In 2017, the LIGO and Virgo detectors recorded the spacetime tremor from the collision of two neutron star merger — event GW170817. After the initial burst came an after-ring: a kilohertz oscillation of the newly formed superdense remnant. This signal is not just noise; it is the voice of matter itself, compressed to unimaginable limits. Its pitch and volume directly depend on how the substance resists compression — its speed of sound.

The speed of sound in ordinary media is familiar: the stiffer the material, the faster the wave travels. For a neutron star, stiffness determines the equation of state — the relation between pressure and density. At densities above nuclear density (2.6×$10^{14}$ g/cm³), this equation remains a mystery. In the star's core, the speed of sound can reach 95% of speed of light — nearly the cosmic limit. As early as 1915, Albert Einstein predicted gravitational waves within the framework of general relativity. And in 1916, Karl Schwarzschild found the first exact solution to the field equations, laying the foundation for the theory of compact objects. The existence of neutron stars was confirmed in 1967, when Jocelyn Bell Burnell discovered pulsars.

To hear the voice of the interior, astrophysicists built numerical symphonies — numerical simulation of mergers with different stiffness profiles. By varying the slope of the squared speed of sound, they traced how the gravitational waves signal changes after the collision. It turned out that the peak frequency increases almost linearly with stiffness, but at extreme values, the behavior becomes non-monotonic. Too soft matter leads to rapid collapse into a black hole, while too stiff matter results in a weak bounce and scant outflow.

Particularly intriguing is the tidal parameter ˜D, which accumulates the contribution of the speed of sound throughout the star's volume. Its relation to the dimensionless frequency turned out to be almost universal across different equations of state. This means that gravitational waves probe densities inaccessible to tidal measurements during the binary star inspiral phase. Future detectors, such as the Einstein Telescope, will be able to capture kilohertz tones from mergers and place real-time constraints on the stiffness of nuclear matter.

In the future, multi-dimensional simulations with phase transitions — from hadrons to quarks — promise an even clearer melody. It is already clear: as they merge, neutron stars sing about their internal structure, and the next concert recorded by the detector network may reveal the nature of the densest matter in the universe. What will we hear? An echo born in the stellar forge of elements.