4 ms·

One “wow” moment that blew me away during my grad physics studies was the realisation that a neutron star is effectively one gigantic atomic nucleus.

BitwiseFool 5y ago
Fascinating, do you know if protons can exist within neutron stars? That would make it each one an element, right? Or if not, they would all be isotopes of Neutronium?
- Florin_Andrei 5y ago
  There are no isotopes with neutron stars because they are not actually nuclei.
- deleted 5y ago
  [deleted]
- Thorondor 5y ago
  Yes, it's thought that neutron stars are about 5-10% composed of protons and electrons. In fact, that's why a neutron star can have a magnetic field – charged particles are required to carry an electric current.
  - ziotom78 5y ago
    However, in principle there should be a (perhaps very small?) component due to the inner magnetic momentum of neutrons [1], right? After all, they are made of quarks, which have nonzero charge. [1] https://en.wikipedia.org/wiki/Neutron_magnetic_moment https://en.wikipedia.org/wiki/Neutron_magnetic_moment
- dredmorbius 5y ago
  There's a gradient of material. At the core, a neutron star is likely very nearly pure neutronium, as other particles simply won't exist long under the conditions present. As depth decreases, other particles can exist, and at the surface there are likely free nuclei and atoms of elements, though my probably with low nuclear potential. Fission and fusion would be accelerated by the energies involved. That's a lay reading / bits of stray collected memories, though quick checking seems to confirm most of this. See: https://heasarc.gsfc.nasa.gov/docs/objects/binaries/neutron_star_structure.html https://heasarc.gsfc.nasa.gov/docs/objects/binaries/neutron_...
Florin_Andrei 5y ago
That's stretching quite a bit the definition of a nucleus. Neutron stars are held together by gravity, not nuclear forces. Also, the mix of particles is very different from an actual nucleus. The physics are pretty different. It's a bit like saying the SpaceX Starship, and my mechanical pencil are the same thing, because they have the overall same shape, and are made of stainless steel.
- jeffwass 5y ago
  > Neutron stars are held together by gravity, not nuclear forces. Neutron stars are governed by both gravitational and the strong force. [edited] Quite different from ealier phases of stars which are held together by gravity. > Also, the mix of particles is very different from an actual nucleus. Which particles? The key difference vs an atomic nucleus other than the immense size is that the neutron star is not charged like an atomic nucleus, as most of the precursor star’s protons and electrons have fused into neutrons.
  - Florin_Andrei 5y ago
    https://en.wikipedia.org/wiki/Neutron_star#Density_and_pressure https://en.wikipedia.org/wiki/Neutron_star#Density_and_press... > In popular scientific writing, neutron stars are therefore sometimes described as "giant nuclei". However, in other respects, neutron stars and atomic nuclei are quite different. A nucleus is held together by the strong interaction, whereas a neutron star is held together by gravity. The density of a nucleus is uniform, while neutron stars are predicted to consist of multiple layers with varying compositions and densities. It's just a pop-sci meme. As for the structure, see here: https://en.wikipedia.org/wiki/Neutron_star#Structure https://en.wikipedia.org/wiki/Neutron_star#Structure
    - jeffwass 5y ago
      I have a feeling you’re just trolling for trolling sake. I’ll only comment on one part of the Wikipedia quote regarding constant densities of atomic nuclei. The neutron star will have a constant density on any length scale as large as an atomic nucleus. The layered structure manifests over length scales of kilometers, which is 10^18 times larger than an atomic nucleus.

BitwiseFool 5y ago

Fascinating, do you know if protons can exist within neutron stars? That would make it each one an element, right? Or if not, they would all be isotopes of Neutronium?

Florin_Andrei 5y ago
There are no isotopes with neutron stars because they are not actually nuclei.
deleted 5y ago
[deleted]
Thorondor 5y ago
Yes, it's thought that neutron stars are about 5-10% composed of protons and electrons. In fact, that's why a neutron star can have a magnetic field – charged particles are required to carry an electric current.
- ziotom78 5y ago
  However, in principle there should be a (perhaps very small?) component due to the inner magnetic momentum of neutrons [1], right? After all, they are made of quarks, which have nonzero charge. [1] https://en.wikipedia.org/wiki/Neutron_magnetic_moment https://en.wikipedia.org/wiki/Neutron_magnetic_moment
dredmorbius 5y ago
There's a gradient of material. At the core, a neutron star is likely very nearly pure neutronium, as other particles simply won't exist long under the conditions present. As depth decreases, other particles can exist, and at the surface there are likely free nuclei and atoms of elements, though my probably with low nuclear potential. Fission and fusion would be accelerated by the energies involved. That's a lay reading / bits of stray collected memories, though quick checking seems to confirm most of this. See: https://heasarc.gsfc.nasa.gov/docs/objects/binaries/neutron_star_structure.html https://heasarc.gsfc.nasa.gov/docs/objects/binaries/neutron_...

Florin_Andrei 5y ago

That's stretching quite a bit the definition of a nucleus. Neutron stars are held together by gravity, not nuclear forces. Also, the mix of particles is very different from an actual nucleus. The physics are pretty different. It's a bit like saying the SpaceX Starship, and my mechanical pencil are the same thing, because they have the overall same shape, and are made of stainless steel.

jeffwass 5y ago
> Neutron stars are held together by gravity, not nuclear forces. Neutron stars are governed by both gravitational and the strong force. [edited] Quite different from ealier phases of stars which are held together by gravity. > Also, the mix of particles is very different from an actual nucleus. Which particles? The key difference vs an atomic nucleus other than the immense size is that the neutron star is not charged like an atomic nucleus, as most of the precursor star’s protons and electrons have fused into neutrons.
- Florin_Andrei 5y ago
  https://en.wikipedia.org/wiki/Neutron_star#Density_and_pressure https://en.wikipedia.org/wiki/Neutron_star#Density_and_press... > In popular scientific writing, neutron stars are therefore sometimes described as "giant nuclei". However, in other respects, neutron stars and atomic nuclei are quite different. A nucleus is held together by the strong interaction, whereas a neutron star is held together by gravity. The density of a nucleus is uniform, while neutron stars are predicted to consist of multiple layers with varying compositions and densities. It's just a pop-sci meme. As for the structure, see here: https://en.wikipedia.org/wiki/Neutron_star#Structure https://en.wikipedia.org/wiki/Neutron_star#Structure
  - jeffwass 5y ago
    I have a feeling you’re just trolling for trolling sake. I’ll only comment on one part of the Wikipedia quote regarding constant densities of atomic nuclei. The neutron star will have a constant density on any length scale as large as an atomic nucleus. The layered structure manifests over length scales of kilometers, which is 10^18 times larger than an atomic nucleus.