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Star of Wonder

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By Roger Snodgrass

Stars don’t always die the same way, but now there is evidence that up to a point, the most massive stars may experience the most spectacular deaths. And there may even be one nearby.An international team of astrophysicists has come up with an explanation that posits a new mechanism to describe the brightest explosions.The research by Stan Woolsey of the University of California Santa Cruz; Sergei Blinnikov, a visiting researcher at UCSC from the Institute of Theoretical and Experimental Physics in Moscow; and Alexander Heger of Los Alamos National Laboratory was published in the Nov. 15 issue of the journal Nature.Their explanation emerged from a study of the earliest stars born out of the cosmic dark ages when very big stars were common.“Almost no two supernovae look identical,” Heger said. “But every once in awhile one looks much different than expected. It depends on the mechanisms by which they explode.”A superlative exception was the phenomenal supernova named SN 2006gy discovered by Robert Quinby, a University of Texas graduate student on Sept. 26, 2006. Located in a galaxy more than 240 million light years away, the supernova blossomed over the course of 70 days into one of the most spectacular explosions in the modern universe.At its peak, according to NASA, the dying star lit up the heavens with a brilliance equal to 50 billion suns – 10 times brighter than the whole galaxy in which it resided and 100 times brighter than a typical nova.“This was probably the most massive star we have ever seen die,” Woolsey said in the Nature article.The event was reported by Time magazine and major science and astronomy journals in May this year, after analysis by a team of astronomers led by Nathan Smith at the University of California declared it “the most luminous supernova ever recorded.”The Woolsey-Blinnikov-Heger team explained the phenomenon, based on their previous studies of the life cycles of massive stars, with a new model based on pulsation pair instability supernova, applicable to very massive but otherwise normal stars that happen to be in the range of 95-130 times the mass of the sun.These stars encounter an instability, the scientists say, that causes some their internal energy to convert into electrons and their anti-matter counterparts, positrons, which in turn precipitates collapse followed by vigorous, explosive thermonuclear burning and energy release, but not enough to unbind the whole star.Depending on factors such as mass, composition and rotation rate, successive expansions, contractions, explosions and collisions of ejected shells over a matter of years can turn the energy released by the explosion of the star into far more light than an ordinary supernova from a smaller star when it makes a black hole or a neutron star.The brightest supernovae are thought to be uncommon in our galaxy, but there is a parallel with the most luminous star in the Milky Way, Eta Carinae.A star between 100-and 150 solar masses and about 7,500 light years distant, Eta Carinae expelled the equivalent of ten solar masses in the 1840’s and is considered unstable.A Chandra fact sheet prepared after all the attention paid to SN 2006gy drew the comparison and observed that Eta Carinae may explode at anytime, “perhaps rivaling the moon in brilliance.”