Neutron stars are among the most extreme objects in the universe. They are incredibly compact, enormously dense, and contain matter unlike anything that can be naturally produced on Earth. Now, a new study by researchers Kantor, Gusakov and Kraav has revealed an important clue about what happens deep inside these mysterious stars during their final moments before collision.
The researchers investigated a process known as hyperon bulk viscosity and its possible influence on gravitational waves produced by pairs of neutron stars spiraling toward each other.
Their conclusion is surprising: hyperon bulk viscosity appears to have almost no measurable effect on the gravitational-wave signal, but it can dramatically heat the inner core of a neutron star to around 500 million kelvin.
What makes neutron stars so extreme?
A neutron star is the incredibly dense leftover core of a massive star that has undergone a supernova explosion. Although a neutron star can have a diameter of only around 20 kilometers, it can contain more mass than the Sun.
The matter inside these objects is compressed to densities greater than those found inside atomic nuclei. Under these conditions, ordinary descriptions of matter no longer work in the same way.
Scientists cannot reproduce such cold, ultra-dense matter in laboratories on Earth. This makes neutron stars natural laboratories for studying the behavior of matter under conditions that are otherwise impossible to create.
One of the best ways to investigate their interiors is through gravitational waves.
Gravitational waves reveal what happens inside
When two neutron stars orbit one another, their immense gravity causes them to spiral closer together. As they accelerate, they produce ripples in spacetime called gravitational waves.
The first confirmed detection of gravitational waves from a binary neutron-star merger, GW170817, was announced in 2017 by the LIGO and Virgo collaborations.
The signal provided scientists with a new way to study neutron stars. Instead of observing only light from these objects, researchers could analyze the gravitational waves themselves and look for subtle signatures of the stars' internal structure.
At first, scientists can approximate the neutron stars as simple point-like objects. But this approximation misses important information about their internal matter.
As the stars approach each other, their enormous gravitational fields produce tidal forces that deform them. These deformations affect the gravitational-wave signal and allow researchers to learn about the properties of ultra-dense matter.
This is why even very small physical effects inside neutron stars can potentially become scientifically important.
The mystery of bulk viscosity
One of the effects researchers are interested in is bulk viscosity.
In simple terms, bulk viscosity describes energy dissipation that occurs when matter is compressed or expanded and its particles cannot immediately return to chemical equilibrium.
During a neutron-star inspiral, the intense tidal forces continuously disturb the matter inside the stars. These disturbances can push particles away from their equilibrium state.
Certain reactions then work to restore equilibrium. During this process, some of the orbital energy of the binary system is converted into heat.
For ordinary neutron-star matter, reactions involving particles called leptons can contribute to this process.
But things become more interesting if the star contains hyperons.
What are hyperons?
Hyperons are particles containing strange quarks. Under the enormous densities found deep inside neutron stars, scientists believe that these exotic particles may appear.
If hyperons are present, they can participate in weak nonleptonic reactions. These reactions can occur much faster than some of the ordinary reactions responsible for bulk viscosity in nucleonic matter.
As a result, hyperons have long been considered a possible source of particularly strong viscous dissipation inside neutron stars.
This raised an important question:
Could hyperon bulk viscosity leave a detectable fingerprint in gravitational waves?
Previous studies had produced different answers.
Some research suggested that even strong bulk viscosity would have only a small influence on the inspiral. Other studies argued that weak nonleptonic reactions involving hyperons could produce substantial heating and potentially detectable changes in the gravitational-wave signal.
Kantor, Gusakov and Kraav decided to revisit this question using detailed models of neutron-star inspirals.
The gravitational-wave effect is surprisingly tiny
The researchers found that hyperon bulk viscosity has only a minor influence on the gravitational-wave phase.
In their models, the resulting phase shift was only around:
10⁻³ radians
In some calculations, even when the researchers used a conservative assumption designed to maximize the possible effect, the phase shift remained below approximately 10⁻² radians up to gravitational-wave frequencies of 1,000 Hz.
That is extraordinarily small.
For comparison, gravitational-wave detectors must distinguish incredibly subtle changes in a signal while also dealing with detector noise, uncertainties in neutron-star models and many other astrophysical effects.
According to the study, the predicted hyperon-viscosity signal is simply too weak for existing gravitational-wave observatories to detect reliably.
And there is another problem.
Even if future observatories become significantly more sensitive, isolating this particular effect could remain extremely difficult because other physical processes may produce similar or larger changes in the signal.
But the real surprise is inside the star
Although hyperon bulk viscosity appears to have almost no observable impact on the gravitational-wave phase, it can have a much stronger effect on the temperature of the neutron star's core.
The dissipative processes convert part of the orbital energy into heat.
In the models studied by the researchers, this viscous heating can raise the temperature of the hyperon-rich core to approximately:
5 × 10⁸ K
That's about 500 million kelvin.
This result changes how scientists may need to think about hyperon bulk viscosity.
It may not be a powerful tool for detecting hyperons through tiny gravitational-wave phase changes. Instead, its importance could lie in the thermal evolution of neutron stars.
Why heating matters
Temperature can influence many physical processes inside a neutron star.
Heating the hyperonic core could affect how the star cools, how particles interact, and how different forms of matter behave under extreme conditions.
This means that even though the gravitational-wave signature is almost invisible, the thermal consequences of hyperon bulk viscosity may be significant.
The finding also demonstrates an important lesson in astrophysics: an effect does not need to be detectable directly in gravitational waves to be physically important.
A process can leave an extremely small imprint on the gravitational-wave phase while simultaneously producing substantial changes inside the star.
What does this mean for future gravitational-wave astronomy?
Next-generation observatories such as the Einstein Telescope and Cosmic Explorer are expected to be far more sensitive than current detectors.
Scientists hope these instruments will detect weaker finite-size effects and extract more information about neutron-star interiors.
However, the new analysis suggests that simply increasing detector sensitivity may not be enough to identify hyperon bulk viscosity through its gravitational-wave phase shift.
The predicted effect is so small that it could be hidden among theoretical uncertainties and other astrophysical effects.
Instead, researchers may need to combine gravitational-wave observations with other information about neutron stars, particularly their thermal behavior.
A new way to study the hidden cores of neutron stars
The study by Kantor, Gusakov and Kraav provides a clearer picture of what hyperon bulk viscosity can—and cannot—do during a neutron-star inspiral.
The process appears to have little influence on the gravitational-wave phase, producing a shift of only about 10⁻³ radians in the models studied.
That makes it extremely challenging to detect with current gravitational-wave observatories and potentially difficult even for future detectors.
But inside the neutron star, the story is very different.
Hyperon bulk viscosity can transform part of the system's orbital energy into heat, potentially pushing the temperature of the hyperonic core to around 500 million kelvin.
This means the most important signature of hyperon viscosity may not be hidden in the gravitational waves themselves. Instead, it could be hidden in the extreme thermal environment deep inside the neutron star.
As gravitational-wave astronomy continues to develop, researchers are increasingly learning that these cosmic collisions contain more than just signals in spacetime. They may also provide clues about exotic particles, ultra-dense matter, and the incredible physical processes occurring inside some of the universe's most extreme objects.
# Neutron Stars Can Hide Cores Hotter Than 500 Million Kelvin
Neutron stars are among the most extreme objects in the universe. Their matter is compressed to densities greater than those inside atomic nuclei, creating conditions that cannot be reproduced on Earth. Scientists study these objects through gravitational waves produced when two neutron stars spiral toward each other.
A new study by Kantor, Gusakov and Kraav has revisited the role of hyperon bulk viscosity, a process that can occur if exotic particles called hyperons exist deep inside neutron stars.
During a neutron-star inspiral, powerful tidal forces disturb the matter inside the stars. Reactions involving hyperons can restore chemical balance while converting some orbital energy into heat. Scientists previously thought this process might significantly alter the gravitational-wave signal.
However, the researchers found that its effect is surprisingly small. Hyperon bulk viscosity produces a gravitational-wave phase shift of only around 10⁻³ radians. This is far too small for current detectors to observe and may remain difficult to isolate even with future observatories.
But the process has another remarkable effect: heating.
The researchers found that hyperon bulk viscosity can raise the temperature of the hyperon-rich core to approximately 5 × 10⁸ kelvin, or 500 million kelvin.
The finding suggests that hyperon viscosity may be more important for understanding the thermal evolution of neutron stars than for detecting subtle changes in gravitational waves.
Reference: Elena M. Kantor, Mikhail E. Gusakov, Kirill Y. Kraav, "Hyperon bulk viscosity effects in neutron-star inspirals", Phys.Rev.D, 2026. https://arxiv.org/abs/2608.28000
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