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. Recent ultraviolet observations have provided crucial evidence that bears on this problem. Emission lines of CNO elements show that the expansion velocity of the emitting gas is <30 km s-1, and that the abundance ratios of N/C and N/O are larger than the solar ratios by factors of ∼30 and 10, respectively1,2, indicating that the ultraviolet emissions come from circumstellar material which had been processed by hydrogen burning in the interior and ejected into space when the progenitor was a red supergiant. The progenitor of SN1987A must therefore have evolved first to a red supergiant and then back to the blue. From calculations of massive star evolution, we show how the 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 M⊙ and almost the whole envelope was uniformly mixed, probably because of rapid rotation.
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