The rate
at which matter accretes is therefore regulated by internal torques and radiative processes. Only if
everywhere, with
being the outside mass supply, can the accretion process be stationary. Since the
internally determined
may change due to instabilities, limit cycles, etc., an occurrence of a really long-term steady
accretion flow should be considered a fine-tuned eigenstate. Note, too, that in many astrophysical situations
is also
genuinely variable.