Imagine a spring being compressed with tremendous force. The harder you press, the denser it gets, but at some point the compression stops being uniform — some parts give way more easily, others are stiffer. That's roughly how the matter inside a neutron star behaves — an object born in the flames of a supernova. It's a sphere the size of a city but with a mass greater than the Sun's. In its depths, matter is squeezed to such an extent that atoms cease to exist, and protons and electrons merge into neutrons — hence the name.

The big mystery of such stars is what happens at the very center, where density goes off the charts. To find out, astrophysicists solve the Tolman–Oppenheimer–Volkoff equations (a sort of "Hooke's law" for stellar matter). But usually, this is like a long hike: you start at the surface and, step by step, layer by layer, calculate pressure and density until you reach the center. Make one mistake, and the whole journey is ruined. The authors of the new work figured out how to turn this hike into an instant leap. They expanded the solution into a series, an infinite sum of simple terms, and showed that with a clever mathematical technique (Padé approximants, which turn bulky expressions into compact fractions), you can get a formula that immediately yields the star's radius and mass.

The trick is that the formula relies not on a ready-made "pressure-density" table but on the function that describes the matter itself — the so-called equation of state. The authors learned to express the series coefficients in terms of derivatives of this function and then stitch together solutions for different layers of the star — for example, the crust and the core. In the end, instead of hours of computer computation, you get a few lines of calculations. The method was tested on two popular models: a fluid with a simple linear equation of state and polytropes (where pressure depends on density via a power law). The results matched numerical calculations within a few percent. Now astrophysicists have a quick way to estimate the sizes and masses of neutron stars, and thus better understand what goes on in the most extreme corners of the Universe. And it all starts with a simple image, a spring that behaves differently under unimaginable pressure.