Neutron stars are the remnants of dead giants, crushed into a sphere the size of a city. Their mass and radius are known from observations of lighthouse-like pulsars and gravitational waves (ripples in space), picked up by the LIGO observatory using giant laser 'rulers'. But the composition of their interiors remains a mystery, like trying to guess the fruit inside a sealed box just by its weight and size. The old method was like brute-forcing every conceivable filling with a computer (computer simulations) until one matched.

Now a neural network trained on thousands of examples instantly spits out pressure and density from mass and radius, like a seasoned gourmand guessing the contents by the box's weight. It strictly obeys the laws of physics, passed down by Einstein, Schwarzschild, and Chandrasekhar. This instant analysis points out which stars to watch first to unravel the secret of the densest matter.