eli5 are neutron stars composed of neutrons, or is it quark-gluon plasma?
Neutron stars have a crust of iron and neutrons. Going down there is less and less iron. We don’t know what the core is like. It could be neutrons all the way down or it could be a quark-gluon plasma in the core. This is an active area of research.
Neutron stars are composed primarily of neutrons, which are subatomic particles that are found in the nucleus of an atom. These neutrons are held together by the strong force, which is one of the four fundamental forces of nature.
At the extremely high densities found in the interior of a neutron star, it is possible that the neutrons themselves may break down into their constituent quarks, forming a state of matter known as quark-gluon plasma. However, this is still an active area of research and it is not yet clear exactly how matter behaves at these extremely high densities.
It’s messier and more complicated than that.
We don’t know for sure, because of course we haven’t been able to create the conditions in a lab, but we have pretty good models of the way quarks, mesons (pairs of quarks) and nucleons (triplets of quarks) interact.
At the surface of a neutron star, there’s likely a crust of “normal” matter. It’s not necessarily the types of atoms common on earth – there will be a lot of radioactive atoms with more neutrons per atom than were used to. And of course a lot of these atoms are actually ionized (there aren’t exactly the same number of electrons attached to a given atom as protons in the atom.)
Now let’s look down at the center. This is probably neutronium. That’s basically a fluid of tightly packed neutrons. It’s probably not a quarks gluon plasma because the temperature is probably low enough that groups of 3 individual valence quarks stay close together (these 3 stick together, and those 3 stick together).
Of course it’s messy because the combination of quarks in a neutron can change into the combination of quarks in a proton by releasing some energy plus an electron and a neutrino. But theres an effect called degeneracy pressure that makes that really unlikely in a neutron star compared to a single neutron.
In between the core and crust, we think there are intermediate phases called “nuclear pasta.” because the phases look like different types of pasta.
Iirc, just below the surface you find the atoms go from “close of normal” types towards “way too many neutrons per atom”.
A little deeper and you start to see long strings of neutrons (spaghetti) mixed in. Even deeper, the strings of neutrons start forming into sheets (lasagna). There’s some other versions of nuclear pasta also. What you have at a given depth depends on the pressure. The higher the pressure, the more neutrons (compared to atoms) and also more neutrons connected together
I’m pretty sure the correct flair would be *Physics* but who cares anyways. The answer is the first one (which should’ve kinda been obvious) as the latter is meant for the hypothetical quark stars.