The neutron star in Cassiopeia A could prove that such stars have superfluid centres. Credit: NASA/CXC/Southampton/W. Ho et al.

Two teams of astrophysicists might have found the first direct evidence that the interiors of neutron stars — the husk left behind after a massive star explodes — exist in a strange, frictionless state known as a superfluid. The teams found that the temperature of a young neutron star in our Galaxy is dropping faster than can be explained by standard cooling theories, matching researchers' expectations for a neutron star on its way to superfluidity.

Superfluids are perfect heat conductors with zero viscosity. On Earth, a superfluid's behaviour is counterintuitive — it stays completely still even if its container is rotating, it can defy gravity by creeping up walls, or it can play Houdini by escaping from an airtight container.

Neutron stars are so dense that most protons within them are crushed together with electrons, forming neutrons. By assuming the stars have a superfluid interior, theorists have successfully described the temperature and magnetic behaviour of existing neutron stars, but confirming such an exotic hypothesis requires more hard evidence.

By 2004, two groups — including Dmitry Yakovlev of the Ioffe Physical Technical Institute in St Petersburg, Russia, and Dany Page of the National Autonomous University of Mexico in Mexico City — had calculated the temperature drop expected if a star was undergoing a transition to a superfluid state. Without knowing the temperature at which that transition occurs, it was difficult to make definite predictions of the cooling rate.

Page says that at the time, "it was more like a theoretical curiosity. What were the odds of finding one of them and being able to observe it?"

Then, Craig Heinke of the University of Alberta in Edmonton, Canada, and Wynn Ho of the University of Southampton, UK, analysed data from the Chandra X-ray Observatory, reporting the rapidly falling temperature of a neutron star in the Cassiopeia A supernova remnant. The star's temperature had dropped by 4% since its discovery in 1999 — normally neutron stars cool too slowly for us to notice. Yakovlev and his colleagues began working with Heinke and Ho on demonstrating the superfluid transition, but Page's group had seen it too.

Fridolin Weber of San Diego State University in California, who was not involved in either team's analysis, calls the research "a very important contribution" because it uses the theory of superfluidity to provide a testable explanation for the rapidly cooling star.

Cool kid

At 330 years old, the neutron star of Cassiopeia A is the youngest known in our Galaxy. After the star's initial explosion, interactions between protons and neutrons would have produced neutrinos, near-massless particles that escaped the star, allowing it to cool.

After the first few days or weeks, the protons, which make up about 10% of the star, would have reached a temperature that allowed them to become a superfluid. In this state, they would have been able to ignore the neutrons, thus stopping the neutrino-emitting process and slowing the star's rate of cooling.

This condition has kept the star hot since its explosion. But sometime in the past century, the star has begun to slip below the temperature that allows neutrons to pair off with each other, allowing them to form a superfluid. Because the research teams dealt with density effects differently, Page's group estimates that critical temperature to be at about half a billion kelvin1, whereas Yakovlev's team sets it at 0.7 billion to 0.9 billion kelvin2.

When they reach this critical temperature, the fledgling neutron pairs repeatedly split apart and try to pair again. They release energy each time they pair up, and when that energy is released in neutrino form, the star is cooled.

This process of coupling and breaking up has driven the rapid cooling of the star for the past few decades, and both teams project that it will continue for a few more. Then, once as many of the neutrons as possible have gone superfluid, the star will return to a slow cooling rate. If Chandra and future telescopes find evidence that bears out this prediction, astrophysicists can be fairly certain that neutron stars really are superfluid on the inside.