How to prevent the Earth from being cooked by the increasingly hot Sun

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I would bet that, as a species, we are quite fond of our planet (despite our unjustified carbon emissions). But the ugly truth is that the Earth is doomed. Someday, the Sun will enter a stage that will make life on the Earth’s surface impossible and eventually reduce the planet to nothing more than a sad, lonely lump of iron and nickel.

The good news is that if we really put our minds to it (and don’t worry, we’ll have hundreds of millions of years to plan) we can keep our homeworld hospitable, even long after our Sun goes crazy.

A waking nightmare

The Sun is slowly but inexorably becoming brighter, hotter and larger over time. Billions of years ago, when groups of molecules began to dance together and consider themselves alive, the Sun was about 20 percent dimmer than today. Even the dinosaurs knew of a fainter, smaller star. And although the Sun is only halfway through the main hydrogen-burning phase of its life, with 4 billion years or more to go until its death throes begin, the peculiar combination of temperature and brightness that makes life on this small world possible of ours will erode in just a few hundred million years. A blink of an eye, astronomically speaking.

The Sun sows the seeds of its own demise through the basic physics of its existence. At this very moment, our star is chewing up something like 600 million metric tons of hydrogen every second, smashing those atoms together in a nuclear inferno that reaches a temperature of more than 27 million degrees Fahrenheit. Of those 600 million metric tons, 4 million are converted into energy, enough to light up the entire Solar System.

However, that fusion reaction is not perfectly clean. There is a leftover byproduct, an ash created by nuclear fires: helium. That helium has nowhere to go, since the deep convection cycles that constantly churn up the material inside the Sun don’t reach the core where helium is formed. So the helium remains there, inert, lifeless, useless, clogging the machine.

At its current age, the Sun does not have high enough temperatures and pressures in its core to fuse helium. Then helium gets in the way, increasing the total mass of the nucleus without giving it anything else to fuse with. Fortunately, the Sun can easily compensate for this, and that compensation occurs through a bit of physics known as hydrostatic balance.

The Sun exists in constant balance, living on the edge of a nuclear knife. On the one hand are the energies released by the fusion process which, if left unchecked, could threaten to explode (or at least expand) the Sun. Counteracting this is the immense gravitational weight of the star itself, pressing inward with all the force that 1,027 tons of hydrogen and helium can muster. If that force were left unchecked, the Sun’s own gravity would crush our star and turn it into a black hole no larger than a medium-sized city.

So what happens when an unstoppable force meets irresistible pressure? Elegant balance and a star that can live for billions of years. If, for some reason, the nuclear inferno randomly increases its temperature, that will heat the rest of the star and inflate its outer layers, relieving gravitational pressure and slowing nuclear reactions. And if the Sun were to contract randomly, more material would be forced into the core, where it would participate in the heady nuclear dance, and the resulting release of energy would conspire to inflate the star back to normal proportions.

But the presence of helium ash, that nuclear waste, upsets that balance by displacing hydrogen that would otherwise fuse. The Sun cannot help but withdraw into itself: gravity is inflexible and indifferent. And when it does, it forces the core’s nuclear reactions to increase in ferocity, raising its temperature, which in turn forces the Sun’s surface to swell and glow.

Slowly, slowly, slowly, as helium continues to build up in the core of the Sun (or any other star of similar mass), it expands and lights up in response. It is difficult to predict exactly when this glow will result in calamity for our planet; That depends on a complex interaction of radiation, atmosphere and ocean. But the general estimate is that we have about 500 million years left before life becomes nearly impossible.

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