing wave is orbitally stable.In this paper, we analyze stability of standing waves for both α < 0 and α > 0. By extending the Sturm theory to Schrödinger operators on the star graph, we give the explicit count of the Morse and degeneracy indices for each standing wave. For α < 0, we prove that all nonsymmetric standing waves in the NLS equation with any positive power nonlinearity are orbitally unstable. For α > 0, we prove the orbital instability of all standing waves.