Our sun has been around about five billion years, and scientists believe it has another five billion years left before it burns out. They can even tell us what will happen when it does. How did they figure that out? The sun has been notoriously difficult to study up until modern telescopes and space probes- after all, you can't even look at it. The state of the sun can be estimated by comparing it to other stars in our galaxy, but those aren't any easier to study, really.
Only a couple hundred years ago, we didn't know how far away the sun is. We didn't know how big it is. And until 1925, we didn't know what kind of fuel it burns. The story of how we learned all those things is as fascinating as the sun itself. Read the steps scientists had to go through to learn about our sun, and how they predict it will all end eventually, at Gizmodo.
(Image credit: Alvesgaspar)