Chandrasekhar–Friedman–Schutz instability or shortly CFS instability refers to an instability that can occur in rapidly rotating stars with which the instability arises for cases where the gravitational radiation reaction is unable to cope with the change in angular momentum associated with the perturbations. The instability was discovered by Subrahmanyan Chandrasekhar in 1970 and later a simple intuitive explanation for the instability was provided by John L. Friedman and Bernard F. Schutz. Specifically, the instability arises when a non-axisymmetric perturbation mode that appears co-rotating in the inertial frame (from which gravitational waves are observed), is in fact is counter-rotating with respect to the rotating star.