This is the golden age of neutron-star physics

In the first moments of a neutron star’s birth, just after a massive star explodes, its iron core shrinks to a sphere the size of Manhattan. What happens next is a mystery. The protons and electrons might fuse into the star’s namesake neutrons all the way down to the centre. Or the unimaginable pressure might compact the material into more exotic particles or states that squish and deform in unusual ways. Answers are coming, thanks to gravitational-wave observatories and an instrument on the International Space Station called the Neutron Star Interior Composition Explorer (NICER).

Nature | 10 min read