Neutron stars are extremely dense objects that form when massive stars run out of nuclear fuel and collapse in on themselves. The enormous pressure within the star forces almost all of the protons and electrons together to form neutrons. Astrophysicists would like to know more about the properties of this ultra-dense matter, and one way to do this is to study exactly how neutron stars cool.
Neutron stars should exhibit both superfluidity and superconductivity, according to two independent groups of scientists. The researchers studied the neutron star in the supernova remnant known as Cassiopeia A, and found that its core should exist in a superfluid state at up to around a billion degrees kelvin, in contrast to the near absolute-zero temperatures required for superfluidity on Earth.
The object at the heart of Cassiopeia A, which is about 11,000 light-years away, is ideally suited to such an exercise because, unusually, it has both a well established age – about 330 years – and a well known surface temperature – around 2 million kelvin.
Last year, Craig Heinke of the University of Alberta in Canada and Wynn Ho of the University of Southampton in the UK analysed 10 years' worth of X-ray data from NASA's Chandra satellite and found that the Cassiopeia A neutron star's surface temperature has dropped more quickly than expected – by about 4% between 2000 and 2009.