Neutron stars are among the strangest objects in the universe. They are created when a massive star runs out of fuel and explodes in a supernova. After the explosion, the star's core is squeezed so tightly that it becomes an incredibly dense object called a neutron star.
Although a neutron star is only about 20 kilometers (12 miles) wide, it can contain more mass than our Sun. In fact, just one teaspoon of material from a neutron star would weigh about a billion tons on Earth.
For many years, scientists have wondered how heavy a neutron star can become before it collapses into a black hole. A new study by researchers Kwon, Kim, and Sekizawa suggests that a special type of rotation could allow neutron stars to become much heavier than previously believed. Their findings may also help solve one of the biggest mysteries in modern astrophysics.
Why Does the Maximum Mass Matter?
Every neutron star has a limit to how much mass it can hold. If it becomes too heavy, its own gravity becomes so strong that it collapses into a black hole.
Scientists use mathematical models called the Equation of State (EoS) to understand how matter behaves inside neutron stars. The EoS describes how matter reacts when it is squeezed to extremely high densities.
Different EoS models predict different maximum masses for neutron stars. This is why discovering very heavy neutron stars helps scientists test whether their theories are correct.
Heavy Neutron Stars Are Challenging Old Ideas
In recent years, astronomers have discovered several neutron stars that are much heavier than expected.
One famous example is PSR J0740+6620, which has a mass of about 2.1 times the mass of the Sun. Any scientific model that cannot support a neutron star this heavy is considered incomplete.
Another even bigger mystery appeared in 2019 when scientists detected gravitational waves from an event called GW190814.
One object involved in this event had a mass between 2.5 and 2.67 times the Sun's mass.
Scientists still don't know what this object was.
It could have been the heaviest neutron star ever discovered, or it could have been the lightest black hole ever found.
This mystery has inspired scientists to search for new ways that neutron stars could support more mass.
The Hyperon Puzzle
Inside a neutron star, pressure becomes incredibly high.
At these extreme densities, scientists believe that new particles called hyperons may appear.
Hyperons are similar to neutrons and protons but contain an extra type of particle called a strange quark.
According to current theories, hyperons should naturally form deep inside very dense neutron stars.
However, they create a serious problem.
When hyperons appear, they make the star's matter softer. This means the star becomes less able to resist its own gravity.
As a result, many computer models predict that neutron stars containing hyperons should collapse before reaching the heavy masses that astronomers have actually observed.
This disagreement between theory and observation is called the hyperon puzzle, and scientists have been trying to solve it for many years.
Can Rotation Help?
The researchers looked at whether rotation could help neutron stars remain stable even with hyperons inside them.
Every neutron star spins, but not all neutron stars rotate in the same way.
Some rotate like a solid ball. In this type of motion, called rigid rotation, every part of the star spins at nearly the same speed.
But another type of rotation is possible.
It is called differential rotation.
In differential rotation, different parts of the star spin at different speeds. The center can rotate much faster than the outer layers.
This creates extra support against gravity.
Because of this extra support, the star may be able to hold much more mass before collapsing.
How the Scientists Studied It
To test this idea, the researchers used advanced computer simulations based on Einstein's theory of general relativity.
They created realistic models of neutron stars using two different Equations of State.
The first model included only ordinary particles like neutrons and protons.
The second model also included hyperons.
They then calculated what would happen if these neutron stars rotated at different speeds.
Their goal was to see whether differential rotation could explain the existence of very heavy neutron stars.
What Did They Find?
The results were exciting.
The researchers found that differential rotation can significantly increase the maximum mass of a neutron star.
In some cases, the increase was large enough to produce neutron stars with masses similar to the mysterious object seen in GW190814.
This means that at least some extremely heavy compact objects might actually be neutron stars instead of black holes.
However, there was one important limitation.
For the hyperon-rich model, the observed rotation speed of PSR J0740+6620, which spins 346 times every second, was not enough to increase the mass sufficiently.
This suggests that rotation alone cannot completely solve the hyperon puzzle.
Scientists will also need better models of how matter behaves at extremely high densities.
A Strange Discovery Inside Neutron Stars
The study also revealed something surprising about the inside of rapidly rotating neutron stars.
Normally, the center of a neutron star is its densest region.
But under strong differential rotation, the highest-density region moved away from the center.
This created a very unusual internal structure.
The researchers found that hyperons could gather in a ring-shaped region surrounding the center of the star.
They called this structure a hyperon ring.
In the most extreme cases, the entire neutron star became almost doughnut-shaped instead of nearly spherical.
Scientists describe these unusual objects as quasi-toroidal neutron stars.
Although these strange stars would probably exist for only a short time, they show how unusual matter can become under extreme conditions.
What Happens After Two Neutron Stars Collide?
When two neutron stars crash into each other, they may temporarily form an extremely massive object called a hypermassive neutron star.
These objects rotate very rapidly and are supported by strong differential rotation.
The new study suggests that such stars could contain every known type of hyperon at the same time.
These short-lived objects may be the only places in the universe where such extreme conditions exist.
Studying them could teach scientists about forms of matter that cannot be created in laboratories on Earth.
Why This Study Is Important
This research gives scientists a better understanding of how neutron stars behave under extreme conditions.
It shows that differential rotation can help neutron stars support much larger masses than previously thought.
Although it does not completely solve the hyperon puzzle, it provides an important piece of the answer.
The study also shows that scientists need better Equations of State that accurately describe matter at very high densities.
Future observations of neutron stars and gravitational waves will help improve these models.
Looking Ahead
The researchers now plan to perform even more detailed computer simulations to see whether these unusual neutron stars are stable or whether they quickly collapse into black holes.
Future telescopes and gravitational-wave detectors will also discover more heavy neutron stars, giving scientists new data to test these ideas.
Every new discovery brings us closer to understanding how matter behaves under the most extreme conditions in the universe.
This research suggests that differential rotation may allow neutron stars to grow much heavier than once thought, opening a new path toward solving one of the biggest mysteries in astrophysics. If future observations confirm these findings, scientists may finally explain how some neutron stars become so massive without collapsing into black holes.
Reference' Hyukjin Kwon, Jinho Kim, Kazuyuki Sekizawa, "Effects of Differential Rotation on the Maximum Mass of Neutron Stars", Physical Review D, 2026. https://arxiv.org/abs/2607.09040
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