1. What Is a Neutron Star?
A neutron star is an extremely dense stellar remnant that can form when the core of a massive star collapses at the end of its life.
During the collapse, protons and electrons largely combine to form neutron-rich matter.
The resulting object contains a stellar-scale mass in a very small volume.
2. Extraordinary Density
The diameter of a neutron star can be only a few tens of kilometers.
Nevertheless, it can contain an amount of matter close to the mass of the Sun.
For this reason, the average density of neutron stars can reach the same order of magnitude as that of an atomic nucleus.
3. What Prevents a Neutron Star from Collapsing?
A neutron star is not only under the influence of gravity.
The extremely compressed matter inside creates quantum mechanical and nuclear-origin pressures that resist collapse.
These pressures can allow the star to remain stable up to a certain mass.
4. But There Is an Upper Limit
As mass increases, the neutron star's own gravity also becomes stronger.
Beyond a certain point, the existing pressures cannot withstand the increasing gravity.
This maximum mass at which a neutron star can remain stable is expressed by the Tolman–Oppenheimer–Volkoff limit.
5. What Is the TOV Limit?
The TOV limit refers to the maximum mass a neutron star can stably support, considering general relativity and the properties of dense nuclear matter.
This value is not a simple fixed number.
Because the behavior of matter at the center of a neutron star is not fully known.
Therefore, different physical models may predict different maximum mass values.
6. Why Is the Exact Mass Unknown?
At the center of a neutron star, matter is compressed far beyond the conditions that can be produced in laboratories on Earth.
At these densities, it is being investigated whether matter is still composed of neutrons or if more exotic particles appear.
For this reason, determining the exact value of the TOV limit is one of the most difficult problems in nuclear physics.
7. What Do Observations Show?
Astronomers have observed neutron stars with very high masses.
In particular, the discovery of neutron stars with masses around 2 solar masses has ruled out some theoretical models.
These observations indicate that neutron star matter must have strongly supportive properties under very high pressure.
8. What Happens If the Mass Increases?
A neutron star can gain matter from outside.
For example, it can draw matter from a nearby star or merge with another neutron star.
If its mass increases enough to exceed the critical limit, it may lose its stability.
9. Rotation Rate Also Matters
A rapidly rotating neutron star gains additional support thanks to the centrifugal effect caused by its rotation.
Therefore, a very rapidly rotating neutron star can carry slightly more mass than a non-rotating star of the same structure.
However, if the rotation rate is not sufficient or if the star slows down over time by losing energy, this extra support decreases.
10. If Rotation Slows Down
Neutron stars can lose part of their rotational energy over time.
Their magnetic fields and electromagnetic radiation play a role in this energy loss.
If the star is very close to the critical limit, the reduction in rotational support can trigger collapse.
11. The Role of Magnetic Field
Some neutron stars have extraordinarily strong magnetic fields.
Magnetic fields can affect the star's outer layers and energy loss.
However, the fundamental issue determining whether a neutron star becomes a black hole is how the total mass and the stability conditions of the internal matter are balanced under general relativity.
12. Mass Accretion in Binary Systems
A neutron star can form a binary system with another star.
In these systems, the neutron star can draw matter from its companion star.
As matter is transferred to the neutron star, its mass can increase.
If enough matter accumulates, it may approach the stability limit.
13. Neutron Star Mergers
The merger of two neutron stars is also a very important scenario.
The total mass of the object formed during the merger can be greater than what a single neutron star can support.
In this case, a short-lived, extremely massive neutron star may form after the merger.
Subsequently, collapse into a black hole may occur.
14. How Does Collapse Begin?
When the critical limit is exceeded, the pressure balancing gravity is not sufficient.
The inner regions of the star begin to compress even further.
The effects of general relativity become increasingly dominant.
When collapse becomes unstoppable, an event horizon may form.
15. Black Hole Formation
When a black hole forms, from the perspective of an external observer, the matter no longer appears as the normal surface of a neutron star.
One of the fundamental features characterizing a black hole is the event horizon.
Not even light can escape from inside the event horizon.
16. The Difference Between a Neutron Star and a Black Hole
A neutron star has a physical surface.
Matter is found extremely densely on or beneath this surface.
In a black hole, however, the event horizon is the fundamental boundary observed from the outside.
One of the critical points distinguishing these two celestial objects is whether matter can be stably supported against gravity.
17. Schwarzschild Radius
When the mass of an object is concentrated within a certain radius, general relativity allows for black hole formation.
For a non-rotating object, this critical radius is called the Schwarzschild radius.
When the radius of a neutron star shrinks to reach this regime, the formation of an event horizon becomes possible.
18. Does Transformation into a Black Hole Always Happen Instantly?
No.
Especially after neutron star mergers, the extremely massive object formed can remain intact for a short time with the help of different physical support mechanisms.
Therefore, in some mergers, black hole formation may not occur immediately.
Subsequent energy loss or changes in rotation rate can trigger collapse.
19. "Critical Mass" Is Not a Single Number
There is an important distinction here.
There is no universally accepted single exact value for the mass at which a neutron star will become a black hole.
The maximum mass depends on the star's rotation, temperature, composition, and especially the equation of state of matter at extreme densities.
For this reason, the TOV limit is considered a physical boundary that includes a range and model dependence.
20. GW170817 and Merger Findings
The GW170817 event observed in 2017 was one of the first direct observations of gravitational waves resulting from the merger of two neutron stars.
This event provided a unique opportunity to study how neutron stars can behave after merging.
The long-term fate of the object formed after the merger was used to constrain models of the maximum mass of neutron star matter.
21. Why Are Gravitational Waves Important?
Gravitational waves produced during neutron star mergers can carry information about the masses and structures of the stars.
The features in the final part of the wave signal help us understand how compressible the merging stars are.
This can provide information about the TOV limit and the equation of state of dense matter.
22. Why Is It Important to Determine the Mass Limit?
Determining this limit is important not only for understanding neutron stars.
It is also critically important for understanding how nuclear physics works under extreme conditions.
At the same time, it helps predict when neutron star mergers will form black holes.
23. The Mystery Inside Neutron Stars
The exact form of matter at the center of a neutron star is still not known for certain.
In addition to neutrons, more exotic particles may be present.
Some models even suggest that quark matter may appear at very high densities.
These different possibilities directly affect the maximum mass value.
24. How Do Observations Constrain Theories?
As astronomers discover heavier neutron stars, some theoretical models are ruled out.
If a model cannot support the mass of an observed neutron star, its description of matter at extreme densities is inadequate.
Therefore, every new high-mass neutron star is an important test for dense matter physics.
25. Conclusion and Evaluation
Neutron stars are among the densest stable celestial objects in the universe. However, there is also a limit to the mass they can support. The Tolman–Oppenheimer–Volkoff limit is the fundamental concept that determines the balance between general relativity and the internal pressure of neutron star matter. If a neutron star exceeds this limit, it may not be able to withstand its own gravity and a black hole may form as a result of collapse.
It is still not possible to know the exact value of this critical limit today, because the behavior of matter at extreme densities in the centers of neutron stars has not yet been fully resolved. Neutron stars with masses of about two solar masses and gravitational waves from neutron star mergers are increasingly constraining theoretical models.
The critical mass that determines the transformation of a neutron star into a black hole is, in fact, the limit of how much resistance matter in the universe can show under the most extreme conditions.