Stellar encore: Dying star keeps coming back big time: Marcia Dunn, AP, 8 November 2017
CAPE CANAVERAL, Fla. (AP) — Death definitely becomes this star.

Astronomers reported Wednesday on a massive, distant star that exploded in 2014 — and also, apparently back in 1954. This is one supernova that refuses to bite the cosmic dust, confounding scientists who thought they knew how dying stars ticked.

The oft-erupting star is 500 million light-years away — one light-year is equal to 5.9 trillion miles (9.5 trillion kilometers) — in the direction of the Big Bear constellation. It was discovered in 2014 and, at the time, resembled your basic supernova that was getting fainter.

But a few months later, astronomers at the California-based Las Cumbres Observatory saw it getting brighter. They’ve seen it grow faint, then bright, then faint again five times. They’ve even found past evidence of an explosion 60 years earlier at the same spot.

Supernovas typically fade over 100 days. This one is still going strong after 1,000 days, although it’s gradually fading.

The finding was published Wednesday in the journal Nature. Nature .

“It’s very surprising and very exciting,” said astrophysicist Iair Arcavi of the University of California, Santa Barbara who led the study. “We thought we’ve seen everything there is to see in supernovae after seeing so many of them, but you always get surprised by the universe. This one just really blew away everything we thought we understood about them.”

The supernova — officially known as iPTF14hls — is believed to have once been a star up to 100 times more massive than our sun. It could well be the biggest stellar explosion ever observed, which might explain its death-defying peculiarity.

It could be multiple explosions occurring so frequently that they run into one another or perhaps a single explosion that repeatedly gets brighter and fainter, though scientists don’t know exactly how this happens.

One possibility is that this star was so massive, and its core so hot, that an explosion blew away the outer layers and left the center intact enough to repeat the entire process. But this pulsating star theory still doesn’t explain everything about this supernova, Arcavi said.

Harvard University’s astronomy chairman, Avi Loeb, who was not involved in the study, speculates a black hole or magnetar — a neutron star with a strong magnetic field — might be at the center of this never-before-seen behavior. Further monitoring may better explain what’s going on, he said.

Las Cumbres, a global network of robotic telescopes, continues to keep watch.

Scientists do not know whether this particular supernova is unique; it appears rare since no others have been detected.

“We could actually have missed plenty of them because it kind of masquerades as a normal supernova if you only look at it once,” Arcavi said.

Nothing lasts forever — not even this super supernova.

“Eventually, this star will go out at some point,” Arcavi said. “I mean, energy has to run out eventually.”
illustration of a supernova

GHOST IN THE SHELL  What looked like an ordinary supernova, shown in this artist’s illustration, might be the result of a single star exploding at least three times, blowing off expanding shells of gas each time.: image via NASA, ESA, G. Bacon/STSci, 8 November 2017

Energetic eruptions leading to a peculiar hydrogen-rich explosion of a massive star: I. Arcavi et al., Nature 551, 9 November 2017 

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 instability. 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.

Stellar encore: Dying star keeps coming back big time.: image via AP Images @AP_Images, 8 November 2017