this is done by studying other stars and the way the sun is now. We know from it’s size and mass and temperature what stage of life it’s at, and we know from other stars what the lifecycle of a star is.
Google. I am joking don’t worry. We find out by looking at other stars in our galaxy and look at how the Sun changes everyday. We work backwards from what we see now and look at how other stars have formed and gone, to try and see what the Sun is doing and how old it is!
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