A friend wanted to make sure something I posted for her would be up for a while. I said it would be up “until the Sun burns out.” She asked me, “Will the Sun really burn out one day?” And I replied,
The huge pressure at the center of the Sun fuses hydrogen atoms together, releasing massive amounts of energy and causing it to shine and give us light and heat. Five billion years from now, the Sun will run out of hydrogen and will start fusing helium atoms (helium is what you get when you fuse two hydrogen atoms together, so there will be a lot handy.) This will cause the Sun to swell up to such a huge size that it envelopes the inner planets, including Earth. After about a billion years in this state, the helium will run out and, devoid of any fuel, the Sun will collapse. It will end up as a tiny, dim ball not much bigger than Earth.
But as it collapses, it will spew off most of its mass into space, creating a huge cloud of gas and dust. And one day, far in the future, some of that gas and dust might get together with some other gas and dust, and gravity might start to pull it together, and if there’s enough, it might compact tighter and tighter until it ignites as a new star. And some of the remaining dust and gas might form into little balls orbiting the star, and they’ll cool and solidify, and chemicals on those little balls will absorb the sunlight and start reacting in really special ways until the point where some of the chemicals start making exact copies of each other. And then, a very long time after that, if conditions are just right on one of those little balls orbiting that star near where our Sun used to be, someone, maybe with 10 arms or just one, with ten eyes or none, will ask a friend if the Sun will burn out one day.