Anyone else question why astronomers are so sure about things that happened 5+ billion years ago ? I guess otherwise there’d be a probably or a maybe in every sentence ? But then how do you tell when they really are sure ?
There is always an implied "as far as we know". But astronomy has a couple reasons to be fairly certain about many things. We can see a lot of stars, and because light takes time to travel the further away they are the older the state we are observing. We only see each star as it is right now, but from looking at a lot of them you get a good idea of how they can develop. Like how you can get a good idea of how humans age by just looking at a lot of people of different ages; you don't need to follow each of them for 90 years. The other advantage is that most of it is well understood physics that can be reasoned about and simulated. And then we can compare those simulations to what we are observing across the universe to see if our simulations make sense.
Science is forming a testable hypothesis and then testing it. So you hypothesize "as stars accumulate fusion products their cores get heavier and the rate of fusion increases, making them brighter" and test this both with modeling and looking if this matches the stars we can see. How is that now science?
They build a model of the universe's laws, as simple as they can get it while matching the evidence. They then look at other evidence they hadn't looked at before. It turns out, that quite simple models can predict a lot of different phenomena, which makes us reasonably confident in our assumptions that (1) the universe is governed by simple, fundamental laws; and (2) those laws are similar to our model.
If we make the assumption that our model applies everywhere and at every time (colloquially, that the laws of physics don't change), we can ask our models what happened in the past. Under our assumption, that's probably quite close to what actually happened. This assumption is called the "principle of induction", the "uniformitarian principle", the "cosmological principle", the "Copernican principle", and many other things besides.
The models are so good that when solar neutrinos were not as predicted, it turned out to be because of new physics (neutrino oscillation) not flaws in the solar model.