  It has recently been shown that a significant slowdown of many stars can be
attributed to the emergence of a strong magnetic field within the radiative
region, where heat is transferred through radiation in a stably stratified
layer. Here, we describe how this transition can be understood as a subcritical
bifurcation to small-scale turbulence in linearly stable flows. The turbulence
is sustained by a nonlinear mean-field dynamo and can be observed down to
relatively small differential rotation, arbitrarily far from the linear onset
of any hydrodynamic instability. In this regime, turbulent fluctuations provide
diffusivity-free transfer of angular momentum that increases the transport
generated by the magnetic field triggering the turbulence. Finally, we present
a simple nonlinear model that captures this scenario and can be used as a
general description of the transition to turbulence in astrophysical flows, as
long as it involves a competition between a large-scale dynamo, and a
small-scale magnetic instability.
