IT JUST keeps on banging. A dying star was caught mid-explosion in September 2014 and still isn鈥檛 done. It has lasted 10 times longer than any other supernova of its type that we鈥檝e seen.
Most supernovae brighten once as they explode and then fade into obscurity. But supernova iPTF14hls has had at least five peaks in brightness since at the University of California, Santa Barbara, and his colleagues began watching it (Nature, ). It finally seems to be fading, he says.
鈥淚t refused to go gentle into that good night. It just kept on exploding and exploding,鈥� says at the University of California, Santa Cruz. Evidence of one of the star鈥檚 past explosions comes through in its light, which reveals a shell of material around the star.
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The light from iPTF14hls has a signature identical to common type II-P supernovae, in which a massive star鈥檚 core collapses and becomes a neutron star, with the resulting shock wave blowing away its hydrogen-rich outer layers. Their bright flash lasts about 100 days before fading. This supernova seems to be acting a little like a type II-P in slow motion. After 600 days of exploding, it looks like a type II-P supernova after 60 days. It is also radiating several times more energy than any type II-P supernova we鈥檝e ever seen.
Arcavi and his team are trying to find a mathematical model that fits the star鈥檚 behaviour, but none has matched up yet.
Woosley and Arcavi agree the most promising model is pulsational pair instability. The centres of very large stars 鈥� about 95 to 130 times the size of the sun 鈥� can reach over a billion degrees Celsius. At these temperatures, gamma rays in the core make pairs of electrons and their antimatter counterparts, positrons.
The radiation pressure from gamma rays stops a star from collapsing under gravity. When the rays turn into particles, the star begins to fall in on itself, igniting an explosion that can blow off the star鈥檚 outer layer but leave the rest intact to begin the process over again.
This could account for iPTF14hls鈥檚 many explosions and for a possible pre-supernova eruption observed in the same spot in 1954. It would also result in multiple shells of expanding material like the one we鈥檝e already seen.
But it鈥檚 not an exact match. Pulsational pair instability supernovae don鈥檛 produce the amount of energy or the mix of elements observed in iPTF14hls.
鈥淚t鈥檚 definitely in the top five weirdest supernovae,鈥� says at the College of Charleston in South Carolina. 鈥淚t鈥檚 too early to tell exactly what it is, but it鈥檚 definitely a weirdo.鈥�
Because the star has exploded several times, it may not even fit the definition of a supernova. 鈥淵ou think of a supernova as a death of a star, and you think of death as something that only happens once. It鈥檚 a very different kind of supernova that can die repeatedly,鈥� says Woosley.
鈥淚t鈥檚 too early to tell what it is, but it鈥檚 definitely a weirdo. It鈥檚 in the top five weirdest supernovae鈥�
This article appeared in print under the headline 鈥淥dd supernova keeps dying again and again鈥�
