Unstable Neutrinos from Gravitational Stellar Collapse and Cygnus X-3

Constraints are surveyed for massive neutrinos which decay without emitting radiation. We point out that the delayed neutrino flux signal at the time of a type-II supernova and high-energy neutrinos from Cygnus X-3 will give information on the scale parameter that causes neutrino decays far beyond the reach of laboratory experiments. The possibility is also shown that for a particular parameter value the decaying neutrinos from a supernova may increase the background events is underground detectors.