[gary]

In a 9 Nov 2017 article in The Guardian, Science Editor Ian Sample reports
on the mysterious case of iPTF14hls, a star in Ursa Major that has
undergone a supernova power explosion multiple times since 1954.
The first star ever observed to have done so.

Originally Posted by Ian Sample, The Guardian

'Zombie star' amazes astronomers by surviving multiple supernovae.

Astronomers have spotted a “zombie star” that refused to die when massive explosions that are normally considered fatal rocked the heavenly body.

The star, which lies half a billion light years away in the constellation of the Great Bear, has exploded multiple times since 1954, but may finally be on its way to the cosmic graveyard.

It is the first time astronomers have seen the same star explode over and over. Until now stellar explosions, or supernovae, have been considered singular events, the dazzling death throes of stars that have burned up all their fuel.

“This is the weirdest supernova we’ve ever seen. It’s the first time we’ve seen multiple explosions in the same place,” said Iair Arcavi, an astronomer at Las Cumbres Observatory in California. There is no known theory that explains the observation.

The curious incident came to light after astronomers detected a supernova in September 2014 with the Intermediate Palomar Transient Factory (iPTF) telescope near San Diego. The exploding star seemed unremarkable at first, but observations four months later showed that rather than dimming over time as expected, the supernova had actually become brighter.

Originally Posted by Ian Sample, The Guardian

The astronomers ruled out the most common supernova theories, but found one that explained some of the star’s odd behaviour. According to the “pulsational pair-instability model”, stars with masses of at least 100 suns can explode multiple times before dying, with each blast sending vast amounts of material into space. Now and again, material rushing away from the star can catch up with older ejected material, producing bright flashes of light as it collides. “The theory doesn’t explain everything, but it’s the only one that comes close,” said Arcavi.

“One thing we can tell from the supernova is how long ago the star exploded,” he added. “The weird thing is that even two years later, it looked like a two-month-old supernova.” It is as if the star exploded in slow motion.

Originally Posted by Ian Sample, The Guardian

More recent observations of the star suggest that the 2014 explosion may be its last, according to details published in Nature. The astronomers have switched to larger instruments to watch the supernova fade and will soon look on with the Hubble space telescope. Before long, the centre of the supernova – where a black hole now lurks – should be visible. “We definitely plan to keep an eye on this one,” Arcavi said.

Stan Woosley, director of the Center for Supernova Research at the University of California, Santa Cruz, said understanding the supernova could throw light on the evolution of the most massive stars in the universe and the birth of certain kinds of black holes. “For now the supernova offers astronomers their greatest thrill: something they do not understand,” he said.

Originally Posted by Arcavi et.al. Nature

"Energetic eruptions leading to a peculiar hydrogen-rich explosion of a massive star"

Abstract
Every supernova so far observed has been considered to be the terminal explosion of a star.

Moreover, all supernovae with absorption lines in their spectra show those lines decreasing in velocity over time, as the ejecta expand and thin, revealing slower-moving material that was previously hidden.

In addition, every supernova that exhibits the absorption lines of hydrogen has one main light-curve peak, or a plateau in luminosity, lasting approximately 100 days before declining.

Here we report observations of iPTF14hls, an event that has spectra identical to a hydrogen-rich core-collapse supernova, but characteristics that differ extensively from those of known supernovae.

The light curve has at least five peaks and remains bright for more than 600 days; the absorption lines show little to no decrease in velocity; and the radius of the line-forming region is more than an order of magnitude bigger than the radius of the photosphere derived from the continuum emission.

These characteristics are consistent with a shell of several tens of solar masses ejected by the progenitor star at supernova-level energies a few hundred days before a terminal explosion.

Another possible eruption was recorded at the same position in 1954. Multiple energetic pre-supernova eruptions are expected to occur in stars of 95 to 130 solar masses, which experience the pulsational pair instability2,3,4,5.

That model, however, does not account for the continued presence of hydrogen, or the energetics observed here.

Another mechanism for the violent ejection of mass in massive stars may be required.

Article here :-
https://www.theguardian.com/science/...ple-supernovae

Abstract and paper in Nature (requires subscription) "Energetic eruptions leading to a peculiar hydrogen-rich explosion of a massive star" by Arcavi et. al. :-
https://www.nature.com/articles/nature24030