This question reminds us of another pop song, Skeeter Davis's 1962 single "The End of the World," in which the singer wonders why the sun keeps on shining after her boyfriend apparently has dumped her. The conceit of the lyrics is that the reality around us -- whether it's the shining sun or the birds singing in the trees -- is more durable than our fragile little feelings. In truth, though, our lovelorn lass had the misfortune to be born too soon -- by about 5.5 billion years, give or take a few. That's the point at which the sun, which like any other star is a gigantic fusion reactor, will run out of the hydrogen in its core that it burns as fuel to create sunshine and will start burning the hydrogen in its surrounding layers.
That'll be the start of the sun's death spiral, in which its core will shrink and its outer layers will expand massively, turning it into a red giant. In a final burst, the sun will roast the solar system with a blast of heat that will temporarily turn even the usually frigid vicinity of Pluto and the Kuiper Belt (out past Neptune) into a celestial sauna. It's likely that the inner planets, including Earth, will be either sucked into the dying giant, or else turned into cinders [source: Overbye].
On the plus side, unless humans manage to colonize the solar systems of other stars, nobody is going to be around to experience this final inferno. The sun, which is about halfway through its expected lifespan, is already gradually heating up, and a billion years from now, it's expected to be about 10 percent brighter than it is now. That increase in solar radiation will be enough to boil away our planet's oceans, leaving us without the water that our species depends upon for survival [source: Overbye].