The star that exploded – at least – twice

The first iPTF14hls explosion (Image courtesy Arcavi et ​al. ​2017, ​Nature)
The first iPTF14hls explosion (Image courtesy Arcavi et ​al. ​2017, ​Nature)

An article published in the journal “Nature” describes the discovery of a supernova that seems to have exploded more than once. Called iPTF14hls, it was identified in 2014 by the Intermediate Palomar Transient Factory but in that position a supernova had already been recorded in 1954. It could be the first case discovered of a type of supernova called a pulsational pair-instability supernova, in which a star is so hot and massive that it produces in its core antimatter that causes periodic explosions.

Until now, the supernovas of all the types observed were final events that led to the death of the exploding stars. The existence of a zombie white dwarf described in an article published in the magazine “Science” is already an event and in the case of type II supernovas a neutron star or a black hole remains. The supernova iPTF14hls was originally cataloged as Type II-P, characterized by a brightness period of around 100 days but when it kept on shining astronomers began to become curious.

To study the anomaly, the astronomers went looking for information in the archive of the observation of that area and discovered that a supernova was already detected in that exact spot in 1954. Iair Arcavi, a​ ​NASA​ ​Einstein​ ​postdoctoral​ ​fellow​ ​at Las Cumbres Observatory​ (​LCO​) ​and​ ​the​ ​University​ ​of​ ​California Santa​ ​Barbara and the first author of the article, stated that this supernova goes against anything astronomers think they know about how they work.

The researchers calculated that the star that generated the supernova iPTF14hls had a mass at least 50 times the Sun’s and probably much higher. It could be the most massive stellar explosion ever seen and that huge mass might be the reason behind its abnormal behavior. It could be a pulsational pair-instability supernova, a possibility that was only theoretical, at least so far.

This type of supernova concerns exactly very massive stars so hot that in their core the energy is converted into matter and antimatter that cause an explosion that ejects the star’s outer layers and leaves its core intact. This process can be repeated over the course of decades until the star is consumed in one last “ordinary” supernova that leaves a dead core that collapses into a black hole.

This hypothesis has some problems, in fact Andy Howell of LCO noted that they expected that kind of supernova to occur when the universe was very young and extremely massive stars formed but iPTF14hls didn’t seem so far away to be from that time. For this reason, he compared it to the discovery of a living dinosaur that would force to wonder if it was really a dinosaur.

The supernova iPTF14hls’ light grew and dimmed several times after it was discovered in September 2014 and a number of telescopes were used to observe it after the 1954 explosion was discovered. It keeps on shining and astronomers keep on studying it in the hope of to establish its nature with certainty.

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