~5 Billion

The End of the Earth

The Sun’s fate is sealed, and it is humbling and perhaps a little sad to realize that our glorious star will not shine forever. The Milky Way has billions of so-called “main sequence” stars with the same basic properties as our Sun, and we can study them all around us in different stages of their predictable life cycles. The destiny of a star is dictated by its initial mass; in the case of a star with the mass of our Sun, its destiny is a short, violent, energetic youth followed by a relatively long, stable, 10-billion-year middle age, and then a relatively gentle, quiet death.

Radioactive dating of primitive meteorites as well as analysis of solar wind particles collected by missions like NASA’s Genesis spacecraft tell us that the Sun is about 4.567 billion years old, or about halfway through its life cycle as a typical low-mass star. As it goes through middle age and uses up more of its hydrogen nuclear-fusion fuel supply, our star is slowly getting hotter—in about a billion years it will be hot enough to evaporate Earth’s oceans. In 5 billion years or so, all the Sun’s hydrogen will be used up and the core will contract and heat up further, expanding the Sun’s outer atmospheric layers until it becomes a red giant star.

The red-giant Sun will eventually swell to about 250 times its present size, engulfing and destroying the inner planets, most likely including the Earth. As the Sun’s helium and other heavier elements are also depleted, enormous pulsating death throes will jettison the Sun’s outer layers (including all the atoms of the former inhabitants of the now-vaporized Earth) into deep space as a planetary nebula, to be recycled into new stars. The remaining ember of the Sun’s core will become a white dwarf star that will slowly cool, eventually fading into the background oblivion of cold space.

Earth will be gone; but will life survive? If we can survive our current challenges and first become a multi-planet and then a multi–solar system species, then perhaps our distant descendants—whatever species they become—will find new habitable worlds around other, younger, Sun-like stars to call home.

Main image: Space artist Don Dixon’s imagined view of the swollen, red-giant Sun about to engulf the Earth and Moon about 5 billion years in the future. Inset: Spitzer Space Telescope infrared image of the Helix Nebula, a shell of debris formed during the death throes of a star that was once similar to our Sun.