Science Adv. 1, e1500578 (2015)

Much like young people, young pulsars don't like to play by the rules. Their very fast rotation period, which is extremely stable for older pulsars, exhibits abrupt changes known as glitches. During these glitches the pulsar suddenly starts to spin faster for a short period of time. This is believed to be the result of the interactions between the normal matter in the outer crust of the star and the superfluid inner crust. But the estimated superfluid reservoir needed to explain the observational data is larger than that available in the crust. To explain this, Wynn Ho and colleagues have suggested that the superfluid extends to the core of the star.

Ho et al. tested several superfluid models, providing the additional moment of inertia needed to explain the pulsar glitches. One such model successfully accounted for the observational data and the temperature dependence of superfluidity. And this turns out to have an unexpected application: using the pulsar glitch data and the interior temperature, one can determine the mass of the star.