If I were asked to name the objects in the Universe that represent the last frontier before the unknown, it would be neutron stars. In this article, we will get to know them better, and it will be fascinating, because we will be talking about the most extreme observable compact bodies in space – the last ones before stepping into the pitch blackness of the black hole singularity.

History of discovery
Neutron stars are among the phenomena predicted by theorists “at the tip of a pen.” Almost immediately after the discovery of the neutron (a subatomic particle that makes up atomic nuclei) in 1932, in 1934, astronomers Fritz Zwicky and Walter Baade published a hypothesis about the existence of objects composed exclusively of the newly discovered particles. The scientists wanted to explain supernovae, short-lived outbursts of stars that release enormous amounts of energy. Although very little was known at the time about nuclear forces and the structure of the atomic nucleus, they made the correct assumption that this energy was converted from gravitational energy when the star sheds its outer shell during the explosion, and its central particle is compressed millions of times. Taking into account the typical masses of luminaries and the estimate of the density of nuclear matter, it also became immediately clear that it would be impossible to find a neutron star with optical telescopes. The fact is that after a supernova explosion, the remaining mass of the supernova, when compressed to nuclear density, would form a sphere with a radius of only a few tens of kilometers, which, even at very high surface temperatures, would be impossible to see due to insufficient brightness.

However, everything changed in the second half of the 1960s. By a strange (though not so rare in science) coincidence, at that time, both theoretical and observational astronomers simultaneously and independently concluded that neutron stars must be sources of powerful radio radiation. In 1967, the Italian astrophysicist Francesco Pacini theorized this. In his article in the journal Nature, he noted that if such an object (at that time only hypothetical) has a strong magnetic field and rotates rapidly, it must convert the kinetic energy of rotation into radio waves. The first observation of a powerful radio source from deep space took place in 1965 in the Crab Nebula, and the confident discovery of the first radio pulsar, which was later identified as a neutron star, was reported in 1967. The American astronomer Anthony Hewish contributed most to both discoveries. He was the one who built the instrument that discovered the first pulsar, PSR B1919+21, with a period of ~1.3 s. However, we should not forget about the contribution of his student Jocelyn Bell, who had to dig through tens of meters of paper records of the instrument’s measurements (an array of radio antennas) in search of signals. Moreover, Hewish received his Nobel Prize for this discovery, while Bell did not, which later caused controversy over the fairness of this award. The detected signal was initially designated LGM-1, which stood for Little Green Man-1. The existence of such a powerful radio source from space seemed so incredible at the time that scientists were ready to attribute it to aliens, jokingly, of course.
The end point of evolution
So, let’s try to figure out how neutron stars have such interesting properties that make them observable. As mentioned earlier, they are the last step before an astrophysical object “falls” into a singularity under the influence of its gravity. A neutron star has a chance to become a sufficiently heavy star, the initial mass of which, however, does not exceed eight solar masses. During its lifetime, it will gradually “burn” fusion fuel in its core: first, hydrogen, turning it into helium, and then helium into carbon. Then, if the core is massive and large enough, it will start fusion of carbon nuclei into heavier ones, such as neon, oxygen, and others. If it is not massive enough for the enormous pressure inside it to allow such reactions to take place, then after the outer shell is shed (this also looks like a supernova flare), the remnant of the star becomes a white dwarf, consisting mainly of carbon atoms.

If the gravity is stronger, i.e. if the stellar core is about 1.4 times heavier than the Sun (the so-called Chandrasekar limit), then after the carbon in the core is “burned out”, electrons begin to tunnel into the nuclei of atoms, interacting with protons to form neutrons and “elusive” neutrino particles. In a short moment, an enormous amount of energy is released, and a single large neutron nucleus is created. If gravity “wins” even the pressure of extremely densely packed neutrons, then the remaining matter collapses irrevocably to its gravitational radius, i.e., into a black hole. This happens when its mass is higher than another limit – the Tolman-Oppenheimer-Volkoff limit (approximately 2.4 solar masses). As you know, in such cases, no signal can get out of there, so we can only theorize about the interior of a black hole. So let’s get back to neutron stars.
During the explosion, the outer shell “falls” to the surface of the star’s core at a relativistic speed that can reach a quarter of the speed of light. Part of it “bounces” off the surface, forming a supernova remnant that will be visible for centuries after the explosion. And the explosion itself is a supernova, or rather, one of its types. Humanity has been lucky enough to observe such phenomena “up close” (in our own or neighboring galaxies) no more than a dozen times in its history.
In this metamorphosis, the neutron star inherits very little from its predecessor, the “ordinary” star. However, one of the quantities that is largely preserved is the momentum, or torque. The laws of conservation of this quantity lead to the fact that the neutron star compensates for the radical decrease in size with an incredible rotation speed. Thus, in some pulsars, the pulsation period can be only a few milliseconds. This means that for an object with a radius of 20-30 km, which makes a complete rotation during this time, the speed of movement of the surface points can be tenths of the speed of light.
This explains one of the amazing properties of neutron stars that makes them visible. Scientists have not yet found an unambiguous answer about the origin of the other, a powerful magnetic field. Perhaps it is also a “legacy” from a dying star. In this case, we are talking about the law of conservation of magnetic flux. If initially the magnetic field created a flux through a significantly larger cross-sectional area (since the star had a much larger radius), then after its reduction from millions to several tens of kilometers, the field strength should have changed inversely to the area. But even this rationale is not enough to explain the monstrous magnitudes of field strengths that astrophysicists estimate neutron stars have near their surface.
Structure and properties
What secrets does the structure of neutron stars hide? Even though they are practically a clump of identical particles (neutrons), their structure can be heterogeneous. For example, the surface layer, which is several percent of the radius, consists of ionized matter – extremely densely packed atomic nuclei and a sea of electrons separated from them. This dense packing allows us to consider the surface as a solid state of matter. As a result, under the influence of centrifugal force and magnetic field, “mountains” several centimeters or millimeters high can form there. When they collapse after “starbursts,” the energy released is enough to be seen from Earth, in the form of powerful gamma-ray bursts coming from neutron stars.

The closer to the center, the higher the density and the fewer protons and electrons that have not yet interacted to form a neutron. The value of the neutron star’s matter density is comparable to the density of atomic nuclei, namely, on the order of 10¹⁷ kg/m³. To describe it, the same formulas and laws are used as in nuclear physics. Astronomical numbers become clearer from the already classical comparison: a teaspoon of this neutron matter will have the same mass as Mount Everest. A substance under such pressure becomes what scientists call a “neutron-proton Fermi gas,” or degenerate matter. In this state, it has extremely unusual properties. For example, its volume almost does not depend on its mass, and the weak dependence that is still present turns everything upside down: most likely, the heavier the neutron star, the smaller its radius.
The nuclei of neutron stars have their secrets. Their density can be twice the average for a star, and their temperature can reach ten billion kelvin. And the conditions there can be such that matter will pass into the state of quark-gluon plasma, when not only neutrons and protons consisting of up and down quarks are in equilibrium, but also more exotic elementary particles with a different quark composition. On Earth, such a state of matter is achievable only for billions of fractions of a second in the most powerful colliders.
Another characteristic feature of the neutron monster is the magnetic field. Its strength near the star can reach values from 10⁴ to 10¹¹ tesla. The energy density (E/c²) of the field at the upper limit of this range is 10 thousand times higher than the density of lead. For comparison, the record that was achieved in terrestrial laboratories was several dozen of the same teslas. And the magnetic field on the surface of our planet has a power of no more than 50 microtesla.
The axis of symmetry of a neutron star’s magnetic field usually does not coincide with its rotation axis. Since this field is markedly inhomogeneous, when rotating at a terrifying speed, it generates radio radiation of sufficient power to be detected hundreds or thousands of light-years away.
This can be considered an almost obligatory attribute of a neutron star: among the 2010 known objects of this type, 2000 are characterized by radio pulsations. Those with particularly strong magnetic fields are called magnetars. These fields are so powerful that they can tear protons from the surface of the star. This leads to rather long (up to 10 s) gamma-ray bursts, during which energy is released comparable to the annual radiation of the Sun. In addition, the force of interaction of such fields with the dipole magnetic moment of an electron can exceed its rest energy, which gives rise to interesting relativistic effects, such as vacuum polarization, particle production from the field, etc. Due to the lengthening of electron clouds around the nucleus, hydrogen atoms in such a field become like wires, they are several hundred times thinner than they are in the usual state.
The most extreme laboratories in the Universe
Finally, let us mention the special role of neutron stars for astrophysics, fundamental physics, and, paradoxically, life itself. We have already noted earlier that the existence of such objects is extreme in many respects: extreme fields, extreme proximity to a black hole, and extreme processes during formation. For example, the gigantic strength of the gravitational field on the surface of a neutron star deforms time and space so that time there slows down by several percent compared to distant regions.

Such extreme properties make it possible to test fundamental theories, in particular, Einstein’s General Theory of Relativity, in the most accurate way. It was thanks to the double pulsar PSR B1913+16 that, in the 1970s, it was possible to indirectly confirm the existence of gravitational waves. Two neutron stars, orbiting one another, slowly lost energy due to the emission of such waves, and this became noticeable due to the predicted and observed decrease in their rotation period. It was only in 2015 that these waves were discovered directly by the LIGO detector on Earth.
In 2017, the same detector helped to observe a unique and rare but important phenomenon – the merger of neutron stars. Scientists were then able to identify two events: the gamma-ray burst of GRB 170817A and the collapse that caused the burst of gravitational waves GW170817. This greatly contributed to the understanding of the physics of such processes. Such explosions are called kilon explosions (by analogy with supernovae), and their importance lies in the fact that they produce massive synthesis of chemical elements heavier than iron, which is impossible in the process of ordinary thermonuclear “burning” in the interior of stars. The chemical diversity of the dust that formed our Solar System, among other things, made it possible for life to emerge on Earth. So, in a sense, we are all children of a couple of neutron stars that fell in love with each other and eventually merged into one!
We can be sure that as astronomical instruments improve, neutron star research will continue to bring us closer to understanding our Universe – by unlocking the secrets of nuclear and relativistic physics, as well as quantum field theory, which play a crucial role in their existence. And perhaps even quantum gravity, which humanity has not even begun to get acquainted with from the practical side. So we are waiting for discoveries!
Author: Maksym Tsizh, PhD in Physics and Mathematics, Research Fellow at the Astronomical Observatory of Lviv University
This article was published in Universe Space Tech magazine #1 (189) 2023. You can buy this issue in the electronic version in our store.