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Brightest Supernova May Reignite

New supernova mechanism would set off repeat explosions
SN 2006gy



NASA/CXC/M.Weiss

A new type of ultrapowerful supernova discovered last year may blow its top again, according to a new study. Researchers report that supernova 2006gy fits a model of star explosion that should have produced two flare-ups already and may culminate in a third before the star fizzles out. A second study proposes that the explosion might have come about from the marriage of multiple stars.

SN 2006gy first caught astronomers' eyes in September. Burning 100 times brighter than a typical supernova, it maintained full strength for an amazing three months, by which point most of its counterparts would have begun fading. Even eight months later it was as brilliant as a so-called type II supernova, the most common variety. The energy unleashed implied that the exploding star was a behemoth of at least 100 solar masses (suns).

Supernovas typically occur after a star bigger than 10 suns has gradually exhausted its supply of hydrogen and helium, which have fused into progressively heavier elements. Unable to muster the heat to support its outer layers, the burnt-out star implodes, blasting its envelope outward in a last gasp of nuclear energy.

To explain SN 2006gy's massive outburst, researchers invoked a competing mechanism called pair-instability, which theoretically kicks in for stars larger than 90 suns. In this scenario, dating to 1967, high-energy gamma rays inside the star convert into pairs of electrons and positrons, draining stellar energy that would normally help maintain its internal pressure, eventually leading to a premature collapse that liberates vast amounts of energy and light.

In a new Nature paper, astrophysicist Stan Woosely of the University of California, Santa Cruz, and his colleagues report that the observed changes in SN 2006gy's brightness fit a model of pulsating pair-instability, assuming the star was initially 95 to 130 solar masses, with a helium-rich core of about 50 suns.

Developed by Woosely and co-author Alexander Heger in 2002, the model predicts that the initial implosion of a 110–solar mass star would shed several sun's worth of mass before igniting the star's carbon and oxygen fuel and temporarily halting the collapse. Roughly seven years later, pair-instability would cause a second breakdown that emits a smaller but faster pulse of material.

Normally, Woosely says, the star itself, bloated with age, would soak up most of such a shockwave's energy without releasing light. But in this case, the two pulses would slam into each other, sparking fireworks.

"Essentially [the first pulse] could act like a sponge to soak up the energy—and then reradiate that energy as light," says Nathan Smith of U.C. Berkeley, a member of the group that first pointed out SN 2006gy's peculiar brightness. The new study "gives a nice numerical confirmation of the empirical model we proposed initially."

Woosely says he "wouldn't want to bet the house" on the model, partly because it requires the initial star to lose mass slower than predicted by calculations of stellar evolution, but adds that "it's the best model going." He notes that if the model is correct, SN 2006gy might flare up again or it might quietly collapse into a black hole.

Whatever the mechanism behind the explosion, a second Nature paper this week proposes that the initial star may have formed from the collision of a large, older star and a small, younger one. This would explain the presence of hydrogen in SN 2006gy, even though big stars are thought to run out of hydrogen hundreds of thousands of years before going supernova, says co-author Simon Portegies Zwart of the University of Amsterdam.

Since SN 2006gy was discovered, astronomers have noticed a second, even brighter supernova that was first observed in 2005 but lasted for an ordinary amount of time. Smith notes that a third explosion nearly matched the brilliance and duration of SN 2006gy, which has hogged the spotlight.

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