Exotic Superfluid Found in Ultra-Dense Stellar Corpse

The ultra-dense meains of the galaxy’s youngest supernova are full of bizarre quantum matter.

Two new studies show for the first time that the core of the neutron star Cassiopeia A, is a superfluid, a friction-free state of matter that normally only exists in ultra-cold laboratory settings.

“The interior of neutron stars is one of the best kept secrets of the universe,” said astrophysicist Dany Page of the National Autonomous University in Mexico, lead author of a paper in the Feb. 25 Physical Review Letters describing the state of the star. “It looks like we broke one of them.”

Cassiopeia A (Cas A) was a massive star 11,000 light-years away whose explosion was observed from Earth about 330 years ago. The supernova left behind a tiny, compact body called a neutron star, in which matter is so densely packed that electrons and protons are forced to fuse into neutrons. Neutron star material is some of the most extreme matter in the universe. Just a teaspoonful of neutron star stuff weighs about 6 billion tons.

The neutron star in Cas A was first spotted in 1999, shortly after the Chandra X-Ray Observatory began scanning the sky for objects that emit X-rays.

Last year, astronomers Craig Heinke of the University of Alberta and Wynn Ho of the University of Southampton noticed something odd: The neutron star was cooling down at an alarmingly fast rate. In just 10 years, the star had cooled from 2.12 million degrees to 2.04 million degrees, a drop of 4 percent.


Theoretical models predicted that neutron stars should cool slowly as the neutrons inside decayed into electrons, protons and nearly-massless particles called neutrinos that flee the star quickly, taking heat with them.

But ordinary neutron decay is too slow. Two competing groups of physicists, one led by Page and one including Heinke and Ho, saw that something else must be going on in Cas A.

Almost simultaneously, both teams came to the same solution: The matter inside the neutron star is converting to a superfluid as astronomers watch. Heinke and Ho’s paper will appear in the Monthly Notices of the Royal Astronomical Society.

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