Years ago I learned something ultimately terrible but still strangly comforting. The bad news was that the Sun eventually was going to quit on us. It would run out of fuel, expand into a red giant in a frantic attempt to compensate, but finally collapse into a white dwarf, dooming the Earth forever.
The good news was that stars of the mass of the Sun keep shining for about 10 billion years. As the Sun happens to be 4.6 billion years old, even a kid could do the math: We had a good five billion years to go! (I knew none of this had any bearing on me, personally, but still ...)
Anyway, it was an idea that kept the monsters securely trapped under my bed at night, and it's been a calming thought in the back of my mind ever since. Oh, foolish me!
It turns out that stars like the sun make their energy by fusing four hydrogen protons into one helium nucleus. That's a big net loss of particles. This has consequences, because the pressure needed to make the Sun's engine work is proportional to the product of the particle density and the temperature. As the numbers of particles go down, the termperature has to go up.
Rats. Within a few hundred million years, the Sun will have heated up so much that people on Earth will really, really notice it. Within, say, half a billion years all the water in all the oceans and lakes and rivers and toilet bowls will be gone. The Earth will far more unlivable than the worst desert you could ever imagine.
We have been robbed. Instead of another 5 billion years of earthly bliss, we get maybe half a billion. Somebody call the police!
Of course we may have options in the future. Maybe we can build a big space tug and tow our planet away from the heat. Maybe by then we can just download ourselves onto computer chips, load them into space ships, and go live somewhere else. Hell, maybe we'll even be able fix the damn Sun.
But wait a minute. Why am I saying "we?"