The Heaviest Neutron Stars Could Have Strange Matter Cores

Physics gets weird at the extremes. Astrophysics usually deals with the extremely large – large energies, large gravities, and lots and lots of stuff. Quantum mechanics, on the other hand, typically deals with the extremely small – quarks and other particles that are completely unseen by the human eye. So far, despite decades of trying, no Grand Unified Theory (or any other theory) combines these two opposed theories. This makes it all the more interesting that a team from the Purple Mountain Observatory of the Chinese Academy of Sciences proposed an idea that the interior cores of neutron stars, one of the most extreme examples of large extremes in the universe, might be made up of a type of tiny particle that makes up part of the “soup” of quantum mechanics called a strange quark. 

Strange quarks are one of the six types of quarks found in the Standard Model of particle physics. They are paired with several other quarks, including Charm, Up, and Bottom. Typically it’s tough to study these quarks in real life, as they almost immediately dissipate after being created to a point where they are measurable. 

However, neutron stars are made out of some of the most dense material in the universe. They are typically formed when a star runs out of fuel and collapses upon itself. Sometimes this phenomenon creates a black hole, whereas other times, depending on the star’s mass, it creates a neutron star. 

Given the exciting physics surrounding their creation, plenty of data has been collected on neutron stars. These include gravitational waves from binary neutron star mergers, their mass, and their radii. All of this data was fed into a model by the researcher at the CAS, and another part of that model gave them a strange quark result. That part is quantum chromodynamics.

Quantum chromodynamics, commonly known as QCD, is a branch of particle physics that deals with how quarks interact with each other. It’s gathered plenty of experimental evidence over the years, but it is still challenging for most non-experts to wrap their heads around. A significant component of it is known as the color force, which scientists believe is responsible for one of the four fundamental forces of physics – the strong nuclear force.

The researchers used QCD to check another series of calculations known as the “equation of state,” which helps determine when matter transitions from “hadronic” (i.e., normal matter that is made up of neutrons, protons, and electrons) to quark matter. Typically hadrons are made up of the quarks themselves, but at high enough pressures, the bonds holding those quarks together into the formation of hadrons break down. According to the new research, the pressure inside neutron stars seems to exceed those levels, causing their internal cores to be made up entirely of quarks. And those quarky cores could be up to 1 km wide. 

All this is another example of the extreme nature of astrophysics and how massive phenomena, like neutron stars, can be powered by extraordinarily tiny phenomena, such as strange quarks. There is still plenty more to learn about neutron stars as they continue to shine as beacons of the extreme physics that will help us better understand our universe.

Learn More:
Chinese Academy of Sciences – A strange quark matter core likely exists in neutron stars
Han et al. – Plausible presence of new state in neutron stars with masses above 0.98MTOV
UT – Astronomy Jargon 101: Neutron Star
UT – Gravitational Waves From Colliding Neutron Stars Matched to a Fast Radio Burst

Lead Image:
Artist’s depiction of a neutron star.
Credit – Pixabay / Public Domain