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Neutron Star Crust Is Stronger than Steel

Neutron stars are deadrelics that have collapsed into very small, dense spheres with tough crusts. Forceswelling from within can crack the crusts during events called star quakes,similar to earthquakes.

The awesome power of thosequakes can blast gamma rays into space, leading scientists to suspect that the stars'crusts must be very hard to break.

A new study suggests howstrong they are: The crust of neutron stars could be 10 billion times strongerthan steel, based on an innovative model of elements compressed as tightly asthey would be on the surface of a neutron star.

In 2004, astronomersspotted a spectaculargamma-ray explosion bursting off a neutron star in the Sagittariusconstellation, 50,000 light years from Earth. The star, SGR 1806-20, is amagnetar, a type of neutron star that has a powerful magnetic field.? NASAand European satellites and astronomers around the world detected the flare,which for a tenth of a second was brighter than anything ever seen beyond oursolar system. It was the biggest such flare ever spotted and one of only fourthat have been seen so far.

"We think that thesegiant flares are coming from really, really bigstar quakes," said Indiana University physicist Charles Horowitz. Onlya super-strong crust could have exploded so forcefully, he explained.

To find out how strong thecrusts of neutron stars really are, Horowitz and a colleague created a computersimulation of a star's surface. Though the interior of the star is a kind offluid mass of mostly neutrons, the crust is composed of broken-up atoms, thenuclei of unknown elements. To simulate this, Horowitz used the computerprogram to squeeze together virtual selenium atoms, pressing them into tinycubes. He determined that the crust is billions of times stronger than even thehardiest metal alloys here on Earth.

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

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

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

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

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

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