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Neutron stars: The star with a soft centre
Posted: Wednesday, June 13, 2001
(New Scientist) TO TRY to understand neutron stars, researchers have abandoned telescopes for a new tool a billion billion times smaller--the nucleus of a lead atom.
Neutron stars, like the one at the heart of the Crab Nebula (above), have a radius of only a dozen kilometres or so but weigh more than the Sun. Despite often pumping out X-rays or radio signals, it is hard to determine the structure of a neutron star just from its radiation.
Researchers think that a neutron star is solid on the outside with a liquid centre. They want to know how thick the solid neutron crust is, as this affects many of the star's properties--from how fast it cools to how well it emits gravitational waves.
Charles Horowitz of Indiana University in Bloomington and Jorge Piekarewicz of Florida State University in Tallahassee believe that they can get an idea of the crust's thickness by measuring the skin of neutrons that covers the nucleus of a lead atom. "We're trying to use lead-208 as a miniature surrogate," Horowitz says.
A lead nucleus is a staggering 55 orders of magnitude less massive than a neutron star, but because both are nuclear matter they are governed by the same physics, which is enshrined in an "equation of state". Physicists do not know the exact form of the equation of state for either a neutron star or lead nucleus. But as Horowitz and Piekarewicz gradually modified the equations, they found a close correlation between the values they got for the star's crust and the nucleus's skin.
"There is definitely a relationship there," says astrophysicist Jim Lattimer of the State University of New York at Stony Brook. "If we can tighten that relationship, it should be a useful tool."
That tool may be put to use in a couple years at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia. Physicists are planning to measure the neutron skin of lead-208 nuclei by bouncing electrons off it. Once they know the skin thickness, neutron stars will be a cinch.
Physical Review Letters More
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