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The time bombs ticking on our doorstep

The universe is a pretty ancient place. Since it was born in the explosion of matter and energy we call the Big Bang, more than 14billion years of time has expanded before it.

Within time’s ever-expanding womb, fertile matter has seeded the birth of countless stars, which have burst with screams of hot fury into the universe’s nursery.

But what fate awaits these children of time? Some are born too small and are doomed to live the shame of a failed star. Others are born too large, burn too bright and die young as supernovae. But many, like our very own Sun, live bright, long and unspectacular lives.

When these stars die, they leave behind a sort of skeletal monument, which is a pale reflection of their former glory, called a white dwarf star. A white dwarf is a stellar corpse, a final glowing cinder that is doomed to slowly fade as the universe reclaims its waning heat.

At least that’s what we thought. New research is showing that some white dwarfs might be stellar time bombs, slowly counting down to an explosive supernova finale.

Factoid: A teaspoonful of typical white dwarf matter would weigh 5.5 tonnes on Earth (about the same as an elephant)

It is thought that a white dwarf explodes as a supernova when it acquires extra mass from its surroundings. It can do this by stripping matter from a nearby star or by wandering too close to another white dwarf and smashing into it in the embrace of their gravitational attraction.

Either way, its end should be pretty swift.

[Above: Artist's impression of a supernova explosion about to engulf an orbiting Saturn-like planet. Credit: David A Aguilar (CfA)]

The key lies in the so-called Chandrasekhar limit, which says that once a white dwarf passes a critical mass (1.4 times the mass of our Sun), it can no longer support itself against the force of gravity and it is crushed like a nectarine under a hammer.

It doesn’t matter how the star gained the extra mass, the result should be a swift and catastrophic fusion chain reaction, ending in a colossal thermonuclear explosion.

Another factoid: The gravity at the surface of a white dwarf is about 100,000 times that of Earth

But the new study, published in The Astrophysical Journal Letters, suggests some of these over-loaded white dwarfs, although primed to explode, are being delayed by their spin.

It suggests that, as a white dwarf gains mass, it also gains angular momentum and its spin speeds up. If the white dwarf rotates fast enough, its spin helps support it, allowing it to exceed the weight limit without immediately blowing up.

But when it stops gaining mass, the white dwarf will gradually slow down. Eventually, its spin won’t be enough to support it, allowing the hammer of gravity to fall at last. The result is a supernova that releases the energy of a billion Suns.

Given that our Milky Way is likely to be littered with billions of white dwarfs, there is a good chance there are thousands of these time bombs ticking away on our solar system’s doorstep right now.

[Graphic: How a dead star becomes a spinning time bomb. Click to super-Chandrasekharise]

You can check out the original release from
Harvard Smithsonian Center for Astrophysics News