- A huge explosion halfway across the galaxy packed so
much power it briefly altered Earth's upper atmosphere in December, astronomers
said Friday.
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- No known eruption beyond our solar system has ever appeared
as bright upon arrival.
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- But you could not have seen it, unless you can top the
X-ray vision of Superman: In gamma rays, the event equaled the brightness
of the full Moon's reflected visible light.
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- The blast originated about 50,000 light-years away and
was detected Dec. 27. A light-year is the distance light travels in a year,
about 6 trillion miles (10 trillion kilometers).
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- The commotion was caused by a special variety of neutron
star known as a magnetar. These fast-spinning, compact stellar corpses
-- no larger than a big city -- create intense magnetic fields that trigger
explosions. The blast was 100 times more powerful than any other similar
eruption witnessed, said David Palmer of Los Alamos National Laboratory,
one of several researchers around the world who monitored the event with
various telescopes.
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- Tsunami Connection?
- Several readers wondered if the magnetar blast could
be related to the December tsunami. Scientists have made no such connection.
The blast affected Earth's ionosphere, which is routinely affected to a
greater extent by changes in solar activity.
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- "Had this happened within 10 light-years of us,
it would have severely damaged our atmosphere and possibly have triggered
a mass extinction," said Bryan Gaensler of the Harvard-Smithsonian
Center for Astrophysics (CfA).
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- There are no magnetars close enough to worry about, however,
Gaensler and two other astronomers told SPACE.com. But the strength of
the tempest has them marveling over the dying star's capabilities while
also wondering if major species die-offs in the past might have been triggered
by stellar explosions.
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- 'Once-in-a-lifetime'
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- The Sun is a middle-aged star about 8 light-minutes from
us. It's tantrums, though cosmically pitiful compared to the magnetar explosion,
routinely squish Earth's protective magnetic field and alter our atmosphere,
lighting up the night sky with colorful lights called aurora.
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- Solar storms also alter the shape of Earth's ionosphere,
a region of the atmosphere 50 miles (80 kilometers) up where gas is so
thin that electrons can be stripped from atoms and molecules -- they are
ionized -- and roam free for short periods. Fluctuations in solar radiation
cause the ionosphere to expand and contract.
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- "The gamma rays hit the ionosphere and created more
ionization, briefly expanding the ionosphere," said Neil Gehrels,
lead scientist for NASA's gamma-ray watching Swift observatory.
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- Gehrels said in an email interview that the effect was
similar to a solar-induced disruption but that the effect was "much
smaller than a big solar flare."
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- Still, scientists were surprised that a magnetar so far
away could alter the ionosphere.
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- "That it can reach out and tap us on the shoulder
like this, reminds us that we really are linked to the cosmos," said
Phil Wilkinson of IPS Australia, that country's space weather service.
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- "This is a once-in-a-lifetime event," said
Rob Fender of Southampton University in the UK. "We have observed
an object only 20 kilometers across [12 miles], on the other side of our
galaxy, releasing more energy in a tenth of a second than the Sun emits
in 100,000 years."
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- Some researchers have speculated that one or more known
mass extinctions hundreds of millions of years ago might have been the
result of a similar blast altering Earth's atmosphere. There is no firm
data to support the idea, however. But astronomers say the Sun might have
been closer to other stars in the past.
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- A similar blast within 10 light-years of Earth "would
destroy the ozone layer," according to a CfA statement, "causing
abrupt climate change and mass extinctions due to increased radiation."
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- The all-clear has been sounded, however.
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- "None of the known sample [of magnetars] are closer
than about 4,000-5,000 light years from us," Gaensler said. "This
is a very safe distance."
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- Cause a mystery
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- Researchers don't know exactly why the burst was so incredible.
The star, named SGR 1806-20, spins once on its axis every 7.5 seconds,
and it is surrounded by a magnetic field more powerful than any other object
in the universe.
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- "We may be seeing a massive release of magnetic
energy during a 'starquake' on the surface of the object," said Maura
McLaughlin of the University of Manchester in the UK.
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- Another possibility is that the magnetic field more or
less snapped in a process scientists call magnetic reconnection.
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- Gamma rays are the highest form of radiation on the electromagnetic
spectrum, which includes X-rays, visible light and radio waves too.
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- The eruption was also recorded by the National Science
Foundation's Very Large Array of radio telescopes, along with other European
satellites and telescopes in Australia.
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- Explosive details
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- A neutron star is the remnant of a star that was once
several times more massive than the Sun. When their nuclear fuel is depleted,
they explode as a supernova. The remaining dense core is slightly more
massive than the Sun but has a diameter typically no more than 12 miles
(20 kilometers).
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- Millions of neutron stars fill the Milky Way galaxy.
A dozen or so are ultra-magnetic neutron stars -- magnetars. The magnetic
field around one is about 1,000 trillion gauss, strong enough to strip
information from a credit card at a distance halfway to the Moon, scientists
say.
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- Of the known magnetars, four are called soft gamma repeaters,
or SGRs, because they flare up randomly and release gamma rays. The flare
on SGR 1806-20 unleashed about 10,000 trillion trillion trillion watts
of power.
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- "The next biggest flare ever seen from any soft
gamma repeater was peanuts compared to this incredible Dec. 27 event,"
said Gaensler of the CfA.
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- http://www.space.com/scienceastronomy/bright_flash_050218.html
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