The motions are nearly radial ( σ σ r ≫ ≫ σ σ t ) for r ≫ ≫ r a and nearly isotropic ( σ σ r ≈ ≈ σ σ t ) for r ≪ ≪ r a . This is a desirable feature, since stellar systems that form via gravitational collapse have isotropic cores and radially-anisotropic envelopes.