A Star That Would Not Die

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

Now, astrophysicists at 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 the journal Nature,​ ​are​ ​challenging​ ​existing​ ​theories​ ​on​ ​these cosmic​ ​catastrophes.

“This​ ​supernova​ ​breaks everything​ ​we​ ​thought​ ​we​ ​knew​ ​about​ ​how​ ​they​ ​work,” said lead author Iair Arcavi, a NASA Einstein postdoctoral fellow in UC Santa Barbara’s Department of Physics and at 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, the scientific team noticed​ that​ ​the​ ​supernova, once faded, was​ ​growing​ ​brighter​. It was a phenomenon 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, the 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, director of UCSB’s Kavli Institute for Theoretical Physics. “​​For me, the most remarkable aspect of this supernova was its long duration, something we have never seen before. It certainly puzzled all of us as it just continued shining.” As part of this effort, Bildsten worked with UC Berkeley astrophysicist Dan Kasen, exploring many possible explanations.

The earlier explosion in 1954 provided an important clue, suggesting that iPTF14hls​ could be the first example of a​ ​pulsational​ ​pair​-instability supernova.​ ​Theory​ ​holds​ ​that​ the 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​ subsequent ​collapse​ ​to​ ​a​ ​black​ ​hole.

“These​ ​explosions​ ​were​ ​only​ ​expected​ ​to​ ​be​ ​seen​ ​in​ ​the​ ​early​ ​universe ​and​ ​should​ ​be extinct​ ​today,” said co-author Andy Howell, a UCSB adjunct faculty member who leads the supernova group at 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 because the ​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, which has allowed 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 global 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.”

This research was supported by the National Science Foundation and the Gordon and Betty Moore Foundation.

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