supernova 1987A(SN 1987A) The first naked-eye supernova since 1604 (see Kepler's star). It occurred in the Large Magellanic Cloud (LMC) on Feb. 23 1987, and reached a visual magnitude of 2.9 by May 18 1987. It belonged to the class of type II supernovae, but because its progenitor (Sanduleak –69°202) was a blue supergiant rather than a red supergiant, as is typical for type II supernovae, its peak luminosity was significantly lower and its spectral evolution much more rapid than expected. Initially it was suggested that the unusual color of the progenitor had to do with the lower metallicity of the LMC. However, it now appears more likely that this is a direct consequence of the binary nature of the progenitor system. In this model, the system initially consisted of two massive stars that merged some 30 000 years before the explosion to form a single, but unusual, star at the time of the explosion. This would also help explain the complex nebula surrounding the supernova, detected with the NTT and HST, consisting of three rings (interlocking in projection). The supernova is located at the center of the inner ring (which has a radius of 0.7 light-years). The outer rings are displaced above and below the plane of the inner ring at roughly twice the distance. This material was ejected by the supernova progenitor, presumably as a by-product of the merger event. The supernova blast wave is about to collide with the inner ring and will ultimately destroy it.
The detection of a burst of neutrinos from the supernova has provided direct confirmation of the long-held belief that type II supernovae are triggered by the collapse of stellar cores. In addition, the detection of gamma-ray lines from the radioactive decay of cobalt (56Co), the decay product of nickel (56Ni), to iron (56Fe) has provided evidence for explosive nucleosynthesis, i.e. the synthesis of iron-peak elements and other heavy nuclei during the supernova explosion.