[ExI] neutron star crusts

p0stfuturist at yahoo.com p0stfuturist at yahoo.com
Tue May 19 02:25:44 UTC 2009

The crust of neutron stars could be 10 billion times stronger
than steel, based on an innovative model of elements compressed as tightly as
they would be on the surface of a neutron star.

                In 2004, astronomers
spotted a spectacular
gamma-ray explosion bursting off a neutron star in the Sagittarius
constellation, 50,000 light years from Earth. The star, SGR 1806-20, is a
magnetar, a type of neutron star that has a powerful magnetic field.  NASA
and European satellites and astronomers around the world detected the flare,
which for a tenth of a second was brighter than anything ever seen beyond our
solar system. It was the biggest such flare ever spotted and one of only four
that have been seen so far. 

                "We think that these
giant flares are coming from really, really big
star quakes," said Indiana University physicist Charles Horowitz. Only
a super-strong crust could have exploded so forcefully, he explained.

                To find out how strong the
crusts of neutron stars really are, Horowitz and a colleague created a computer
simulation of a star's surface. Though the interior of the star is a kind of
fluid mass of mostly neutrons, the crust is composed of broken-up atoms, the
nuclei of unknown elements. To simulate this, Horowitz used the computer
program to squeeze together virtual selenium atoms, pressing them into tiny
cubes. He determined that the crust is billions of times stronger than even the
hardiest metal alloys here on Earth. 

                "You can't produce
anything like these conditions on Earth, which is why we did not know the
strength before," he said. His results were published May 8 in the journal
Physical Review Letters. 

                Not just any old relics, neutron
stars are the leftover cores of huge stars that exploded in
supernovae. In a massive star's death throes, it can blast most of its
outer material into space. When the fireworks are over, the core collapses in
on itself under the weight of its own gravity. Like an ice skater pulling in
her arms, the star spins faster as it shrinks, Horowitz explained. 

                The stars are usually
tiny, about 15 miles in diameter. But within that small ball, there is the mass
of about one and a half suns. A black hole is the only thing denser.

                Neutron stars are so dense
that if you could dip a teaspoon into one of them and scoop out some of its
neutrons the spoon would weigh 100 million tons. If you were to hold that empty
teaspoon just one yard above the star's surface and drop it, it would strike
the surface at 4.3 million mph. 

                Though their surfaces are
generally smooth, mountains made of super-dense star stuff rise from the crust.
The mountains' height depends on how strong the crust is. Horowitz takes
creative license in calling them mountains, he said, because they are only a
few inches high. When they are too high they sag under the stars' gravity and
sink back into the ground. The highest mountain a crust could support would
only be about 4 inches in altitude, Horowitz estimated. Even so, the gravity of
neutron stars is immense. It radiates into space in weird
ways around the star, warping space-time and slowing the star's spin.



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