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Supernova Wipeout

While doing research for a term paper, Karin Sandstrom, a student at Harvard University, discovered a star in our own backyard that is on the brink of exploding in a supernova. It?s so close that if it were to blow up before moving away from us, it could wipe out life on Earth.

Most supernovae occur when large stars run out of fuel and then collapse under their own weight. As the atoms in the star are squeezed together, they rebound outwards, blowing off energy in a dazzling and dangerous display lasting several weeks. But this one is different. Called HR 8210, it is a humble white dwarf, a star that has run out of fuel and should be too small to produce a supernova. But it may not stay that way. First, it's not alone, but is orbiting a companion star in a binary system. And it?s 1.15 times the mass of our Sun, which is huge for a white dwarf.

The system was first discovered in 1993 but little attention was paid to it. Then when Sandstrom investigated HR 8210, she discovered that it is only a little short of the Chandrasekar limit - the mass at which it would be big enough to go supernova. That makes it the best and by far the closest supernova candidate discovered so far.

The problem will come when HR 8210's companion begins to run out of fuel. As it expands to form a red giant star, its outer layers will be dumped onto HR 8210, pushing it over the Chandrasekar limit. "Our initial idea was that this might happen very soon," says Sandstrom's supervisor Dave Latham.

"Very soon" could mean hundreds of millions of years in the future, which is good, because right now we?re only 150 light years away from HR 8210 - well short of the 160 to 200 light years which is the minimum safe distance from a supernova. If it did collapse, the high-energy electromagnetic radiation and cosmic rays it released would destroy Earth's ozone layer within minutes, wiping out all life.

This would not be the first time a supernova has changed the course of life on Earth. In 2001, Jesus Maiz-Apellaniz and colleagues from the Space Telescope Science Institute in Baltimore found a supernova remnant in the group of stars known as the Scorpius- Centaurus association. The timing of the supernova corresponds to an otherwise mysterious deposit of heavy isotopes in deep Earth cores and to a mass marine extinction two million years ago. At the time, Scorpius-Centaurus was around twice as far away from Earth as HR 1820 is now.

It will take time for HR 8210 to accumulate the mass it needs. Preliminary calculations by Rosanne di Stefano at the Harvard-Smithsonian Center suggest this may take hundreds of millions of years. By that time it will be much further away, she says, although she still needs to confirm exactly how far. "I want to be sure I'm right."

Could there be similar threats out there that we don?t yet know about? Latham says, "The fact that there's such a system so close to us suggests maybe these objects are not so rare."

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