Is a neutron star basically an isotope?

Question

Since a neutron star is basically just a vast amount of densely packed neutrons, I was wondering whether those neutrons form a single "atom" (of atomic number zero) or whether they are further apart and in a QFT-sense isolated?

Answer 1

It is a misnomer (at best) to characterize a neutron star as all neutrons. There are protons and electrons too.

Imagine compressing a bunch of regular matter at some point it requires less energy for a proton and electron to combine to form another neutron rather than for the electron to try to fill a very high energy state. That means there are so many electrons so as to fill all the states with energy up to the difference in energy of a neutron compared to a proton plus an electron. So those electrons stay around, as do an equal number of protons.

So if you want to think of it as an isotope, it wouldn't be a Z=0 isotope. And if it is so large you could even have protons entering one side and leaving another in a spacelike separated way, there is no clean sense where it is a single object with a certain Z. It is a system of many interacting parts.

Answer 2

Even if it is pure neutrons, I doubt that it can be called one single atom. Strong forces are short ranged and I don't think a macroscopic number of neutrons can form one single bound state. Instead they will form into many bound states each with a few neutrons.

Calling a neutron star a giant $Z=0$ atom would be like calling the earth, which is made of electrons, neutrons and protons, a giant atom with a giant $Z\neq 0$.

Answer 3

A neutron star is kind of like a giant atomic nucleus, but its sheer mass changes the rules of the game. Quantum forces that usually control atoms get overwhelmed by extreme gravity, leading to some pretty wild differences in how matter behaves.

The similarity you're probably thinking of is how both a neutron star and an atomic nucleus are packed with particles, with hardly any free electrons. Theories suggest electrons mostly gather on the surface of a neutron star rather than inside, unlike how they orbit the nucleus in an atom. Protons are present too, but the star's immense gravity means neutrons might behave more like a strange plasma state of matter.

For context, the density of an atomic nucleus is around 2.3 × 10^17 kg/m³, while a neutron star takes it even further, ranging between 3.7 × 10^17 and 5.9 × 10^17 kg/m³. That extra density comes from gravity's pressure squeezing matter tighter than anything we typically see.

Think of it this way: when atoms get too massive, they break apart because gravity can't hold them together. But a neutron star is so massive that gravity starts interfering with both atomic and subatomic forces, creating conditions you wouldn't see anywhere on Earth.

We still don't fully understand how protons and neutrons behave under these extreme conditions. Gravity could completely alter how their inner particles, called hadrons, interact. Some scientists even speculate about the existence of "quark stars," where gravity is so intense that protons and neutrons break down into their quark components, forming an even denser state of matter—though not quite a black hole.

So, while neutron stars share some traits with atomic nuclei, their extreme gravity takes matter to a whole other level, far beyond anything we experience in normal atoms.