When two neutron star whirl in a merger dance, their tidal pull acts like a hammer striking a bell. If the striking frequency matches the star's natural frequency, it begins to resonate, emitting gravitational waves — ripples spreading through the fabric of spacetime. The future Einstein Telescope will catch this 'ringing' and reveal the star's internal structure with a precision unattainable by ordinary astronomy.

Such resonance will show up in about one in three powerful mergers per year. Without correcting for it, the mass and size of the star would be measured incorrectly. The Einstein Telescope will surpass the first LIGO detector and complement observations of pulsar — rapidly spinning neutron star lighthouses. Gravitational waves travel at the speed of light through the expansion of the universe, carrying the imprint of monstrous gravity: near the star, the spacetime curvature is so extreme that Time dilation becomes noticeable. Amazingly, the frequency of this ringing often falls into the audible range — hundreds of hertz, like the note 'A'. Although there is no sound in a vacuum, gravitational waves can be converted into an audio signal to hear the 'voice' of the star.