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Our galaxy full of ‘time bombs’?

india Updated: Sep 07, 2011 13:54 IST
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A new study has suggested that thousands of “time bombs” could be scattered throughout our galaxy.

Scientists have revealed that some old stars might be held up by their rapid spins, and when they slow down, they explode as supernovae.

“We haven’t found one of these ‘time bomb’ stars yet in the Milky Way, but this research suggests that we’ve been looking for the wrong signs. Our work points to a new way of searching for supernova precursors,” said astrophysicist Rosanne Di Stefano of the Harvard-Smithsonian Centre for Astrophysics.

The specific type of stellar explosion Di Stefano and her colleagues studied is called a Type Ia supernova. It occurs when an old, compact star known as a white dwarf destabilizes.

A white dwarf is a stellar remnant that has ceased nuclear fusion. It typically can weigh up to 1.4 times as much as our Sun - a figure called the Chandrasekhar mass after the astronomer who first calculated it.

Any heavier, and gravity overwhelms the forces supporting the white dwarf, compacting it and igniting runaway nuclear fusion that blows the star apart.

There are two possible ways for a white dwarf to exceed the Chandrasekhar mass and explode as a Type Ia supernova. It can accrete gas from a donor star, or two white dwarfs can collide. Most astronomers favour the first scenario as the more likely explanation.

Di Stefano and her colleagues suggest that white dwarf spin might solve this puzzle.

A spin-up/spin-down process would introduce a long delay between the time of accretion and the explosion.

As a white dwarf gains mass, it also gains angular momentum, which speeds up its spin. If the white dwarf rotates fast enough, its spin can help support it, allowing it to cross the 1.4-solar-mass barrier and become a super-Chandrasekhar-mass star.

Once accretion stops, the white dwarf will gradually slow down.

Eventually, the spin isn’t enough to counteract gravity, leading to a

Type Ia supernova.

This research appeared in the Sept. 1 issue of The Astrophysical Journal Letters.