Biggest Star Explosion Seen; Was Rare, "Clean" Death

The biggest star explosion yet seen may be the best known example of a rare type of star death that leaves no "body" behind, astronomers say. The unusual blast, dubbed SN 2007bi, appears to be a textbook example of a pair-instability supernova, a theoretical type of explosion proposed for very massive stars—those more than 140 times the mass of the sun.

Although most supernovae leave behind black holes or dense stellar corpses called neutron stars, pair-instability explosions would be so intense that the whole star would be obliterated.

Pair-instability supernovae have been hard to spot, however, because stars more than a hundred times the sun's mass are extremely rare.

Spied in images of a distant dwarf galaxy taken by an automated telescope, SN 2007bi was about 40 times brighter than a typical supernova, and it took about three times longer to reach its maximum brightness.

"Anything that takes that long to rise and is that bright has to have a lot of mass," said study co-author Peter Nugent, an astrophysicist at the Lawrence Berkley National Laboratory in California.

Hot Core, Unstable Star

Massive stars normally die when they run out of material to sustain nuclear fusion, and all that's left in their cores is inert iron.

This means the core is no longer producing a steady stream of photons, which in a living star creates outward pressure, keeping the star from being crushed by its own gravity.

Without this stable outward pressure, the star collapses, generating a supernova in which the core gets crunched down to form a black hole or a neutron star.

(Related: "New Type of Supernova Discovered.")

But for even more massive stellar titans, astronomers think the cores quickly get so hot that their photons start to split apart into pairs of electrons and positrons.

This leads to an instability between the star's temperature and pressure, sparking a devastating explosion that flings the star's remains into space.

The star effectively vanishes, although a lingering cloud of expanding gas can remain visible for a while.


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