Stellar evolution in astronomy is a sequence of changes that a star undergoes during its life, that is, over millions or billions of years, while it emits light and heat. During such colossal periods of time, the changes are very significant.
A star begins its life as a cold, rarefied cloud of interstellar gas, contracting under the influence of gravitational instability and gradually assuming the shape of a ball. When compressed, the energy of the gravitational field is transferred mainly into heat and radiation, and the temperature of the object increases. When the temperature in the center reaches 15-20 million K, thermonuclear reactions begin and the compression stops. The object becomes a full-fledged star. The first stage of a star’s life is similar to that of the sun – it is dominated by the reactions of the hydrogen cycle. It stays in this state for most of its life, until the reserves of fuel in its core run out. When in the center of the star all the hydrogen turns into helium, a helium core is formed, and the thermonuclear combustion of hydrogen continues at the periphery of the core.
During this period, the structure of the star begins to change. Its luminosity increases, the outer layers expand, and the surface temperature decreases – the star becomes a red giant. The star spends much less time on this branch than on the main sequence. When the accumulated mass of the helium core becomes significant, it cannot withstand its own weight and begins to shrink; if the star is massive enough, the temperature rising at the same time can cause further thermonuclear conversion of helium into heavier elements (helium – into carbon, carbon – into oxygen, oxygen – into silicon, and finally – silicon into iron).
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