The dense core of a nearby collapsed star is undergoing a rapid chill, providing the first direct evidence that such stars can produce a superfluid of neutrons – a state of matter that cannot be created in laboratories on Earth. Neutron stars are the remnants of exploded stars. Their cores are so dense that atomic nuclei dissolve, and protons and electrons combine to form a soup dominated by neutrons. If conditions are right, these neutrons ought to be able to pair up to form a superfluid – a substance with quantum properties that mean it flows with zero friction.
It has long been assumed that neutrons in the cores of neutron stars become superfluid, but without any direct evidence that they do so. That changed in 2010, when astrophysicists examined measurements taken by NASA's orbiting Chandra X-ray Observatory of the 330-year-old neutron star at the heart of the dusty supernova remnant Cassiopeia A. These measurements show the star has dimmed by 20 per cent since it was discovered in 1999, corresponding to an estimated temperature drop of 4 per cent. Now colleagues have calculated that this rapid cooling can be explained if a fraction of the neutrons in the core are undergoing a transition to superfluidity.