Neutron stars are the ultra-dense remnants of stars, where matter is compressed to the limit described by Subrahmanyan Chandrasekhar. Magnetars are a type of neutron star with a magnetic field a quadrillion times stronger than Earth's. Normally, light passing a massive body bends because spacetime is warped: it acts like a gravitational lens. But in magnetars, the field adds a second lens: the light bends even more. It's like looking through glasses with double lenses: one from gravity, one from magnetism. Without accounting for the second lens, the error in radius reaches 10%.

Future telescopes, analyzing the flicker of pulsars (discovered by Jocelyn Bell Burnell), will refine neutron star sizes. This will unlock the secrets of ultra-dense matter inaccessible on Earth. And 10% isn't just a number: for a neutron star, it's the line between stability and collapse into a black hole.