A Star That Would Not Die

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Supernovae,​ ​the​ ​explosions​ ​of​ ​stars,​ ​have​ ​been​ ​observed​ ​by​ ​the​ ​thousands.​ ​And in all cases,​ ​the transitory astronomical events​ ​signaled​ ​the​ ​death​ ​of​ ​those​ ​stars.​ ​

Now, astrophysicists during UC Santa Barbara and astronomers​ ​at​ ​Las​ ​Cumbres​ ​Observatory​ (LCO) ​have reported​ ​a​ ​remarkable​ ​exception​:​ ​a​ ​star​ ​that​ ​exploded​ ​multiple​ ​times​ ​over​ ​a​ ​period​ ​of more​ ​than​ ​50​ ​years.​ ​Their​ ​observations, published in a journal Nature,​ ​are​ ​challenging​ ​existing​ ​theories​ ​on​ ​these cosmic​ ​catastrophes.

An artist’s sense of a supernova.
Image credit: NASA, ESA and G. Bacon.

“This​ ​supernova​ ​breaks everything​ ​we​ ​thought​ ​we​ ​knew​ ​about​ ​how​ ​they​ ​work,” pronounced lead author Iair Arcavi, a NASA Einstein postdoctoral associate in UC Santa Barbara’s Department of Physics and during LCO. “It’s​ ​the​ ​biggest​ ​puzzle​ ​I’ve encountered​ ​in​ ​almost​ ​a​ ​decade​ ​of​ ​studying​ ​stellar​ ​explosions.”

When​ ​​iPTF14hls was​ ​discovered​ ​in​ ​September​​ ​2014​ ​by​ ​the Caltech-led Palomar​ ​Transient​ ​Factory,​ ​it​ ​looked​ ​like​ ​an​ ​ordinary​ ​supernova.​ ​But several​ ​months​ ​later, a systematic organisation noticed​ that​ ​the​ ​supernova, once faded, was​ ​growing​ ​brighter​. It was a materialisation they had never seen before.

A​ ​normal​ ​supernova​ ​rises​ ​to​ ​peak​ ​brightness​ ​and​ ​fades​ ​over​ ​100​ ​days.​ ​Supernova iPTF14hls,​ ​on​ ​the​ ​other​ ​hand,​ ​grew​ ​brighter​ ​and​ ​dimmer​ ​at​ ​least​ ​five​ ​times​ ​over​ ​three years.

When​ ​the scientists​ ​examined ​archival​ ​data,​ ​they​ ​were​ ​astonished​ ​to​ ​find evidence​ ​of​ ​an​ ​explosion​ ​in​ ​1954​ ​at​ ​the​ ​same​ ​location.​ ​Somehow this​ ​star​ survived​ ​that explosion​ ​and​ ​then exploded​ ​again​ ​in​ ​2014. In the​ ​study, a authors​ ​calculated​ ​that​ ​the​ ​exploding star​ ​was​ ​at​ ​least​ ​50​ ​times​ ​more​ ​massive than​ ​the​ ​sun​ ​and​ ​probably​ ​much​ ​larger.​ ​

“Supernova​ ​iPTF14hls​ ​may​ ​be​ ​the​ ​most massive​ ​stellar​ ​explosion​ ​ever​ ​seen,” explained co-author Lars Bildsten, executive of UCSB’s Kavli Institute for Theoretical Physics. “​​For me, a many conspicuous aspect of this supernova was a prolonged duration, something we have never seen before. It positively undetermined all of us as it only continued shining.” As partial of this effort, Bildsten worked with UC Berkeley astrophysicist Dan Kasen, exploring many probable explanations.

The progressing blast in 1954 supposing an critical clue, suggesting that iPTF14hls​ could be a initial instance of a​ ​pulsational​ ​pair​-instability supernova.​ ​Theory​ ​holds​ ​that​ a cores of ​massive​ ​stars​ ​become​ ​so​ ​hot​ ​that energy​ ​is​ ​converted​ ​into​ ​matter​ ​and​ ​antimatter.​ ​This​ ​causes​ ​an​ ​explosion​ ​that blows​ ​off​ ​the​ ​star’s outer​ ​layers​ ​and​ ​leaves​ ​the​ ​core​ ​intact. Such a​ ​process​ ​can​ ​repeat over​ ​decades​ ​before​ ​the​ ​final​ explosion​ ​and​ successive ​collapse​ ​to​ ​a​ ​black​ ​hole.

“These​ ​explosions​ ​were​ ​only​ ​expected​ ​to​ ​be​ ​seen​ ​in​ ​the​ ​early​ ​universe ​and​ ​should​ ​be extinct​ ​today,” pronounced co-author Andy Howell, a UCSB accessory expertise member who leads a supernova organisation during LCO. “This​ ​is​ ​like​ ​finding​ ​a​ ​dinosaur​ ​still​ ​alive​ ​today.​ ​If​ ​you​ ​found​ ​one,​ ​you would​ ​question​ ​whether​ ​it​ ​truly​ ​was​ ​a​ ​dinosaur.”

The pulsational​ ​pair​-instability ​theory​ ​may​ ​not​ ​fully​ ​explain​ ​all​ ​the​ ​data obtained​ ​for​ ​this​ ​event since a ​energy​ ​released​ ​by​ ​the​ ​supernova ​​is​ ​more than​ ​the​ ​theory​ ​predicts. This means​ iPTF14hls​​ ​may​ ​be​ a completely​ ​new kind of supernova.

LCO’s ​supernova​ ​group ​continues​ ​to​ ​monitor​ ​iPTF14hls,​ ​which​ ​remains​ ​bright three​ ​years​ ​after​ ​it​ ​was​ ​discovered. Their​ ​global​ ​telescope​ ​network​ ​is​ ​uniquely​ ​designed​ ​for​ ​this type​ ​of​ ​sustained​ ​observation, that has authorised researchers to observe​ ​ iPTF14hls ​every​ ​few​ ​days​ ​for several​ ​years.​ ​Such​ ​long-term​ ​consistent​ ​monitoring​ ​is​ ​essential​ ​for​ ​the​ ​study​ ​of​ ​this​ ​very unusual​ ​event.

“We​ ​could​ ​not​ ​have​ ​kept​ ​tabs​ ​on​ ​iPTF14hls​ ​for​ ​this​ ​long ​and​ ​collected​ ​data​ ​that challenges​ ​all​ ​existing​ ​supernova​ ​theories​ ​if​ ​it​ ​weren’t​ ​for​ ​the tellurian telescope network,” Arcavi​ ​said.​ ​“I can’t​ ​wait​ ​to​ ​see​ ​what​ ​we’ll​ ​find​ ​by​ ​continuing to look​ ​at​ ​the​ ​sky​ ​in​ ​the​ ​new​ ​ways​ ​that​ ​such a setup​ ​allows.”

Source: UC Santa Barbara

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