How do astronomers determine the approximate age of stars, given that changes to stars are measured on scales that far exceed the length of time humans have even been around to observe changes in stars?
Henceforth, I shall Google questions to save time.
A sampling of some of the things we do know:
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We know how old the Earth is, by radioisotope dating (most notably U-238, whose half-life is close to the age of the Earth), and we presume that the Solar System is mostly all the same age. So we know how old the Sun is, which lets us estimate the ages of similar stars.
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We know how much power stars are emitting, and we know how much energy fusion releases, so we know how fast they’re burning through their fuel. Combine that with knowledge of a star’s mass, and what fraction of a star’s fuel it burns in its lifespan, and you can determine maximum ages.
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We know how old some star clusters are, because they form together, but drift apart with age, and we can see how much they’ve drifted. Any star in that cluster can then be assumed to be approximately the same age as the cluster.
Yes, there’s the “Main sequence” which roughly classifies stars by size. The bigger they are, the brighter they burn; when they first contracct to the point where fusion starts, they are mainly burning hydrogen to helium; as they age, there is fuel for other processes, burning helium etc. Eventually the contract and implode-explode to novae, or supernova if big enough. All the heavy elements are the byproduct of old supernova explosions (from large stars) spewed across the universe. The bigger stars burn brighter but faster.
One way of estimating the age of a star is the ratio of hydrogen to helium (how far along the process of burning H vs He vs other elements is it?) based on spectrum lines relative strength, compared to apparent size based on the overall spectrum of the star.
But then too, there are things like globular clusters inside and orbiting the Milky Way; most of the stars in a globular cluster formed at the same time, so if a few agree it seems the rest match that age…
I didn’t mean to imply “you should have just googled this”. I know from experience that there’s often a Wikipedia article on the exact topic, but it’s hard to figure out what the title would be. And in this case the Wiki article is brief and clear and directly on point, so I thought there wasn’t much point in me trying to summarize it since I have no more than casual amateur knowledge of this stuff.
It’s true that massive stars (those starting out with at least 8 solar masses) will supernova at the end of their life, but ordinary novae are not the end-of-life phenomenon of less massive stars. That’s an idea that long been tossed by astronomers. Instead, intermediate mass stars, such as the Sun, will end life as a red giant before losing their atmosphere and settling down to white-dwarfism. We don’t know for sure what the least massive stars (red dwarfs) will do since the universe hasn’t existed long enough for any of them to reach the end of their life.
This would only work on stars that are fully convective, which as far as I know, are just the red dwarfs. Larger stars like the Sun do not convect the helium formed in their core to the surface, so their ratio of hydrogen to helium is thought to be essentially that of the universe as a whole.