  Central spin systems, in which a {\it central} spin is singled out and
interacts nonlocally with several {\it bath} spins, are paradigmatic models for
nitrogen-vacancy centers and quantum dots. They show complex emergent dynamics
and stationary phenomena which, despite the collective nature of their
interaction, are still largely not understood. Here, we derive exact results on
the emergent behavior of open quantum central spin systems. The latter
crucially depends on the scaling of the interaction strength with the bath
size. For scalings with the inverse square root of the bath size (typical of
one-to-many interactions), the system behaves, in the thermodynamic limit, as
an open quantum Jaynes-Cummings model, whose bosonic mode encodes the quantum
fluctuations of the bath spins. In this case, non-Gaussian correlations are
dynamically generated and persist at stationarity. For scalings with the
inverse bath size, the emergent dynamics is instead of mean-field type. Our
work provides a fundamental understanding of the different dynamical regimes of
central spin systems and a simple theory for efficiently exploring their
nonequilibrium behavior. Our findings may become relevant for developing fully
quantum descriptions of many-body solid-state devices and their applications.
