How do scientists measure the life of the sun?

How do scientists measure the life of the sun?

We can't actually observe any single star evolving because the time scales involved are far too long. Our sun is sort of middle aged at the moment, and is about 5 billion years old. It should be kicking around for another 5 billion years or so. The way that people understand stars is really by modelling their structure and then observing lots of examples of different stars in different stages of their evolution and seeing if the collection of observations you make fits the theory. We believe that we understand the structure of these stars and so it's really a theoretical construct. We have a model for these stars, it matches all the stars that we see. When we look at the sun, we know what kind of star it is and its properties, and we can say that it's about 5 billion years old.