Neutron starThe neutron star (illustration) at the center of the remains of the Cassiopeia A supernova may have a core filled with a strange frictionless fluid, new research suggests. 

Neutron stars, after the black holes, are the densest objects in the Universe. Observations of a dead star shrouded in the debris of a distant supernova suggest, that may have liquid in their cores.
Two separate teams of scientists say that a frictionless state of matter called a superfluid is the only reasonable explanation for temperature changes recently observed in the youngest known neutron star.
“This the first direct evidence for superfluidity in neutron stars,” says Wynn Ho, an astrophysicist at the University of Southampton in England and coauthor of a paper that will appear in the Monthly Notices of the Royal Astronomical Society describing one team’s findings. The other team’s results will appear in an upcoming Physical Review Letters.

Located 11,000 light-years away in the constellation Cassiopeia, the neutron star formed when a larger star collapsed in a brilliant explosion that was visible from Earth about 300 years ago. The orbiting Chandra X-Ray Observatory first spotted the neutron star created by this supernova in 1999.
Subsequent measurements revealed that its 2-million-degree surface had cooled by 4 percent over the decade since its discovery. “This was the first time anyone found a young neutron star clearly changing temperature,” says Craig Heinke, an astrophysicist at the University of Alberta in Edmonton, Canada, who reported the observations last year in Astrophysical Journal Letters.

Theorists had long speculated that a young neutron star should cool for the first 100 years after its creation. Neutrons can break down into protons, ejecting nearly massless particles called neutrinos that carry energy away from the star. But this energy-sapping Urca process (named for a money-sapping casino in Brazil) couldn’t account for the steep temperature drop seen by Chandra hundreds of years after the Cassiopeia supernova.

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