Artist concept of a red giant star burning a planet in orbit around it. Image via ESO / L. Road.
The Earth exists thanks to our sun, which has formed in orbit around it from a huge cloud of gas and dust in space, 4.5 billion years ago. Similarly, the sun will ruin the Earth for living things, in about 5 billion years. As the sun evolves, it will expand to become a red giant star and fry our planet to ashes. In addition, the death of the Earth will occur in a context of changes on a galactic scale. Our Milky Way galaxy and Andromeda galaxy will be in the middle of a colossal collision that will forever alter our galactic home in space.
Our sun is a G-type star currently about halfway through its life cycle. This type of star is very stable for most of its life, quietly fusing hydrogen with helium inside it for billions of years. One day, the hydrogen inside the sun will run out. At this point, the inner pressure of gravity will gain the outer pressure of the sun’s internal melting, until the sun heats up enough to start melting helium. At this point, the sun will rise in the open air to become a red giant star.
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As the sun heats up and expands, its outer layers will envelop the innermost planets, Mercury and Venus. The outer edge of the sun will grow until it reaches approximately the orbit of the Earth. The water and atmosphere of our planet will boil, leaving only a charred, lifeless rock. Mars will take a while to warm up, but eventually Mars will be out of the human habitable zone. At this time, the moons of the outer planets, such as Jupiter and Saturn, will be the only places left in our solar system for human colonies.
But these locations will also be just temporary solutions in search of a new home.
As our sun expands toward the giant red phase, the living area around it will be pushed outward into the solar system. Image via NASA / Wendy Kenigsburg.
The giant red phase of the sun could continue for a billion years or so, but eventually helium will also run out. Then the sun will blow a gas wrapper. Astronomers looking through telescopes at other stellar systems will see our sun as what we call a planetary nebula, a large shell of gas that surrounds a dying star. Ultimately, this shell will dissipate into space and what is left of our sun will become a white dwarf star.
Earth astronomers can look out into space to glimpse the future of the Earth. For example, 400 light-years away, a star known to astronomers as SDSS J1228 + 1040 is a white dwarf with its gas nebula funerary cover, and inside it astronomers have found the signature of a planetesimal that still orbits the sun of its house long after its calamitous death.
And what about the Milky Way galaxy itself, the large island of stars that contains our Earth and our Sun? When our sun rises towards the red giant phase, long before it is established as a white dwarf, the Milky Way will go through a long process of inevitable collision with the giant spiral galaxy next door, the Andromeda Galaxy. The last humans on Earth, if there are any humans left in a couple of million years, will see the Andromeda galaxy growing and shining in the night sky. Currently, it is barely visible to the naked eye from a dark sky location. But in a couple of million years, the Andromeda galaxy will be an impressive, unmistakable whirlpool of stars easily visible in the night sky of all the inhabitants of Earth.
As Andromeda and the Milky Way come closer together, the large mass of the Andromeda galaxy will begin to affect the stars in our Milky Way. Our galaxy is wide and flat, like a panel. Currently, on Earth, we see its stars on a dark August night like a large nebula in the sky. But as Andromeda’s gravity distorts its paths, the stars in the Milky Way will scatter across our skies.
It may sound amazing, but the stars in the galaxies are so far apart that even when the two giant spirals collide, there will be few fireworks from collisions between stars. However, the gas clouds in the two galaxies are likely to collide to form vast conglomerations of new stars.
This series of illustrations shows the planned fusion between our Milky Way galaxy and the neighboring Andromeda galaxy. Image via NASA.
Ultimately, the Milky Way and Andromeda galaxies will be established to form a new massive blob-shaped galaxy. At this point, the Earth, our sun, and the rest of our solar system could be in a completely new place with respect to the galactic center. Today, the Earth is about 25,000 light-years from the center of the Milky Way. After the fusion of Andromeda and the Milky Way, astronomers believe that our home in space will have been launched into a new galactic orbit about 100,000 light-years from the center of the new large combined Andromeda-Milky Way galaxy. As theorist TJ Cox of the Harvard-Smithsonian Center for Astrophysics said:
You could say they send us to a country residence.
And what will be the destiny of humanity? That is impossible to say. If it survives, the future of humanity will depend on our ability to move away from our dying sun and settle in another camp.
Fortunately, we have several million years to figure out how to carry out this monumental task.
Summary: What will be the fate of the Earth? The Earth will become a dry, burnt rock as our sun becomes a red giant star. Moreover, as the Milky Way and the Andromeda galaxies collide, our Sun and Earth (and the rest of our solar system) are expected to be launched outward, away from the galactic center, into the outskirts of a new large galaxy created in the collision. .
