Evolution of stars

Although the human time scale, and the stars seem to be eternal, they, like all things in nature, are born, live and die. According to the generally accepted hypothesis dust cloud, a star is born as a result of gravitational compression of interstellar gas and dust clouds. As the seal of the cloud is first formed protostar, the temperature at the center has grown steadily until it reaches the limit required for the speed of the thermal motion of particles exceeded the threshold at which the protons are able to overcome the macroscopic forces of mutual electrostatic repulsion and join the reaction of thermonuclear fusion.

As a result, a multi-stage reaction of thermonuclear fusion of four protons eventually formed the nucleus of helium (2 protons + 2 neutrons) and stands a fountain of different elementary particles. In the final state the total mass of the produced particles smaller than that of the original four protons, and thus in the process of reaction produces free energy. Because of this, the inner core of the new-born star quickly heated to extremely high temperatures, and the excess energy begins to spill toward it less hot surface - and out. At the same time the pressure in the center of the star begins to rise. Thus, the "burning" of hydrogen in fusion reactions, the star does not give the forces of gravitational attraction to squeeze themselves to a superdense state, opposing gravitational collapse continuously renewable internal thermal pressure, resulting in an sustainable energy balance. About the stars on the stage of active hydrogen combustion say they are on the "main phase" of their life cycle or evolution (see. Hertzsprung-Russell). The transformation of one chemical element into another inside a star is called nuclear fusion and nucleosynthesis.

In particular, the Sun is in the active phase of the combustion of hydrogen in the process of nucleosynthesis active for about 5 billion years, and stocks of hydrogen in the core for its continuation of our luminary should last another 5.5 billion years. The more massive the star, the greater the margin of hydrogen fuel at its disposal, but to counter the force of the gravitational collapse of hydrogen, it is necessary to burn with the intensity that exceeds the rate of growth of the rate of growth of reserves of hydrogen with increasing mass of the star. Thus, the more massive the star, the shorter its lifetime, determined by depletion of hydrogen, and the biggest stars are literally burn for "some" tens of millions of years. The smallest stars, on the other hand, the "comfortably" there are hundreds of billions of years. So on this scale our Sun belongs to "a strong middle peasants".

Sooner or later, however, any star will spend the whole suitable for burning in its thermonuclear furnace hydrogen. What's next? It also depends on the mass of the star. Sun (and all the stars, does not exceed the weight of more than eight times) ends his life rather banal way. With the depletion of hydrogen in the interior of the star power of gravitational contraction, waited patiently for this hour since the inception of the lights begin to prevail - and under the influence of the star begins to shrink and thicken. This process leads to a twofold effect: temperature in the layers immediately surrounding the core of the star rises to a level at which the hydrogen contained therein comes, finally, to the reaction of thermonuclear fusion to form helium. At the same time, the temperature in the core, consisting Now almost one helium rises so much that the mere helium - a kind of "ashes" decaying primary reaction nucleosynthesis - enters a new fusion reaction: three helium nuclei form one carbon nucleus. This process is a secondary reaction of thermonuclear fusion, the fuel for which the products are the primary reaction - one of the key moments of the life cycle of stars.

When the secondary combustion of helium in the core is released so much energy that the star begins to literally swell. In particular, the shell of the sun at this stage of life will extend beyond the orbit of Venus. Thus the total energy of the radiation of the star remains approximately at the same level as during the main phase of her life, but, as this energy is emitted now through a much greater surface area, the outer layer of the star cools down to the red part of the spectrum. Star turns into a red giant.

For star-class Sun after the depletion of fuel supply secondary reaction nucleosynthesis, again comes the stage of gravitational collapse - this time final. The temperature inside the core is no longer able to rise to the level required to start the fusion reaction the next level. Therefore, the star contracts until such time as the forces of gravitational attraction will not be balanced by the following power barrier. In his role acts the pressure of the degenerate electron gas (see. Chandrasekhar limit). Electrons to this stage to play the role of unemployed extras in the evolution of stars, not engaging in nuclear fusion and moving freely between the nuclei in the process of synthesis, at a certain stage of compression are deprived of "living space" and begin to "resist" further gravitational compression of the star. Status star stabilizes, and it turns into a degenerate white dwarf, which will radiate into space the residual heat, until cool completely.

Stars more massive than the sun, waiting for a much more spectacular end. After combustion of helium mass in compression is sufficient to heat the core and shell to the temperatures required to run the following reactions of nucleosynthesis - carbon, then silicon, magnesium - and so on, the growth of nuclear masses. Thus at the beginning of each new reaction in the core continues its previous envelope. In fact, all chemical elements up to iron, of which the universe was formed as a result of nucleosynthesis in the interior of the dying star of this type. But iron - is the limit; it can not serve as fuel for nuclear fusion reactions, or the collapse at any temperature and pressure, as for its collapse, and to add to it additional nucleons needed influx of external energy. As a result of a massive star gradually accumulates inside the iron core, incapable serve as fuel for any further nuclear reactions.

Once the temperature and pressure inside the core reaches a certain level, the electrons begin to react with the protons of iron nuclei, whereby the neutrons produced. And in a very short period of time - some theorists believe that this takes a matter of seconds - available throughout the previous evolution of the star, electrons literally dissolve in the proton nuclei of iron, all the material core of the star turns into a solid bunch of neutrons and begins to rapidly shrink in gravitational collapse because to oppose it degenerate electron gas pressure drops to zero. The outer shell of the star, from under which is stamped every prop, falls towards the center. The energy of the collision with the outer shell of the collapsed core of a neutron is so high that it is with great speed bounce and scatter in all directions from the nucleus - and the star literally explodes in a blinding flash of a supernova. In a matter of seconds in a supernova explosion can be released into space more energy than is emitted at the same time all the stars of the galaxy together.

After supernova shell expansion and the stars of a mass of about 10-30 solar masses ongoing gravitational collapse leads to the formation of a neutron star, a substance which is compressed until until begins to be felt degeneracy pressure of neutrons - in other words, now the neutrons (like Moreover, as previously did the electrons) begin to resist further compression, demanding currently living space. It usually occurs on reaching the size of a star about 15 km in diameter. The result is a rapidly spinning neutron star emitting electromagnetic pulses at a frequency of rotation; Such stars are called pulsars. Finally, if the mass of the star's core is more than 30 solar masses, nothing can stop its further gravitational collapse, and as a result of a supernova a black hole.