In 2017, the LIGO and Virgo detectors recorded the trembling of spacetime from the neutron star merger of two stars in a binary star system. After the impact, the newborn ultradense remnant rang like a bell. Its vibrations are gravitational waves at frequencies of thousands of hertz. The pitch of this 'ring' tells us how much the material resists compression: its speed of sound.

In everyday life, it’s well known: the stiffer a material, the faster sound travels through it. Faster in steel than in rubber. Inside a neutron star, the stiffness is beyond imagination, and the speed of sound there can almost catch up with the speed of light. The idea of gravitational waves was first proposed by Albert Einstein, and the exact solution for compact objects was found by Karl Schwarzschild. Neutron stars themselves were discovered in 1967 when Jocelyn Bell Burnell noticed pulsars.

Building on these facts, astrophysicists created computer numerical simulations with different material stiffness. It turned out that the frequency of the gravitational signal after the collision is directly linked to the speed of sound. Too soft material quickly collapses into a black hole, ejecting almost nothing. Too stiff material gives a weak rebound and little ejecta. But with moderate stiffness, a lot of ejected material is born, and a bright kilonova (the glow from the ejecta) appears, where nucleosynthesis of heavy elements (gold, platinum, uranium) takes place.

Future detectors, such as the Einstein Telescope, will be able to pick up the kilohertz tones of mergers and determine the stiffness of nuclear matter in real time. Thus, the song of neutron stars will unlock the secrets of their interiors.