Quantum fields in compact stars can be amplified due to a semiclassical instability. This generic feature of scalar fields coupled to curvature may affect the birth and the equilibrium structure of relativistic stars. We point out that the semiclassi
cal instability has a classical counterpart, which occurs exactly in the same region of the parameter space. For negative values of the coupling parameter the instability is equivalent to the well-known spontaneous scalarization effect: the plausible end-state of the instability is a static, asymptotically flat equilibrium configuration with nonzero expectation value for the quantum fields, which is compatible with experiments in the weak-field regime and energetically favored over stellar solutions in general relativity. For positive values of the coupling parameter the new configurations are energetically disfavored, and the end-point of the instability remains an open and interesting issue. The vacuum instability may provide a natural mechanism to produce spontaneous scalarization, leading to new experimental opportunities to probe the nature of vacuum energy via astrophysical observations of compact stars.
