SELF

12

O.N. Karavashkina and S.B. Karavashkin

2.7. Chemical evolution of star

To understand all aspects of further star evolution, we also have to enlighten (and mostly to recall) the chemical transformation in it and their interrelation with other revelations of star evolution. In this item we will consider the chemical evolution of an ideal star, disregarding individual peculiarities of evolution which we will touch in the next items.

Just as in previous items, let us begin with the conventional understanding of this process. Shklovsky writes: "Understanding of the meaning of energy sources is absolutely necessary to explain the duration of stars existence in almost constant state. Even more important is to know the nature of star energy for the star evolution… Mayer believed permanent fall of meteorites onto the Sun surface to be the source of Sun radiation. However, the calculation showed this source obviously insufficient to provide the observed Sun radiation. Helmholtz and Kelvin tried to explain long duration of Sun radiation by its slow contraction, which is accompanied by the gravity energy release. This highly important even (and especially!) for today astronomy hypothesis appeared, however, unable to explain the Sun radiation during billions years. … Already to 20th years of our century it became clear, we have to seek the energy source of Sun and stars in nuclear transformations. Eddington also thought so, but then there was impossible to indicate specific nuclear processes occurring in real star depths and accompanied by release of necessary amount of energy. … Jeans … believed such source in … radioactivity. … A. Epick … came to conclusion that the energy source of Sun and stars can be only in thermonuclear reactions of synthesis. Only in 1939 famous American physicist Bete provided the quantitative theory of nuclear sources of star energy" [1, p. 128- 129].

Consider, how nucleosynthesis occurs in Gorbatsky's description. "Thermonuclear reactions occur in plasma only under definite conditions that are unrealisable, for example, in outer layers of stars or in interstellar medium. ... Protons being like-charged particles experience the more electrostatic repel the closer they are located to each other. Proton is as if surrounded by a hindrance that impedes him to be approaching to another positively charged particle. … Intranuclear attracting forces very fast decrease with the distance and only at the distances between the particles about 10^-13 cm or less they exceed the forces of electrostatic repel. Consequently, the nucleus of deuterium atom will form only in case if relative velocity of colliding protons if so large that kinetic energy corresponding to it is enough to get the potential barrier over. … If the temperature of plasma is not sufficient - say, 3- 5 million degrees, - it almost does not contain the protons having the velocity necessary to get the potential barrier over, and proton-proton reaction does not go. But at temperatures about 15 million degrees, comparatively large part of particles has an energy allowing them to approach another particle quite close to form more complex nucleus. … On the other hand, in order to facilitate more often meetings of protons, high concentration of these particles is needed. Both these conditions are fulfilled in the central regions of Sun and alike stars" [11, p. 81].

In the star nucleus the particles approach each other to the distance less than the potential barrier, on the account of medium pressure. Thermal excitation adds them speed, and the star nucleus depleted of electrons allows the substance to be effectively compacting. This is why the concentration of particles able to make an association is so great in the star, and the amount of parameters of pressure and temperature is achievable much simpler than in the artificial synthesis. So we cannot exclude that Shklovsky was not so far from the truth when thought that nucleosynthesis begins already at first millions of degrees. And this exact value of temperature is not so much important in our consideration. It is important that

(i) in the star nucleus, there occur nuclear reactions that form new structural level of stellar substance, and

(ii) in the nuclear synthesis, there form not atoms but atomic nuclei with some quantity of electrons suppressed into the nucleus and formed in this way neutrons. And perhaps this is the only distinction from the conventional theory. Let us recall, how it describes the process.

First reactions of nuclear synthesis are called proton-proton; they have three branches. To the first branch there relate reactions in which deuterium either helium are produced; they are usually described as follows (in the right-hand parts we show the value of released energy, and in brackets - the calculated duration of reaction):