It is my understanding that for something to be scientific fact it has to be repeatable and observable.
How is the age of a star able to be proved scientifically?
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Imagine you are looking at a forest. There are a bunch of big trees, a load of small trees and a few dead trees lying on the ground. Without chopping them down to count tree rings etc, you can assume that the small trees are younger versions of the big trees, and the dead trees were once living trees. We don’t have to sit and watch the tree grow for years to ‘prove’ this. It is a reasonable assumption to make.
We find the same thing with stars. We see new ones being formed, we see the remains of stars that are dead, and ones in between. We also sometimes get to see one die, which is very exciting.
By observing and classifying stars together we can build predictive models of how stars form, live and eventually die even though these happen on timeframes of millions or billions of years and impossible to observe in real time. Like with the trees, we don’t have to watch one die and fall over in a storm to know that the dead tree in the forest wasn’t always dead.
Stellar evolutionary models predict that stars on the main sequence get brighter as they get older. The rate that the brightness increases is determined by the mass of the star, so if you measure the mass of the star and the current brightness, you can tell how long it has been on the main sequence.
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