The density of neutron stars

Neutron stars are extremely dense planets in the universe. Even according to the most conservative estimates, the density of neutron stars can be as high as 100 trillion tons per cubic meter, which, it must be said, is unimaginable, so why do neutron stars exist at such high densities? Let's take a look at how empty the atoms are.

Take the hydrogen atom as an example. The hydrogen atom is the simplest in the universe, possessing only one electron, and its nucleus is a proton. The radius of a hydrogen atom in the ground state is about 0.528 x 10^(-10) meters, while the radius of a proton is about 0.84 x 10^(-15) meters. As for the radius of the electron, we do not have a definite value yet, but, certainly, the radius of the electron order of magnitude will not be larger than 10^(-18) meters.

That is, the total radius of a hydrogen atom is roughly 62,857 times that of its nucleus. If the nucleus radius of a hydrogen atom is enlarged to the size of soccer, then after the same scale of enlargement, the hydrogen atom has a radius of about 7 km, while the electron is a particle of dust with a radius of almost 0.1 mm.

Yes, that is how empty the inside of an atom is. For the matter of the Earth, the atoms are not close together, for example, the average atomic spacing in solid matter is usually between 10^(-9) meters and 10^(-10), and the average atomic spacing in the liquid and gaseous matter is even larger than in solid matter if we compress all the atomic spacing of the Earth's matter and the space inside the atoms, then the radius of the Earth would be reduced to The density of the Earth in this state is the density of a neutron star.

So we can simply understand that the reason why neutron stars are so dense is that the matter that makes up the neutron star is in a highly compressed state, so why are there such dense planets in the universe? The answer is gravitational collapse.

Since gravity is a long-range force and has only an "attractive" force without a "repulsive" force, it can be superimposed infinitely. For the various planets in the universe, their gravitational forces not only "attract" nearby matter, but also cause their gravitational collapse.

If a planet wants to exist stably, it must have a mechanism to resist gravitational collapse inside it. If the mass of the planet is greater than or equal to this threshold, then the electromagnetic force cannot resist.

All stars in the universe have masses greater than or equal to 8% of the mass of the Sun, and they resist their collapse by the nuclear fusion reactions at their cores, but the "nuclear fuel" of a star is, after all, finite, and after the "nuclear fuel" is depleted, the star will inevitably continue to collapse.

For less massive stars, their cores can resist gravitational collapse and eventually evolve into a white dwarf by the "abbreviation pressure" between electrons.

(Note: "Simplicity pressure" can be simply understood as the force by which some microscopic particles do not allow other particles of the same kind to occupy their own space, and thus repel each other)

But if the mass of the star is large enough, the "simple merger pressure" between the electrons can not resist the gravitational collapse, in this case, the core of the star will continue to collapse, and under the huge pressure, the electrons will be pressed into the nucleus and combined with the protons to form neutrons, these neutrons and the original neutrons in the nucleus will be close to each other These neutrons and the original neutrons in the nucleus are pressed together, creating a "neutron simplification pressure".

Generally speaking, when a star reaches this point in its evolution, it undergoes a supernova explosion of incredible power, after which, if its remaining core can resist gravitational collapse by the "neutron recombination pressure," it evolves into an extremely dense planet where all the microscopic particles are close together, with a density of at least Since most of the microscopic particles that make up such a planet are neutrons, they are called neutron stars.

One may ask, what happens after a star goes supernova if the mass of its residual core is so large that even the "neutron simplification pressure" cannot resist the gravitational collapse?

According to modern physics, neutrons are made up of quarks, and there is a "comoving pressure" between quarks, so reasonable speculation is that if the "neutron comoving pressure" cannot resist the gravitational collapse, then The neutrons will be "crushed", and then the "quark simplex pressure" will take the responsibility of "resisting the gravitational collapse".

If the "quark-simplex" can resist the gravitational collapse, the residual core of the star evolves into a denser planet than a neutron star, which we can call a quark star.

If it cannot resist, then there is no force left in the framework of modern physics that can resist the gravitational collapse, in which case the residual core of the star will become an infinitely small and dense "singularity" due to the unstoppable gravitational collapse, and this "singularity This "singularity" will bend the nearby space-time to the extreme, forming a completely closed space-time, thus evolving into the universe's most daunting existence - black holes.

It is worth mentioning that since scientists are not sure whether there are quark stars in the universe, we usually still think that neutron stars are the second most dense objects in the universe after black holes.