Neutron stars , despite their name, are not stars . A star is a gigantic ball of gas and plasma that accumulates enough mass that the tremendous pressures in its interior are capable of raising the temperature of its core enough to make possible the nuclear fusion of, at least, the hydrogen nuclei. that are there. At least because hydrogen is the most common element in the universe and, among those found in a certain amount, the one that produces the most energy after fusing, perpetuating the chain of reactions that makes star after star.
It is this same criterion that is followed to not classify brown dwarfs as true stars , since they do not accumulate enough mass and do not generate pressures large enough for nuclear fusion to take place in their interior in a sustained manner over time. These brown dwarfs are substellar objects , halfway between the giant planets like Jupiter and the smallest stars, the red dwarfs. Stars like Proxima Centauri where the right conditions do exist . This is why, technically, we shouldn’t consider neutron stars to be proper stars. These stars are composed of neutrons and reach densities similar to those of an atomic nucleus despite being trillions of times larger. In other words, these stars accumulate the mass of the Sun in a sphere of just 10 kilometers in radius . But despite the very great pressures that must be experienced inside, there is no atomic nucleus to fuse.
Of course neutron stars and brown dwarfs have very different origins . Brown dwarfs form, like any other star, after the slow compression of a gigantic cloud of gas and dust . The difference from real stars is simply that that cloud, or the part of the cloud that eventually gave rise to the brown dwarf, did not contain enough mass to form a real star. Neutron stars, however, have a much more sudden and violent origin , since they form after the death of a very massive star in the form of a supernova explosion .
These explosions do not all have the same origin. A good percentage of them, known as type Ia (one a) supernovae, the explosion occurs after a chain reaction produced by the fusion of carbon that makes the star explode, leaving behind nothing but the explosion itself . Type II (two) supernovae, on the other hand, happen when a star implodes in its last moments of life, expelling its outermost and lightest layers in this very powerful explosion. The explosion completely destroys the original star, but the implosion appears to leave behind an extremely compact object, what we know as a neutron star. These stars are among the densest macroscopic objects in the universe, only surpassed by black holes. A neutron star is denser than the core of any star and even white dwarfs.
During the implosion of the massive star’s inner layers just before the supernova explosion, electrons collide violently with protons, forming neutrons and emitting neutrinos in the process. The neutrinos leave the star quickly, passing intact through vast distances of dense material, but the neutrons are trapped in the collapsing core. It is the degeneracy pressure of the neutrons themselves, the most direct consequence of the Pauli exclusion principle, the only one capable of stopping the contraction of this nucleus before it becomes a black hole. The shock wave that destroys the outer layers does not reach the core of the star, leaving behind all the destruction what we know as a neutron star.
As we have already said, these objects accumulate masses similar to that of the Sun in a size of just a few kilometers in radius, thus being smaller than comets such as Swift-Tuttle , the cause of the Perseid meteor shower. Neutron stars are so dense that a small teaspoon of the material that makes them up accumulates millions of tons of mass. Another difference with brown dwarfs is that neutron stars do shine , although they do not do so because of the temperatures reached during the nuclear fusion of hydrogen, but because of the residual heat produced by the violent process that formed them .
In addition to compressing the core, the collapse that occurs after the supernova will also speed up the rotation until the neutron star rotates several hundred times per second . It will also boost their magnetic field, with newly formed neutron stars having magnetic fields billions of times stronger than Earth’s .
All these characteristics, however, will decay over time . The star will cool down, its rotation will slow down and its magnetic field will attenuate, in an extremely slow process that can take billions of years to complete.
James M. Lattimer, 2015, Introduction to neutron stars, AIP Conference Proceedings 1645, 61 (2015); https://doi.org/10.1063/1.4909560