Atoms are composed of protons, electrons, and neutrons. The electron is a type of elementary particle, but the proton and neutron are complex particles: they are made up of so-called “up” and “down” quarks. A proton has two up quarks and one down quark, while a neutron has two down quarks and one up quark. Due to the special nature of the strong interaction, these quarks are always bound together, so they can never be truly free particles like electrons, at least in the vacuum of space. It was recently published in Nature Communications Stady In the hearts of neutron stars they may finally be free.
Neutron stars are the remains of massive stars. Their presence is a last-ditch effort to prevent the star's core from collapsing into a black hole. After the dense nucleus runs out of nuclear fuel, only the quantum pressure of the neutrons can resist gravity. This is where things get complicated.
According to the simplified model of the neutron star, the core of the celestial body consists of neutrons, which are about to collapse. They can collide with each other with tremendous energy, but they are still neutrons. The quarks in them are bound together so tightly that neutrons cannot tear them apart. Some researchers theorize that at these gravitational limits, neutrons can break apart and their quarks can fuse into a kind of quark soup. This means that neutron stars have a dense quark core.
Unfortunately, we cannot directly examine the nucleus of neutron stars, nor can we create similar dense matter on Earth, but we do have an idea of how this dense nuclear matter behaves if we look at its equation of state. Examining the equation of state is a way to calculate the main properties of a substance. For neutron stars, this equation of state is the Tolman-Oppenheimer-Volkoff equation. The only problem with it is that it's very complex, and if you use it to calculate whether neutron stars have quark nuclei, you get the answer: Maybe.
This time, the researchers took a different approach. Instead of performing equation of state calculations, Bayesian statistics are applied to observational data on neutron star masses and sizes. This statistical method looks for patterns in observational data and extrapolates likely probabilities in a precise but efficient way. In this case, if neutron stars contain quark nuclei, they are somewhat denser than neutron stars that do not contain quark nuclei. Since small neutron stars are unlikely to contain quark nuclei, but larger ones do, a shift in the relationship between mass and density should appear in the Bayesian analysis.
The researchers found that massive neutron stars, with a mass greater than two solar masses, have an 80-90% probability of having a quark nucleus. It seems that the real question is not whether quark stars exist, but where the transition between quark stars and conventional neutron stars occurs.
In fact, this analysis is based on a fairly small data sample. The mass and radius of most neutron stars are currently unknown, but this will change over time. If we had more data, we could determine the exact conditions for the phase transition between quark matter and neutron dense matter. However, for now, all we can say for sure is that some neutron stars are different than previously thought.
source: The universe today