An Almost Beautiful Friendship
Notwithstanding pesky little details like constantly blasting us with radiation and giving us skin cancer, the sun has been a pretty good friend to Earth. After all, from it Earth draws a little thing called life. We're in what astronomers call a habitable zone. Think of it as an astronomical sweet spot; we're close enough to a star to get what we need from it without getting melted by it, but not so far away that we're the ice station zebra of the Milky Way. It's certainly an unbalanced relationship though: We need it more than it needs us. In fact, it doesn't need us at all. If we disappeared tomorrow, the sun would just keep on cooking up the vast store of hydrogen in its core.
It's a shame that this successful partnership of unequals is slated to end in about five billion years. And it's not going to be pretty. One day the sun is going to completely run out of hydrogen in its core, contracting but growing hotter as it switches its day job (the sun has no night shift) to burning helium. Meanwhile, the outer layers of the sun will still be burning off hydrogen, and the whole star will expand, so much so that it will envelope Earth (the other inner planets shouldn't make long-term plans either). It will puff out its chest and take its little blue sidekick with it. (It was a nice planet while it lasted!)
And what happens to the sun after that? Having chosen not to explode, its retirement years will be quieter. Its fate is ultimately to become a typical, cooling-off, "white dwarf" star -- very dense, but only about the size of Earth (talk about a humbling downfall, considering it used to be more than a million times our size). With not enough heat left to burn anything in its chemistry lab, the sun won't kick off any energy; it will just continue to cool. The outer layer, which swallowed us so greedily, will eventually dissipate into the interstellar landscape as planetary nebulae.
Poor Earth obviously doesn't fare very well in this story, paying the ultimate price for being, well, a bit needy. Long before it gets eminent-domained by the sun, our planet will be slowly evaporated by the expanding star. Goodbye oceans, hello unspeakable heat. A molten rock will be all that's left. Perhaps humans will have long since hopped in a nearby wormhole and moved away to a safer neighborhood. Or maybe our species will have destroyed itself, or been destroyed by, say, a killer disease or a large rock from outer space that Bruce Willis and friends weren't able to destroy in time.
We're told from our youth to choose our friends wisely. But sometimes there is no choice -- sometimes friends are sort of stuck with each other. That's just how it is here in the habitable zone. Earth can't help being where it is and neither can the sun.