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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It’s actually not simple to determine the age of a given star. They are estimates based on indirect observations and/or rotation comparisons (rotation slows over time) using our sun as a baseline, since we do know its age pretty accurately. In the case of the sun, we can date really old rocks and extrapolate from that about how old the sun is.
Someone with more knowledge my be able to explain more clearly than me. But the larger issue is what ‘knowing’ means in science. Theories are tested through experimentation; observations are not.
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.
Physicist made models of how stars works. With time elements content in a atar differs and so sight spectrum ot sensnds out. knowing light spectrum of elements and star life cycle we can estimate it age. For distant stars there’s other methods that are based on our distance to a star
We understand the process of nuclear fusion, and [how stars consume](https://en.wikipedia.org/wiki/Stellar_nucleosynthesis) the material that they have (hydrogen, helium, etc.) in order to shine.
We also understand how materials glow when heated; it’s not white light, it’s only certain frequencies based on the atom and how its electrons absorb and release this heat energy as photons. This lets scientists use [spectroscopy](https://en.wikipedia.org/wiki/Astronomical_spectroscopy) to detect what element a star has, and thus what nuclear reactions are going on inside the star.
So the combination of all this knowledge has led to scientists being able to [classify stars](https://en.wikipedia.org/wiki/Stellar_classification). So using just the brightness and spectral lines, astronomers can tell what kind of star it is, and get a pretty good estimate of its size and thus how much of the fuel has been consumed (the age of the star).
We don’t *know* in the way we *know* that 2+2=4. What we *know* is their position in the sky, the spectrum of the light from them, their parallax, couple of other things. The rest is inference.
We have a model of how stars work that is consistent with all known observations (basically based on the *exact* light we see from them, and their surrounding environment) which gives us a prediction of their age. If we got a new model we would have a different prediction. But it’s really pretty settled, probably settled enough to count as a fact for all intents and purposes if you put the right error bars on the age.
Generally speaking science is in the business of inferring things or disproving inferences, not proving them the way a mathematician would understand it – we can say ‘if this is not true, X and Y things would also not be true, and we are very sure of those things’ but there are a whole swathe of things that are ‘this is how we understand and talk about the world, but they could in theory still be disproved’.
> It is my understanding that for something to be scientific fact it has to be repeatable and observable.
There isn’t really widespread agreement on what makes something scientific. If science were limited to things that can be repeatedly observed in controlled experiments, then we wouldn’t be able to make any scientific claims about other stars, or evolution, or ancient civilizations, or any large-scale social or economic phenomena.
Ages of stars are based on a combination of observations, inference, and modelling. We know that they emit radiation with spectra that match that of light that has passed through a sample of hydrogen on earth. We can model a big ball of hydrogen and see that it would behave in a similar way to stars. We can model how this big ball of hydrogen changes over time. We can match different stages of this process, for different sizes of hydrogen balls, to the different kinds of stars we can see. There have been various attempts to come up with alternative models (e.g. models in which stars have a small outer layer of hydrogen but are mostly made up of something else) but none of these have worked very well.
The lifetime of a star is fairly reliably connected to its mass and its brightness (for 90% of stars, known as “main sequence” stars) because its brightness depends on how fast it uses its fuel (hydrogen). Slightly counterintuitively, larger, more massive stars are shorter lived than smaller ones, because they burn fuel so much faster.
We have fancy equipment that can detect the “fingerprints” of different elements in a star and these help us work out how much hydrogen it has compared to how much helium, the product from hydrogen fusion. This ratio of hydrogen to helium changes predictably over the life of a star so we can determine its age by measuring the ratio.
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