How do we know what kinds of stars explode as supernovae in distant galaxies? A new in-depth study has attempted to tackle this thorny problem by searching for the suspected progenitor stars of type Ib/c supernovae. This supernovae are the collapse of masses stars that seem to possess no hydrogen. There are stars like this, called Wolf-Rayet stars, which are some of the most massive stars in the Universe (above 20 solar masses), and have strong stellar winds that blow away their outer envelope of hydrogen. This exposes the inner layers of carbon and nitrogen.
During a session of lectures at NAM today, entitled 'Explosions in the Distant Universe', Joanne Bibby of the University of Sheffield presented results of a survey of 11 galaxies all within a distance of 33 million light years that are being searched in detail for Wolf-Rayet stars. For one galaxy in particular, NGC 7793, which is 13 million light years away 52 Wolf-Rayet stars were found, 27 being Wolf-Rayet stars with exposed nitrogen that are believed to be the progenitors of Type Ib supernovae, and 25 with exposed carbon layers that make Type Ic supernovae. In total Bibby estimates that 80 percent of all nitrogen Wolf-Rayet stars, and 90 percent of all carbon Wolf-Rayet stars, have now been found in NGC 7793. The upshot of this is that the next time there is a Type Ib/c supernova in NGC 7743, the chances are that we will know which star it is that has exploded, allowing us to put more constraints on our understanding of which stars explode, and why. Image: ESO.