Illustration of Cassiopeia A Neutron Star
This is an artist's impression of the neutron star at the center of the Cassiopeia A supernova remnant. The different colored layers in the cutout region show the crust (orange), the higher density core (red) and the part of the core where the neutrons are thought to be in a superfluid state (inner red ball). The blue rays emanating from the center of the star represent the copious numbers of neutrinos that are created as the core temperature falls below a critical level and a superfluid is formed.
(Credit: Illustration: NASA/CXC/M.Weiss)

X-ray and Optical Images of Cassiopeia A
Two independent research teams studied the supernova remnant Cassiopeia A, the remains of a massive star, 11,000 light years away that would have appeared to explode about 330 years as observed from Earth. Chandra data are shown in red, green and blue along with optical data from Hubble in gold. The Chandra data revealed a rapid decline in the temperature of the ultra-dense neutron star that remained after the supernova. The data showed that it had cooled by about 4% over a ten-year period, indicating that a superfluid is forming in its core.
(Credit: X-ray: NASA/CXC/UNAM/Ioffe/D.Page,P.Shternin et al; Optical:
NASA/STScI)

The Formation of a Superfluid in a Neutron Star
This set of three artist's impressions shows some details of how a neutron superfluid begins to form in a neutron star. It shows a schematic close-up of the sea of neutrons in the ultra-dense core of a neutron star. We follow the two neutrons in the foreground that are moving towards each other in the first panel. The motion of both neutrons is slow because of interactions, via the strong nuclear force, that they have with the neutron sea. If the temperature in the core falls below a critical value, the two neutrons can form a pair, resulting in the emission of a pair of neutrinos (2nd panel). This neutron pair then moves off at high speed because it has different physical properties and is no longer held back by interactions with the surrounding sea of neutrons (3rd panel). It now has superfluid properties. The neutrinos are lost by the neutron star, causing the star to lose energy and cool down.
(Credit: NASA/CXC/M.Weiss)