The Sun, without which life would never be possible on Earth, is not eternal. This is what the end of our star, which is 4.6 billion years old today, should look like.
The lifespan of stars is determined by their mass. The more massive the star, the shorter its life and it consists of several million years to end with a furious flame of space fireworks – a supernova. Things will be different for our Sun. Our star is a yellow dwarf with a total lifespan (330,000 times the mass of the Earth) of about 10 billion years. The good news about life on Earth is that the Sun is now only 4.6 billion years old, so it should shine for another five billion years, according to astronomers. However, it will be necessary to think about relocation, as the Earth will become uninhabitable in less than a billion years (despite the current global warming). Why? Because the brightness of the Sun will continue to increase by about 10% every billion years. Thus, our Pale Blue Dot will gradually turn into a furnace, similar to perhaps the hell that now reigns on our neighbor Venus. Therefore, the habitable zone in our solar system will shift, and ice worlds like Pluto have begun to melt. We can also imagine that Homo sapiens is migrating to Mars, which will become softer. But if that happens, it won’t last long. Therefore, we may need to go to other stars if the human race can afford it.
What will happen when the Sun begins to die? Over time, the reserves of hydrogen accumulated by the Sun at birth will be depleted. With a relatively small mass, the star has maintained a balance between gravity and radiation for billions of years (as a result of hydrogen synthesis). But the “day” will come when the fuel will run out. Then the star’s heart will be ruled by the helium nucleus (created by the fusion of hydrogen), which will collapse on itself. The temperature that will rise during the contraction will release energy that will repel 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 (up to 3000 K). According to a study published in 2008, the radius of our star will increase from about 700,000 kilometers today to over 170 million kilometers! In other words, Mercury, Venus, and also the Earth will be inside and will eventually be destroyed (if the orbit of our planet is not pushed out, which will depend on the mass lost by the red giant). During shrinkage, the helium nucleus will heat up and reach a temperature of 100 million degrees. Therefore, helium will burn faster and faster, producing carbon and oxygen, and ultimately releasing as much energy in a few seconds as all the stars in a galaxy normally produce together. All this energy will be released with a light that astrophysicists call “helium lightning”. But this will not last long, as the residual mass of the Sun is not enough to reach 600 million degrees in the center – the temperature needed to ignite the core.
Hydrogen and helium will burn around it for several thousand years, as a result of which our star will stratify and its outer shell will expand even more. Ultimately, since there will be no radiation to balance it, gravity will prevail. The rest of the core of our star will shrink and turn into a white dwarf – a body the size of Earth, which is extremely dense and hot (about 30,000 ° C). At the same time, the outer layers, which will be too far away, will dissolve into space. For about 10,000 years, the shell will shine from within, shining in the light of the still warm central hearth. As a result, a planetary nebula will form.
Will it be visible from other planetary systems, similar to those we can see through our telescopes? Like the Ring Nebula in the constellation Lyra? Will our descendants in exile be able to observe the rest of the Sun, around which life has flourished? A study published in May 2018 found, using models of stellar evolution, that this is possible. Due to its mass, the Sun would be at the limit of luminosity. Thus the planetary nebula will be visible, but very weak. What next? Will this really be the end? No. It will take billions of years for the fiery heart to cool down and become a black dwarf. Meanwhile, there are still many beautiful days before the Sun.
