What Will Happen to the Sun? A Timetable for the Next Eight Billion Years
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The sun is about 4.6 billion years old and roughly halfway through the stable part of its life. What happens after that is not speculation in the loose sense: stellar evolution is one of the better-constrained parts of astrophysics, because the whole sky is full of stars at every stage and their behaviour follows from the physics of gravity, pressure and nuclear fusion. The sequence is a slow brightening, a dramatic expansion, a brief and violent late phase, and eight billion years of cooling.
The stable present
The sun is a main sequence star, meaning it is fusing hydrogen into helium in its core and is in equilibrium: the outward pressure from radiation and hot gas exactly balances the inward pull of gravity, and any disturbance is self-correcting, since a core that contracts slightly gets hotter, fuses faster and pushes back. It converts about 600 million tonnes of hydrogen a second, of which roughly four million tonnes becomes energy. This state lasts about ten billion years for a star of this mass, which is set almost entirely by mass: a star twice as heavy burns through its fuel in a fraction of the time because the higher core pressure drives fusion far faster, while a red dwarf a tenth the sun's mass will last for trillions of years. The sun is not constant even now, since it has brightened by about thirty percent since it formed, as helium accumulates in the core and the core contracts and heats to compensate.
The slow brightening, which matters first
The event that ends life on Earth is not the dramatic one. The steady increase in luminosity continues at roughly one percent every hundred million years, and the consequences arrive long before the sun leaves the main sequence:
- •In about 600 million years to a billion, rising temperatures accelerate the weathering of silicate rock, which consumes carbon dioxide, and levels are expected to fall below what most plants need for photosynthesis
- •That removes the base of the food chain, and complex life goes with it, leaving microbial life in whatever refuges remain
- •In around one to one and a half billion years the oceans begin to evaporate substantially, and water vapour, a powerful greenhouse gas, accelerates the heating in a runaway
- •By roughly two billion years the surface is expected to be dry and sterile, with Earth resembling Venus
- •The habitable zone moves outward as the sun brightens, so Mars warms during this period, and by the late stages the moons of the outer planets are the warmest places in the system
The red giant
At about five billion years from now the core runs out of hydrogen. What follows is counterintuitive: the core contracts and heats while the outer layers expand and cool. Hydrogen fusion continues in a shell around the inert helium core, the energy output rises enormously, and the sun swells into a red giant, reaching perhaps two hundred times its present radius and over a thousand times its present brightness while its surface cools to a dull orange-red. Mercury and Venus are certainly engulfed. Earth's fate is genuinely uncertain and depends on a competition: the sun loses a substantial fraction of its mass as a stellar wind during this phase, which weakens its gravity and lets the planets spiral outward, while tidal interaction drags Earth inward. Most current models have Earth being swallowed, and some have it surviving as a scorched cinder orbiting further out. Either way the surface is molten long before.
The last acts
When the contracting helium core reaches about a hundred million degrees, helium begins fusing into carbon and oxygen. In a star of this mass the ignition happens almost explosively in a matter of minutes, an event called the helium flash, absorbed internally and not visible from outside, after which the sun settles into a shorter stable phase of perhaps a hundred million years burning helium in its core. That fuel exhausts in turn, and the sun expands again, larger and more unstable than before, pulsing and shedding its outer layers in a series of ejections. It never becomes hot enough to fuse carbon, which requires a much more massive star, so there is no supernova. What is expelled forms a glowing shell illuminated by the exposed core, which is what a planetary nebula is, and the name is a historical accident from early telescopes that showed a round disc resembling a planet. That shell disperses into the interstellar medium within a few tens of thousands of years, carrying the carbon and oxygen the sun made.
The long ending
What remains is the naked core: a white dwarf about the size of Earth containing more than half the sun's original mass, so dense that a teaspoon would weigh several tonnes, held up not by fusion but by the quantum resistance of electrons to being squeezed into the same state. It has no energy source at all and simply radiates away its stored heat, starting at over a hundred thousand degrees and cooling over billions of years. The theoretical endpoint is a black dwarf, a cold dark object emitting nothing, and the cooling time is so long that the universe is not yet old enough for any to exist. The material the sun sheds will eventually be incorporated into new stars and planets, which is the same process that supplied the carbon, oxygen, nitrogen and iron in every living thing on Earth, all of it manufactured inside earlier generations of stars and released when they died.
The takeaway
The sun has roughly five billion years of stable hydrogen fusion left, but its steady brightening will end complex life on Earth within about a billion years by stripping carbon dioxide from the atmosphere and then evaporating the oceans. When the core's hydrogen runs out it will swell into a red giant two hundred times its present size, certainly swallowing Mercury and Venus and probably Earth, then burn helium, shed its outer layers as a planetary nebula, and leave a white dwarf that cools for billions of years.