system
Given the following post, respond as if you're commenting on Reddit:
human
With such a small window of observation, how do we know how stars evolve and age?

We've been here for such a small amount of time. How can we possibly know how things work having been exposed to such a tiny fraction of the process?
gpt
We've been exposed to quite a lot of the process because we can see so very many stars at various points along their life-cycle. Consider a five-year-old child attending a reunion of their very large family. This child will see many people of all ages, ranging from the newest crop of infants to the grandparents that probably won't be at next year's reunion. More importantly, because the family is so large, the child will see many instances of people in a given age group. From this, the child will be able to get a sense of how humans change as they age.

Now, in the case of stars, we also have a whole lot of other information that we can draw on. We understand how chemistry works on Earth. We understand quite a bit about how nuclear processes occur and how fundamental particles interact on Earth. It's not unreasonable to think that chemistry and particle interactions don't depend on your planet, so we think those processes probably work the same elsewhere in the universe (like in stars). And we see a lot of stuff out there that *matches* what we know about the results of chemical and nuclear processes, so we figure those processes are probably the source. Similarly, we understand quite a bit about how gravity works on Earth, and our models appear to be quite good at describing the behavior of other planets and stars, so again we expect them to be relevant throughout the universe. Combining these models with the vast array of stars we see, we can put together a rather impressively detailed model for the life-cycle of a star.