Red giants, supergiants and the future of our Sun

(ORDO NEWS) — A red giant is formed after a star with a mass of 0.8 to 10 solar masses has run out of hydrogen fuel for nuclear fusion, and the process of self-destruction has started.

A star maintains its stability through a delicate balance between its own gravity, which tends to push all matter toward the center, and internal resistance (expansion) from the continuous fusion processes occurring in its core.

However, as soon as the star’s core runs out of hydrogen, the balance is disturbed and the core begins to collapse.

When the nucleus “collapses”, the plasma shell surrounding it begins to synthesize residual hydrogen.

This leads to additional heating of the star and a rapid expansion of the outer layers; the star acquires an isothermal helium core and “swells” several hundred times its previous size.

The energy on the surface of the star is dissipated much more strongly than in the depths, as a result of which the swollen shell cools rather quickly, changing color from white or yellow to red.

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Red giant R Sculptor, 1500 light-years from Earth. The image was taken using the ALMA radio telescope complex on October 10, 2012

A red giant is forming. After several thousand years, the red giant sheds all its outer shells, forming a planetary nebula around its cooling core (white dwarf).

If the star was initially massive enough (from 10 to 70 solar masses), then it becomes not a red giant, but a red supergiant, which not only sheds shells, but bursts into a supernova (explodes), leaving behind an extremely dense core and nebula. In some cases, a supernova is considered a “failed” as the evolution ends with the appearance of a black hole.

The future of our sun

In about 4.6-7.1 billion years, our Sun will turn into a red giant, swallowing up some of the terrestrial planets (Mercury, Venus and Earth).

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Planetary nebula NGC 2371, 4500 light-years from Earth. Image taken by the NASA/ESA Hubble Space Telescope March 4, 2008

Then, when the gas shells are shed and a planetary nebula is formed, only a tiny (several thousand kilometers in diameter) core will remain, which will continue to cool, and in 1.5-2 billion years its temperature will be equal to the temperature of the surrounding space.

The history of the star will be finished.

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