What is a quark star?

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In wide circles, neutron stars have long and firmly secured the title of the densest and most compact objects in the Universe, with the exception, of course, of black holes. However, calculations show that there may be stars that will be even smaller in size than neutron ones, but will have a greater mass. They are called quark stars. What is it and where do they come from? Let’s figure it out together.

From the school physics course, most people know that the matter around us consists of atoms, they, in turn, consist of a nucleus and electrons, and in the nucleus there are protons and neutrons. The knowledge of most people about microparticles usually ends there, but protons and neutrons also have constituent parts, each of them consists of three smaller particles — quarks.

According to modern concepts, quarks under ordinary conditions cannot exist alone, they always form, a particle consisting of several quarks. But sometimes some conditions develop in a neutron star and this rule can be violated, the so-called quark deconfinement occurs.

When a sufficiently massive star dies, a neutron star is formed from it, it has a huge mass, extremely small size and rotates at a tremendous speed, sometimes making thousands of revolutions per second. The maximum mass of a neutron star is, according to various estimates, from 2.5 to 3 solar masses; with a larger mass, a neutron star will simply collapse into a black hole.

Quark stars can form from the most massive neutron stars close to becoming a black hole. When such neutron stars slow down their rotation, the centrifugal force becomes less, and gravity compresses the core of the neutron star more strongly. Due to the high pressure and temperature in the nucleus, neutrons there begin to decay into quarks, forming quark matter. This process may be limited to the core, but under certain circumstances it should cover the entire neutron star and all of its matter will consist of individual quarks. In any case, the star is considered a quark star.

During the transition to the quark state, the star shrinks significantly; according to various estimates, its diameter should decrease by 1.5-2.5 times. Also, in this case, a quark nova should occur — an explosive release of energy, according to calculations, it should be 100,000 times higher than the energy of an ordinary supernova explosion and be the brightest event in the Universe.

At the moment, there is no confirmation of the existence of quark stars, but there are several candidates for them. This may be due to the fact that quark stars should be extremely rare, according to calculations, only two quark novae should be formed every day in the entire observable Universe, and it is incredibly difficult to observe the already formed quark stars, and it is even more difficult to distinguish them from neutron stars.

Author: Fedor Karasenko.

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