A STUDENT at Harvard University has stumbled across the terrifying spectacle of a star in our galactic backyard that鈥檚 on the brink of exploding in a supernova. It is 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 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鈥檚 not alone, but is orbiting a companion star in a typical binary system. And it鈥檚 1.15 times the mass of our Sun, which for a white dwarf is a whopper.
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The system was first logged in 1993 but little attention was paid to it. Then when Harvard student Karin Sandstrom investigated HR 8210 for a college paper this year, she discovered that it is only just shy of the Chandrasekar limit鈥攖he 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 crunch will come when HR 8210鈥檚 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. 鈥淥ur initial idea was that this might happen very soon,鈥� says Sandstrom鈥檚 supervisor Dave Latham.
But don鈥檛 panic yet. 鈥淰ery soon鈥� could mean hundreds of millions of years in the future. And that鈥檚 just as well, because we are only 150 light years away from HR 8210 at present鈥攚ell short of the 160 to 200 light years thought to be the minimum safe distance from a supernova. If it did let fly, the high-energy electromagnetic radiation and cosmic rays it released would destroy Earth鈥檚 ozone layer within minutes, giving life little chance of survival.
This wouldn鈥檛 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, Maryland, found a 鈥渟moking gun鈥� 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 2 million years ago. At the time, Scorpius-Centaurus was around twice as far away from Earth as HR 1820 is now.
Fortunately, 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, though she still needs to confirm exactly how far. 鈥淚 want to be sure I鈥檓 right.鈥�
But will similar stars threaten us before then? 鈥淭he fact that there鈥檚 such a system so close to us suggests maybe these objects are not so rare,鈥� says Latham.