Astronomy
How Gravitational Waves Reveal Neutron Star Interiors
Quick fact
The gravitational-wave signal from a neutron star merger is so sensitive to the stars' internal structure that it can reveal whether the matter inside is 'squishy' or 'stiff', allowing astronomers to infer the kind of particles that exist at the core.
Why this is interesting
When two neutron stars spiral together and collide, they briefly shake the fabric of space itself. Could those ripples carry the secret of what's inside these city-sized stars?
Read the full explanation
Understanding How Gravitational Waves Reveal Neutron Star Interiors
Neutron stars are the remnants of massive stars after supernova explosions. They pack more mass than the Sun into a sphere roughly the size of a city, making them the densest observable matter. When two such stars orbit each other, their immense gravity pulls them into a dance. As they get closer, they move faster and faster, and according to Einstein's general relativity, this accelerating motion causes ripples in spacetime itself—gravitational waves—that carry energy away, making the stars spiral together even faster. Just before they collide, the gravitational pull of each star causes the other to bulge, creating a tidal deformation. The size of that bulge depends on how rigid the star is. A soft, squashy star deforms easily; a stiff one resists. That deformation is imprinted on the gravitational-wave signal, slightly changing its frequency and amplitude. By carefully measuring these changes, scientists can deduce the deformability and thus learn about the internal pressure and composition.
A deeper explanation
The deep connection lies in the concept of the tidal deformability, often denoted by the Greek letter lambda. When a neutron star is in a binary, the companion's gravitational field creates a tidal force that stretches the star. The star's response is governed by its internal structure, specifically by its equation of state—the relationship between pressure, density, and temperature. A stiff equation of state means high pressure for a given density, making the star harder to deform, such a star will have a small tidal deformability. Conversely, a soft equation of state makes the star more deformable. This deformability directly affects the orbital motion in the final few orbits before merger, imprinting a phase shift in the gravitational waveform. By comparing the observed waveform with theoretical model templates that assume different equations of state, astronomers can identify which model best matches the data. This method is a powerful new way to probe the extreme physics of nuclear matter at densities far beyond what we can achieve in laboratories on Earth. It even constrains the maximum mass of a neutron star and provides clues about whether they can contain exotic particles like hyperons or quarks.