Oversize supernova.

Although they can't fully explain why a star blows up, astronomers thought that they had accurately determined the maximum mass that a star could have before exploding as a connnon type of supernova. Now, they've found a star that breaks the rule.

According to prevailing theory, a type la supernova is triggered by a compact star called a white dwarf piling up matter stolen from a bloated companion star. When the white dwarf becomes as heavy as 1.4 times the mass of the sun, the high density and heat generated by the matter spark a thermonuclear explosion.

Researchers have found a type la supernova so luminous that it must have been produced by a much heavier star than the standard theory allows. Andy Howell of the University of Toronto and his colleagues report in the Sept. 21 Nature that the supernova, called SNLS-O3D3bb, is 2.2 times as bright as expected and that the star that exploded had to be 50 percent heavier than the 1.4-solar-mass limit.

The most likely explanation for the overweight star, comments David Branch of the University of Oklahoma in Norman, is that the matter accumulated by the white dwarf revved up the star's rotation, creating a centrifugal force that provided extra support against the crushing force of gravity. Then the dwarf could pack on additional mass before exploding.

Because most type la supernovas have about the same peak brightness and can be seen from afar, astronomers have used the explosions to measure cosmic distances. The new finding doesn't undermine that practice, but astronomers need to be aware of extra-bright supernovas, the team says.