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N6946-BH1, story of a failed supernova

– News of March 24, 2019

N6946-BH1 is a star that had a rather singular end. It was a very big star, about 25 times more massive than the sun. It has been observed in the NGC 6946 galaxy located about 22 million light-years away from us. We can think that such a massive star ends its life in supernova. Indeed, in 2009 the luminosity of N6946-BH1 suddenly increased to a million times brighter than the sun. It lasted a few months before the star disappeared completely. It may seem very bright but in reality it’s too little for a supernova.

N6946-BH1 is thought to have missed its supernova explosion. It would have collapsed directly into a black hole without going through an explosive phase. While searching the infrared area with the Spitzer Space Telescope, a small shape was observed which is probably due to the black hole accretion disk that formed. But we do not yet know how often this type of death of a star occurs.

The team that followed the death of N6946-BH1 estimates that 10% to 30% of massive stars could dead as failed supernovas. This would perfectly explain why we observe fewer supernovas than we would expect given the massive number of stars. These failed supernovas could also be phenomena that give rise to black holes of several tens of solar masses such as those observed in gravitational waves by LIGO.

Typically, a supernova expels most of the outer layers of the star, thus reducing the size of the central black hole. In the case of N6946-BH1, more of the mass of the star is found in the final black hole, which allows to form more massive black holes. We will probably improve our knowledge of this phenomenon by observing other failed supernovae.

Picture by L. Calçada, ESO

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