  Constraints on the lifetime of the heavy sterile neutrino, that was proposed
as a possible interpretation of the KARMEN anomaly, are derived from primordial
nucleosynthesis and SN 1987A. Together with the recent experimental bounds on
the nu_s lifetime, SN 1987A completely excludes this interpretation.
Nucleosynthesis arguments permit a narrow window for the lifetime in the
interval 0.1-0.2 sec. If nu_s possesses an anomalous interaction with nucleons,
the SN bounds may not apply, while the nucleosynthesis ones would remain valid.
