Stars evolve over millions of years. In reality, they never stop evolving and changing, from birth to death.
They are born when a large amount of matter accumulates in a place in space. The material is compressed and heated until a nuclear reaction begins, which consumes matter, converting it into energy. Small stars spend it slowly and last longer than large stars.
The theories about the evolution of stars are based on evidence obtained from studies of the spectra related to luminosity. The observations show that many stars can be classified in a regular sequence in which the brightest are the hottest and the smallest, the coldest.
This series of stars forms a band known as the main sequence in the temperature-luminosity diagram known as the Hertzsprung-Russell diagram. Other groups of stars that appear in the diagram include the giant and dwarf stars mentioned above.
The life of a star
The life cycle of a star begins as a large mass of relatively cold gas. The contraction of the gas raises the temperature until the interior of the star reaches 1,000,000 ° C. At this point nuclear reactions take place, the result of which is that the nuclei of the hydrogen atoms combine with those of deuterium to form helium nuclei. This reaction releases large amounts of energy, and the contraction of the star stops. For a while it seems to stabilize.
But when the energy release ends, the contraction begins again and the temperature of the star increases again. At a given moment, a reaction begins between hydrogen, lithium and other light metals present in the star's body. Energy is released again and the contraction stops.
When lithium and other light materials are consumed, the contraction resumes and the star enters the final stage of development in which hydrogen is transformed into helium at very high temperatures thanks to the catalytic action of carbon and nitrogen. This thermonuclear reaction is characteristic of the main sequence of stars and continues until all the hydrogen is consumed.
The star becomes a red giant and reaches its largest size when all its central hydrogen has become helium. If it continues to shine, the core temperature should rise enough to cause the fusion of helium nuclei. During this process it is likely that the star will become much smaller and therefore denser.
When it has spent all possible sources of nuclear energy, it contracts again and becomes a white dwarf. This final stage may be marked by explosions known as "novas". When a star is released from its outer shell exploding as a nova or supernova, it returns to the interstellar medium heavier elements than the hydrogen it has synthesized inside.
Future generations of stars formed from this material will begin their life with a richer assortment of heavy elements than previous generations. Stars that shed their outer layers in a non-explosive way become planetary nebulae, old stars surrounded by spheres of gas that radiate in a multiple range of wavelengths.
From star to black hole
Stars with a mass much greater than that of the Sun undergo a faster evolution, a few million years from birth to the explosion of a supernova. The remains of the star can be a neutron star.
However, there is a limit to the size of neutron stars, beyond which these bodies are forced to contract until they become a black hole, from which no radiation can escape.
Typical stars such as the Sun can persist for many billions of years. The final destination of low mass dwarfs is unknown, except that they cease to radiate appreciably. Most likely they become ashes or black dwarfs.
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