The image at top of page shows a city-sized neutron star that powers the vast Crab Nebula. The Chandra X-ray Observatory and the Hubble Space Telescope showed wisps of gas It has long been assumed that neutrons in the cores of neutron stars become superfluid, but without any direct evidence that they do so until 2010, when astrophysicists Craig Heinke and Wynn Ho 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 cooled tremendously fast, dimming by 20 per cent since it was discovered in 1999, corresponding to an estimated temperature drop of 4 per cent.