From time immemorial, the stars overhead have stirred the consciousness of mankind, been a source of inspiration and scientific interest. Observations of them and the laws of physics have allowed scientists to build a theory of stellar evolution that describes the different stages of their life cycle. One of the key stages is the stage when the star's matter is compressed to a density characteristic of the atomic nucleus. Such states provide unique opportunities for studying matter in extreme conditions, allowing scientists to go beyond the standard models of the fundamental forces of nature.

Precision data from modern cosmology confirms that our universe is dominated by hidden mass and dark energy. Their description requires a revision of current physical models. As Maxim Khlopov, chief researcher at the SFedU Research Institute of Physics, Doctor of Physico-Mathematical Sciences, Professor, notes: "Such an output is also necessary to describe the initial conditions of the evolution of the Universe, which attracts inflation and bariosynthesis, the mechanisms of which cannot be based only on known physical laws. Thus, the now standard cosmological model attracts physics beyond the framework of standard models of fundamental interactions. The research of this physics is based on a combination of experimental physical and astrophysical studies. In the latter, the analysis of the natural conditions of the state of matter in compact stars occupies an important place."

What are compact stars and their evolution?

In the process of evolution, stars go through stages in which their matter is compressed to extremely high densities. This leads to a significant decrease in the size of the stars, making them more compact. This compression of massive stars leads to their collapse into black holes. The result of the evolution of stars whose mass is less than 2-3 times the mass of the Sun is considered to be the formation of neutron stars, the substance of which is compressed to the density of the atomic nucleus. However, the description of the macroscopic state of matter at such densities may differ significantly from the structure of atomic nuclei.

It is generally believed that at such densities, the matter of neutron stars consists of stable neutrons that do not decay, as happens with free neutrons. But the scientists of the Southern Federal University in their study proposed an alternative explanation — at these densities, a transition to the so-called "color-fragrant" state may occur. In this state, protons and neutrons, which make up atomic nuclei, can decompose into quarks, forming a giant "superfluid quark drop". Interestingly, not only light quarks are involved in this state, but also heavier "strangeness" quarks, which turn out to be stable under such conditions.

The term "color-fragrant state" may seem unusual, but it hides a deep physical meaning. In quantum chromodynamics, the theory describing the strong interaction between quarks— special characteristics of particles are introduced, which physicists figuratively called "color" and "aroma".

The "color" of a quark is a specific quantum charge that takes one of three values, conventionally called red, green and blue (there are also anti—colors). It is important to note that these "colors" have nothing to do with visible colors. It's just an analogy.: Just as the combination of all the colors of the visible spectrum gives a white color, so the combination of three quark "colors" forms a "colorless" state that can exist in nature, such as in protons and neutrons.

"Flavor" is another fundamental characteristic of quarks that determines their type. There are six "flavors": upper, lower, enchanted, strange, true and charming. These names are conventions that have nothing to do with odors. In ordinary matter, only upper and lower quarks are found, but at high densities and temperatures, heavier "flavors" can also appear.

When physicists talk about the "color-fragrant" state of matter in superdense stars, they mean a special quantum state in which quarks of various "flavors" and "colors" form complex structures. Such a state is fundamentally different from ordinary nuclear matter and can lead to the appearance of completely new properties of matter.

The aim of the study, the results of which are published in the Chinese Journal of Physics, was to study the possibility of the existence of such a color-fragrant quark state in compact stars.

"Based on the solutions of the equations of general relativity, a description of the quark state of matter and the structure of compact stars has been obtained. We came to the conclusion that such a description is not only realistic, but also leads to new conclusions about the possible properties of compact stars," said Maxim Khlopov.

The results obtained by scientists open up new horizons for astronomical research. The developed model makes it possible to predict the existence of stars with masses noticeably exceeding the limits accepted for neutron stars. This discovery may lead to a revision of the observational data for such objects and their interpretation.

"New solutions for describing the structure of stars lead to the prediction of new types of celestial objects and the interpretation of both astronomical observations and fusion processes of compact stars available to multichannel astronomy methods," the scientist emphasizes.

As part of this work, scientists have compiled graphs describing the dependence of the mass and radius of stars on their density, which made it possible to more accurately model the possible properties of compact objects.

These studies of the SFedU were conducted with the financial support of the Ministry of Science and Higher Education of the Russian Federation (State Order GZ0110/23-10-IF). The results of the work may be important both for the interpretation of gravitational wave signals from mergers of compact stars, and for the analysis of processes occurring in such systems.

According to Maxim Khlopov, cooperation with Indian colleagues, which has been going on for many years, provides excellent opportunities for further research. This concerns not only the study of new forms of matter, but also their evolution and astrophysical manifestations. The importance of these studies lies in the fact that they expand the methods of studying physics beyond the standard models of fundamental interactions, providing new approaches to the development of this field. At the same time, the experience of a group of Indian scientists led by the Deputy director of the Center for Cosmology, Astrophysics and Space Sciences (CCASS) of GLA University (Mathura, Uttar Pradesh), Professor Saibal Ray, plays an important role in solving equations of both the standard (GRT) and modified theory of gravity.

Thus, the proposed model of compact stars opens up broad prospects for studying new physical phenomena, and also helps to clarify modern ideas about the structure and evolution of the Universe.