Imagine this: You stand at the shore of a desolate ocean on a desolate world. As the dawn breaks, a strangely red star, 50 times larger than our familiar day star, shines blood-red on the waves, and the sky turns from jet black to pale blue. Vapors rise from the hot ocean, and the water begins to boil.
The place is planet Earth. The time is five billion years hence. The sun, the source of life on our planet, is dying, having reached its red-giant phase. Earth is dying with it.
How do we know Earth’s fate so long from now?
The Milky Way is a massive place. We have but to look upward to see sun-like stars at various stages of their development.
One such star is Mira in the constellation Cetus, the Sea Monster. Look for orange Mira on Mira’s neck. Right now, Mira is visible to the unaided eye but will fade to invisibility by year’s end. Mira’s variable brightness is the key to understanding the sun’s future.
Mira isn’t exactly like the sun will be at the end of its life in 5 billion years. For one thing, Mira is slightly more massive than the sun at 1.18 solar masses.
It was probably somewhat larger before its death throes. Dying stars tend to shed their outer layers as they expire.
Stars are enormous thermonuclear furnaces. Because of their cores’ extreme pressures and temperatures, they convert hydrogen to helium like unimaginably large hydrogen bombs.
Because of its slightly larger mass, Mira consumed its hydrogen fuel and reached its bloated red-giant phase in only 6 billion years. The sun is already 5 billion years old, a relatively sedate middle age. From start to finish, the sun will live 10 billion years.
Still, the processes are similar. Mira pulsates in its death throes, which accounts for its variable brightness. So, too, will the sun.
Even now, the conditions that spell doom for the sun are firmly in place. The hydrogen deep in its core is compressed to form helium. Because helium is heavier than hydrogen, the helium falls to the sun’s center, creating an inert ball inside the nuclear reaction.
As the helium presses outward against the thermonuclear reaction around it, the sun will gradually get hotter and larger until it shines perhaps 100 times more brightly than it does now.
And then the sun goes berserk. Its helium core will suddenly shrink, and helium will begin to fuse into carbon. The thermonuclear reaction around the helium core will grow out of control.
The outward pressure will expand the sun inexorably past the orbit of the first planet, Mercury. Earth’s surface temperature will rise well above 212 degrees F. Its atmosphere will be blown away. Its oceans will begin to boil.
The sun will then shrink and expand again, this time past the orbit of Venus, the second planet. Earth’s rocks will glow red from the heat.
Yet again, the sun will contract and then grow more voluminous. As its hot gasses begin to lick the atmosphere of Earth, the sun will fill Earth’s sky from horizon to horizon.
As the millennia pass and the sun continues to contract and expand, the temperature of our planet will exceed 6,000 degrees. The surface rocks will melt and then boil.
What happens next depends on factors contemporary astronomers are still determining. At the minimum prediction of the sun’s expansion, the sun’s outer shell will reach near Earth, and our planet will french fry.
At the maximum prediction, Earth will be inside the sun and must plow through its thin, hot gasses. Consequently, our planet’s momentum through space will slow. For a century or longer, it will spiral slowly toward the sun’s center and vaporize into hot gasses that will quickly dissipate into the raging inferno surrounding it.
Five billion years is a long time. By the time the sun consumes our planet, we will certainly have gone the way of the dodo and the dinosaur, extinct because of our inability to adapt to the changing circumstances of our planet.
Or perhaps we will have found a way to flee to the outer reaches of our solar system or even the stars. But the world we leave behind will surely die, consumed by the star that gave it life.
Tom Burns is the former director of the Perkins Observatory in Delaware.
