Free Astronomy Magazine November-December 2014

25 SUPERNOVAE NOVEMBER-DECEMBER 2014 he mystery II b SN R ecent estimates made by astronomers suggest that a supernova explodes every second somewhere in the uni- verse. From our observing point inside the Milky Way that phenomenon does not in reality seem so frequent, but since there are at least 100 billion other galaxies, on a cos- mic scale, such a high number of events is certainly plausible. The physical mechanisms that can lead a star to explode as a supernova are more than one, and it is not easy to verify in prac- tice what the theoretical models predict. Some types of supernovae are the product of the evolution of single stars, while oth- ers originate in binary systems in which the companion star of the progenitor plays a key role in determining the type of result- ing supernova. It might be imagined that I n the background, the spiral galaxy M81. The small white arrow indi- cates the position where 21 years ago the supernova 1993J exploded. [NASA, ESA and the Hubble Heritage Team (STScI/AURA)] Left, the compari- son between be- fore and after the explosion with two images taken in October 1992 and April 1993 [Max- Planck-Institut für extra-terrestrische Physik (MPE)] distinguishing between events related to a single star and events related to a double star is relatively simple, but in reality it is not −or not always at least. There is in fact a class of supernovae, called Type II b, that in some respects appear related to both sce- narios: when they explode they show spec- troscopic characteristics typical of Type II supernovae, derived from a single massive star (more than 10 solar masses) with hy- drogen-rich outer layers; but in the de- scending phase of the light curve they look more like those of Type I b, produced by the interaction between pairs of stars not nec- essarily massive, in which the supernova’s progenitor loses much of its hydrogen be- fore exploding. The Type II b is therefore a kind of hybrid characterized by the lack of hydrogen in its light during the months fol-