A neutron star is a sphere the size of a city, but it weighs as much as one and a half Suns. Inside such a sphere, matter is so compressed that a teaspoon of it would weigh billions of tons. But physicists don't fully know what this matter is made of. There are dozens of hypotheses, and each gives a different answer to a simple question: what is the star's radius for a given mass?
Usually, scientists look for universal relations between mass, radius, and other characteristics of a star. It's like finding patterns in people's height and weight without knowing what's inside them. But these relations have a drawback: they are always approximate. The authors of the new study took a different approach — training on many computer models. They created thousands of virtual neutron stars with different interiors and taught an algorithm to guess the radius from indirect signs.
The new method doesn't just give a single number. It shows how reliable the answer is and accounts for errors that were previously overlooked. Moreover, the algorithm itself found a new useful relation: the star's radius can be calculated from its mass and two frequencies at which it oscillates. These frequencies can be measured from Earth — via gravitational waves or flares. So soon we'll be able to determine the sizes of neutron stars with unprecedented precision, even without knowing their internal structure.
A neutron star spins so fast that its surface moves at up to a quarter of the speed of light — about 75,000 kilometers per second.
In Bruce Sterling's novel Schismatrix, neutron stars serve as beacons for interstellar navigation: their precise sizes and masses are critical for calculating jumps.