Star streams in galactic halos are long , thin , unbound structures that will be disturbed by the thousands dark matter sub-halos that are predicted to be orbiting within the main halo . A sub-halo generally induces a localized wave in the stream which often evolves into a “ z-fold ” as an initially trailing innermost part rotates faster than an initially leading outermost part . The folding , which becomes increasingly complex with time , leads to an apparent velocity dispersion increase and thickening of the stream . We measure the equivalent velocity dispersion around the local mean in the simulations , finding that it rises to about 10 km s ^ { -1 } after 5 Gyr and 20 km s ^ { -1 } after 13 Gyr . The currently available measurements of the velocity dispersion of halo star streams range from as small as 2 km s ^ { -1 } to slightly over 20 km s ^ { -1 } . The streams with velocity dispersions of 15-20 km s ^ { -1 } are compatible with what sub-halo heating would produce . A dynamical understanding of the low velocity dispersion streams depends on the time since the progenitor ’ s tidal disruption into a thin stream . If the streams are nearly as old as their stars then sub-halos can not be present with the predicted numbers and masses . However , the dynamical age of the streams can be significantly less than the stars . If the three lowest velocity streams are assigned ages of 3 Gyr , they are in conflict with the sub-halo heating . The main conclusion is that star stream heating is a powerful and simple test for sub-halo structure .