NASA's Chandra X-ray Observatory¹ has discovered the first direct evidence for a superfluid（超流体） , a bizarre, friction²（摩擦） -free state of matter, at the core of a neutron³ star. Superfluids created in laboratories on Earth exhibit remarkable⁴ properties, such as the ability to climb upward and escape airtight containers（密封容器） . The finding has important implications for understanding nuclear interactions in matter at the highest known densities⁵. Neutron stars contain the densest⁶ known matter that is directly observable. One teaspoon⁷ of neutron star material weighs six billion tons. The pressure in the star's core is so high that most of the charged particles, electrons and protons, merge⁸ resulting in a star composed mostly of uncharged particles called neutrons⁹.

Two independent research teams studied the supernova remnant Cassiopeia A, or Cas A for short, the remains¹⁰ of a massive star 11,000 light years away that would have appeared to explode about 330 years ago as observed from Earth. Chandra data found a rapid decline in the temperature of the ultra-dense neutron star that remained after the supernova（超新星） , showing that it had cooled by about four percent over a 10-year period.

"This drop in temperature, although it sounds small, was really dramatic and surprising to see," said Dany Page of the National Autonomous¹¹ University in Mexico, leader of a team with a paper published in the February 25, 2011 issue of the journal Physical Review Letters. "This means that something unusual is happening within this neutron star."

Superfluids containing charged particles are also superconductors, meaning they act as perfect electrical conductors and never lose energy. The new results strongly suggest that the remaining protons in the star's core are in a superfluid state and, because they carry a charge, also form a superconductor.

"The rapid cooling in Cas A's neutron star, seen with Chandra, is the first direct evidence that the cores of these neutron stars are, in fact, made of superfluid and superconducting material," said Peter Shternin of the Ioffe Institute in St Petersburg, Russia, leader of a team with a paper accepted in the journal Monthly Notices of the Royal Astronomical¹² Society.

Both teams show that this rapid cooling is explained by the formation of a neutron superfluid in the core of the neutron star within about the last 100 years as seen from Earth. The rapid cooling is expected to continue for a few decades and then it should slow down.

"It turns out that Cas A may be a gift from the Universe because we would have to catch a very young neutron star at just the right point in time," said Page's co-author Madappa Prakash, from Ohio University. "Sometimes a little good fortune can go a long way in science."

The onset¹³ of superfluidity in materials on Earth occurs at extremely low temperatures near absolute zero, but in neutron stars, it can occur at temperatures near a billion degrees Celsius¹⁴. Until now there was a very large uncertainty¹⁵ in estimates of this critical temperature. This new research constrains¹⁶ the critical temperature to between one half a billion to just under a billion degrees.

Cas A will allow researchers to test models of how the strong nuclear force, which binds¹⁷ subatomic（亚原子的） particles, behaves in ultradense（高密度的） matter. These results are also important for understanding a range of behavior in neutron stars, including "glitches¹⁸," neutron star precession and pulsation¹⁹, magnetar outbursts and the evolution of neutron star magnetic fields.

Small sudden changes in the spin rate of rotating neutron stars, called glitches, have previously²⁰ given evidence for superfluid neutrons in the crust of a neutron star, where densities are much lower than seen in the core of the star. This latest news from Cas A unveils new information about the ultra-dense inner region of the neutron star.

"Previously we had no idea how extended superconductivity of protons was in a neutron star," said Shternin's co-author Dmitry Yakovlev, also from the Ioffe Institute.

The cooling in the Cas A neutron star was first discovered by co-author Craig Heinke, from the University of Alberta, Canada, and Wynn Ho from the University of Southampton, UK, in 2010. It was the first time that astronomers²¹ have measured the rate of cooling of a young neutron star.