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A neutron star is a sphere the size of a city but with the mass of an entire Sun. Its magnetic field can tear atoms apart from thousands of kilometers away. Yet until now, it was a mystery how this field is arranged inside: whether it is twisted into tight ropes or more like neat lines radiating from the poles.

The authors built a computer model of such a star and let time run — tracking the magnetic field's behavior over many cycles of its own "swings." Inside, they placed a mixture of two field types: one direct, like a regular magnet (dipole), and the other twisted into rings (toroidal). The fraction of the twisted part varied across experiments from zero to 90 percent of the total magnetic energy.

The result was unambiguous: no matter what fraction of twisted field was set initially, the star itself brought it to the same stable configuration. Whirls and instabilities, like ripples from a thrown stone, quickly died out — in about the time of one "magnetic pulse" (the so-called Alfvén time). In the end, the twisted part always amounted to no more than ten percent of the total field energy — both outside and inside the star.

It's like a tight rubber band that, when released, twists itself into neat rings: no matter what knots you tie, it still settles into the same simple shape. For astrophysicists, this is a key clue: long-lived neutron stars almost always have exactly this mixed but strictly dosed magnetic structure. And that, in turn, explains why pulsars, rapidly rotating neutron stars, blink so rhythmically: their external field is as stable as a pendulum.