Free Astronomy Magazine November-December 2014

allow to get a glimpse of the spectral signatures of the possible companion star, it is self-evident that the best candidate for a research in this direc- tion is the oldest explo- sion, hence that of SN 1993J (discovered on 28 March 1993 by the Span- ish amateur astronomer Francisco Garcia). Be- sides temporal factors, the latter is also an ex- cellent target due to its relative proximity, seeing that it exploded in M81, a well-known galaxy in the constellation Ursa Major, “just” 11 million light-years away from Earth. In fact, it is the closest supernova of Type II b observed so far by astronomers, and the pros- pect of discovering its compan- ion star has stimulated several studies in the last two decades. In having astronomers identified the progenitor of SN 1993J as a su- pergiant star of spectral type K (sur- face temperature not far from 4000 kelvin), it was predicted that as from 2006 the radiation coming from the ex- plosion residue would have become suffi- ciently weak to allow to identify the contri- bution of the companion star in the form of an excess of ultraviolet radiation. A flux of that kind, usually expected in the presence of a very hot star, had already been partially detected in 2002 and 2004, thanks to observations made with SUPERNOVAE NOVEMBER-DECEMBER 2014 28 H ere, in summary, is how a Type II b supernova forms: two giant stars orbit around the com- mon centre of gravity until one of them starts to strip away large amounts of hydrogen from the companion star that has become a red super- giant. The latter explodes leaving a residue composed mainly of helium −element that ends up characterizing also the surviving star. [NASA, ESA, and A. Feild (STScI)] An artistic full-page vision of the rem- nant of SN 1993J. [NASA, ESA, and G. Bacon (STScI)]