Neutron stars represent one of the most extreme endpoints in the life cycle of a massive star. Born from the catastrophic core collapse that follows a supernova explosion, these stellar remnants are objects of immense gravitational power. They pack extraordinary amounts of matter into an incredibly small volume, creating a density surpassed only by a black hole’s singularity. To understand how heavy a neutron star is requires exploring the physics of matter under truly cosmic pressure.
The Typical Mass Range
The mass of a neutron star is most conveniently measured in solar masses (\(M_{\odot}\)), the unit representing the mass of our Sun. Most observed neutron stars cluster around \(1.4 M_{\odot}\), a value close to the theoretical maximum for a white dwarf star. This \(1.4 M_{\odot}\) figure indicates the minimum mass required for a star to collapse and form a neutron star. The typical observed mass range extends from approximately \(1.1 M_{\odot}\) up to about \(2.25 M_{\odot}\).
An average neutron star contains about 40% more mass than the entire Sun. All this material is compressed into a sphere only about 20 kilometers across, roughly the size of a major city.
Extreme Density and Composition
The reason for a neutron star’s tremendous mass concentration is its defining characteristic: extreme density. The gravitational collapse that creates the star forces electrons and protons to combine through a process called electron capture, resulting in a star composed almost entirely of neutrons. This process effectively bypasses the force that supports a white dwarf, known as electron degeneracy pressure.
The collapse stops only when the neutrons themselves resist further compression, a quantum mechanical effect known as neutron degeneracy pressure. This pressure, arising from the Pauli exclusion principle, prevents any two neutrons from occupying the exact same quantum state. This resistance is far greater than the electron degeneracy pressure that supports white dwarfs.
The resulting density is so high that a single teaspoon of neutron star material would have a mass exceeding five billion tons, which is comparable to the mass of all the cars on Earth combined. Deep within the star, where the pressure is highest, matter may exist in exotic states that stretch atomic nuclei into complex shapes, sometimes referred to as “nuclear pasta”. The core of the star reaches densities several times that of an atomic nucleus, pushing the boundaries of our understanding of matter.
The Theoretical Maximum Mass Limit
The stability of a neutron star against its own gravity is not limitless, and there is an upper boundary to its mass. This maximum stable mass is referred to as the Tolman-Oppenheimer-Volkoff (TOV) limit. It represents the point at which the neutron degeneracy pressure, even combined with nuclear forces, can no longer counteract the inward pull of gravity.
While the exact value depends on the poorly understood physics of matter at such extreme densities, current estimates place the TOV limit between approximately \(2.01 M_{\odot}\) and \(2.25 M_{\odot}\). Recent observations, particularly from the gravitational waves produced by merging neutron stars, have helped constrain this value, suggesting a tight upper bound around \(2.17 M_{\odot}\).
If a neutron star accumulates mass beyond this limit, perhaps by merging with another compact object or by accreting material from a companion star, it will undergo an instantaneous gravitational collapse. The immense pressure overcomes the quantum resistance of the neutrons, and the object implodes to form a black hole. The TOV limit defines the ceiling for how heavy a neutron star can be before it ceases to exist as a neutron star.
How Scientists Measure Neutron Star Mass
Determining the mass of these distant, tiny, and dense stellar objects requires indirect observational methods. The most accurate mass measurements for neutron stars come from studying them within binary systems. When a neutron star orbits another star, the dynamics of that orbit are governed by the laws of gravity, which depend directly on the masses of both objects.
Astronomers observe the orbital motions of these binary companions, often by detecting the periodic radio pulses from a spinning neutron star, known as a pulsar. By precisely timing these pulses, scientists can track the movement of the neutron star and its partner, allowing them to apply principles of orbital mechanics to calculate the masses. Highly precise measurements, such as those involving the general relativistic effect known as the Shapiro delay, have yielded the most accurate mass determinations for some of the heaviest known neutron stars.
A newer method for mass determination comes from the detection of gravitational waves produced by the merger of two neutron stars. The waveform of the gravitational signal carries information about the masses of the two spiraling objects, providing constraints on their internal structure and the maximum stable mass limit. These multi-messenger observations combine gravitational wave and electromagnetic data to refine our understanding of these cosmic remnants.