Sun: when and how will our star die?

The sun, without which life would never be possible on Earth, is not eternal. This is what the end of our star, now 4.6 billion years old, should look like. For stars, the length of life is determined by their mass. The more massive it is, the more life is short and consists of a few million years to finally end in a furious blaze of cosmic fireworks, a supernova. For our Sun, it will be different. Our star is a yellow dwarf with a total life span of about 10 billion years by mass (330,000 times the mass of Earth). The good news for life on Earth is that it is now only 4.6 billion years old, so it should still shine for another five billion years, according to astronomers' estimates. Nevertheless, it will still be necessary to think about relocation because Earth will become uninhabitable in less than a billion years (despite current global warming). Why? Because the Sun's luminosity will continue to increase by about 10% every billion years. Thus, our gentle blue will gradually turn into an oven, similar, perhaps, to the hell that now reigns on our neighbor Venus. Consequently, the habitable zone in our solar system will shift. It is also conceivable that Homo sapiens would migrate to Mars, which would become milder. But if that were to happen, it would not last long. So we might have to go to other stars, if the human race can then afford it. What happens when the Sun dies? Over time, the hydrogen reserves accumulated by the Sun at birth will run out. With its relatively small mass, the star has maintained a balance between gravity and radiation (as a result of hydrogen fusion) for billions of years. But there will come a "day" when the fuel runs out. Then a helium core (created by hydrogen fusion) will rule the heart of the star and collapse on itself. The temperature, which increases as it compresses, releases energy that pushes away the outer layers of the sun. In just five million years, the star will grow significantly and become what astrophysicists call a red giant. Red because its surface temperature will drop (to 3,000 K).
According to a study published in 2008, its radius will move from about 700,000 kilometers today to over 170 million kilometers! In other words, Mercury, Venus, and also the Earth will be moved inward, finally they will be destroyed (if our planet's orbit is not pushed back, it will depend on the mass lost by the red giant). During compression, the core of helium heats up and manages to reach a temperature of 100 million degrees. Under these conditions, helium can begin to forge carbon. This is known as a "helium flash." But it won't last long. And it won't go any further, because the remaining mass of the Sun won't be enough to reach 600 million degrees at the center, the temperature needed to ignite the carbon core. Hydrogen and helium will burn around it for several thousand years, causing our star to peel off and further expand its outer shell. Eventually, because there will be no radiation to balance it out, gravity will prevail. The rest of our star's core shrinks and becomes a white dwarf, a body the size of the Earth, extremely dense and hot (about 30,000 °C). At the same time, the outer layers, which are too far away, will dissolve into space. For about 10,000 years, the shell will shine from within, glowing in the light of the still warm central hearth. The result will be a planetary nebula. Will it become visible from other planetary systems like those we can see through our telescopes? Among the best known are the Ring Nebula, in the constellation Lyra, and the Dumbbell Nebula in the constellation Foxy. Will our descendants in exile be able to observe the rest of the Sun, around which life flourished? A study published in May 2018 showed, using stellar evolution models, that it would be possible. Because of its mass, the Sun would have been at its luminosity limit. Thus, a planetary nebula would be visible but very faint. What's next? Will it really be the end? No. It would take billions of years for the fiery heart to cool down and become a black dwarf. In the meantime, the Sun has many more beautiful days ahead.


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