SYDNEY:
A spectacular, rotating binary star system is a ticking time bomb,
ready to throw out a searing beam of high-energy gamma rays – and Earth
may be right in the line of fire.
Astronomers at the University of Sydney, in Australia, first
discovered the unusual and beguilingly beautiful star system eight
years ago in the Constellation Sagittarius. One member of the pair is a
highly unstable star known as a Wolf-Rayet, thought to be the final
stage of stellar evolution to precede a cataclysmic supernova
explosion.
"When it finally explodes as a supernova, it could emit an intense
beam of gamma rays coming our way", said Peter Tuthill, lead researcher
of the team who report their findings in the current Astrophysical Journal.
Vast and glowing plume
At a distance of 8,000 light years from Earth, the pair of stars are
a short hop away in galactic terms, and just one quarter of the way to
the centre of our Milky Way galaxy.
The researchers took images of the system, known as WR 104, over a
period of eight years using Hawaii's Keck Telescope. These images
reveal a vast and glowing plume of heated dust and gas, billowing out
in a spiral as the stars rotate once every eight months. This 'tail' is
up to 30 billion kilometres long.
But something curious about the images caught the attention of the experts.
"Viewed from Earth, the rotating tail appears to be laid out on the
sky in an almost perfect spiral. It could only appear like that if we
are looking nearly exactly down on the axis of the binary system," said
Tuthill.
This means we are peering down the barrel of the gun, as when binary
supernovae go off, all their energy is focussed into a narrow beam of
wildly destructive gamma ray radiation that emanates (both up and down)
from the poles of the system.
"If such a gamma-ray burst happens, we really do not want Earth to
be in the way," he said. "I used to appreciate this spiral just for its
beautiful form, but now I can't help a twinge of feeling that it is
uncannily like looking down a rifle barrel."
Sterilising effect
Though the risk may be remote, there is evidence that gamma ray
bursts have swept over the planet at various points in Earth's history
with a devastating effect on life.
A 2005 study showed that a gamma-ray burst originating within 6,500
light years of Earth could be enough to strip away the ozone layer and
cause a mass extinction. Researchers led by Adrian Melott at the
University of Kansas in Lawrence, U.S., suggest that such an event may
have been responsible for a mass extinction 443 million years ago, in
the late Ordovician period, which wiped out 60 per cent of life and
cooled the planet.
Further research would be required to determine if we are exactly in
line with the axis of the system – but even if we are, we probably
still have hundreds of thousands of years to come up with a solution,
said Tuthill.
with the University of Sydney