Nitrogen and helium enhancement in the progenitor of supernova 1987A

Why the progenitor of supernova 1987A was a blue supergiant has been a fundamental question, because the occurrence of a type II supernova from such a progenitor had not been known before. From calculations of massive star evolution, we show how the progenitor star can undergo blue-red-blue evolution. By comparing these theoretical models with the abundance information from the ultraviolet observations, we conclude that the hydrogen-rich envelope of the progenitor was as massive as 7-11 solar masses and almost the whole envelope was uniformly mixed, probably because of rapid rotation. (author)