Free Astronomy Magazine November-December 2014

26 SUPERNOVAE lowing the explosion an element that grad- ually becomes more and more rarefied (and hence less detectable) due to the expansion of the supernova remnant. Concurrently, in- stead, helium begins to abound, which is a clear indication that the progenitor had ear- lier lost its outer layers where hydrogen dominates. This does not happen in a classic Type II supernova, where hydrogen remains easily detectable for a long time. Why this difference? What is the mechanism that strips away from the progeni- tors of Type II b their outer hy- drogen layers? Astronomers have started ask- ing themselves these que- stions since the earliest discov- ery of that variant, and the possible answers came from the theoretical front with the construction of models agree- ing on a scenario where a com- panion star subtracts hydro- gen from the progenitor near the super- nova. When the progenitor reaches the end of its lifetime, it expands as a red super- giant and its hydrogen-rich outer layers reach the companion star (less massive and thus still in “good health”), which siphons them through the intersection of the two Roche lobes (the gravitational spheres of in- fluence of the two stars). A n artistic re- presentation of the binary sys- tem at the origin of SN 1993J, where a red su- pergiant that has reached the end of its lifetime ex- plodes while the companion star is still stripping away its outer hydrogen layers. Left, a video showing in more details that same scenario. [ESA/Hubble (M. Kornmesser & L. L. Christensen)]