Think of a merry-go-round: it's easier to spin a light one, harder to spin a heavy one. Neutron stars, the ultra-dense remnants of exploded stars, behave in a similar way. Astronomers looked at fifty binary systems where one neutron star orbits another and noticed: the heavier the star, the longer its spin period. The link isn't strict, but it's statistically noticeable. The correlation coefficient (a measure of statistical association) is about –0.26, and with 96% probability it's less than zero.

This gentle anti-correlation fits nicely with the picture where the star is spun up by matter falling from its companion. But there's a catch: the moment of inertia (a measure of resistance to spinning) for such stars is almost directly proportional to mass in the studied range. So, based on mass and period data alone, you can't confidently say what plays the leading role — the amount of fallen matter or the star's own "build."

The work also showed that neutron stars with helium white dwarf companions are slightly more massive than those with carbon-oxygen companions — by about 0.06 solar masses. This aligns with the idea that in the first case, more matter fell onto the star. So the spin and mass data piece together the life story of each system, like an old carousel that remembers every push.