Although stars appear to be eternal on the human time scale, they are born, live and die. Despite the fact that the life span of a star is extremely long, scientists have managed to analyze the evolution of celestial bodies and synthesize the knowledge gained. Thus, several stages of the evolution of stars are known, which differ depending on the mass of celestial bodies. It is necessary to analyze which way a star goes from birth to extinction, as well as how white dwarfs appear.
The first stage in the life of a star is its birth and formation. A star is born as a result of gravitational compression of an interstellar gas and dust cloud. After a multi-stage thermonuclear fusion reaction of four protons, a helium nucleus is eventually formed and elementary particles are released. In the final state, the total mass of the formed particles is less than the mass of four initial protons, which means that free energy is released during the reaction (Eldridge and Tout 56). Because of this, the inner core of a newborn star quickly warms up to ultra-high temperatures. Its excess energy begins to slosh toward its less hot surface—and out (Hill 78). By burning hydrogen in the process of a thermonuclear reaction, the star does not allow the forces of gravitational attraction to compress itself to a superdense state (Eldridge and Tout 34). In particular, the Sun has been in the active stage of burning hydrogen in the process of active nucleosynthesis for about 5 billion years (Hill 232). The more massive the star, the more hydrogen fuel it has. To counteract the forces of gravitational collapse, it has to burn hydrogen at a rate that exceeds the growth rate of hydrogen reserves as the mass of the star increases (Hill 101). The more massive the star, the shorter its lifetime, determined by the depletion of hydrogen reserves.
Sooner or later, any star will use up all the hydrogen available for combustion in its fusion furnace. The future of a celestial body depends on the mass, for example, the Sun ends its life in a specific way. As the hydrogen reserves in the interior of the star are depleted, gravitational contraction forces begin to gain the upper hand, and the star begins to shrink and condense (Hill 307). This process of secondary reaction of thermonuclear fusion, fueled by the products of the primary reaction, is one of the key moments in the life cycle of stars. During the secondary combustion of helium in the core of a star, so much energy is released that it begins to swell (Eldridge and Tout 83). In particular, the shell of the Sun at this stage of life will expand beyond the orbit of Venus (Hill 321). As this energy is now radiated across a much larger surface area, the outer layer of the star cools to the red end of the spectrum and the star becomes a red giant (Eldridge and Tout 102). However, this is not the final event, since large stars are characterized by a two-stage outcome.
First of all, the previous process leads to the next stage in the evolution of the star. For stars of the Sun class, after the depletion of fuel, the stage of gravitational collapse again sets in. The temperature inside the core is no longer able to rise to the level necessary to start the next level of fusion (Hill 84). The star contracts until the forces of gravitational attraction are balanced by the next force barrier (Hill 96). The state of the star stabilizes, and it turns into a degenerate white dwarf, which will radiate residual heat into space until it cools down completely.
Thus, the mass of a star is a key factor in determining what it will become at the end of its evolution. On the example of the Sun, it was analyzed that a celestial body is born due to the concentration of gravitational compression. Then a star is formed that lives for a certain amount of time, evolving permanently. As it was indicated, the stars of the class of the Sun, that is, having a similar mass, eventually turn into white dwarfs, and then gradually disappear.
Eldridge, J. J. and Tout, Christopher A. The structure and evolution of stars. World Scientific Publishing Company, 2018.
Hill, Graham. The evolution of stars. From birth to death. Cambridge Scholars Publishing, 2020.