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Multiple generations is perhaps an overstatement. The first oxygen in the Universe came from what we call Population III stars which is the first generation of
by astroH 2y ago
Multiple generations is perhaps an overstatement. The first oxygen in the Universe came from what we call Population III stars which is the first generation of stars to form after the Big Bang and what separates these from other stellar populations is that they do not have elements heavier than hydrogen or helium (except for minuscule traces left over from the Big Bang but these are insignificant). Now we don't know much about Population III stars but many models predict they are massive and when they die, can release 60 times the mass of our sun in the form of oxygen. That's really a lot of oxygen so you don't need too many of these to go off to pollute the early Universe and probably one of the reasons why we haven't yet found Population III stars.
- malfist 2y agoBig stars burn hot and fast, the more mass, the shorter their lives
- throwawaymaths 2y agoPop III stars (if they existed) are really a mystery, we can't easily extrapolate. These stars would be purely hydrogen and helium so it would take them a surprisingly long time to get to CNO cycle, for example.
- astroH 2y agoSo I think it is fair to say they did exist. If we believe in Big Bang Nucleosynthesis then heavy elements had to come from somewhere making the first generation of stars (whatever their properties may be) be Population III. I agree that without a catalyst it's hard to initiate the CNO cycle but indeed models predict that it is possible even under these circumstances.
- dredmorbius 2y agoWhy would it take a Pop III star longer than an equivalent-mass Pop I or II star to reach the end of its H / He fusion cycle?
- astro-throw 2y agoBecause of the lack of C, N, and O, which are catalysts in the CNO cycle. They aren't produced by it.
- dredmorbius 2y agoThanks!
- dredmorbius 2y agoNB: based on some quick searches, it seems that low-metalicity Pop III stars would rely on the pp (proton-proton) fusion chain. That's going to slow reaction somewhat, and extend lifetime. But for high-mass stars with only a few millions of years expected lifetime in a Pop I/II class, that's ... still a relatively modest difference compared to the several hundred million year lifespan of the early Universe. Or am I missing something?
- astro-throw 2y agoAnother interesting quirk of Pop III stars is that their initial mass function is expected to form much more massive stars than Pop II or Pop I. So even if Pop III stars are longer lived at the same mass as Pop II or Pop I, there will be a lot more supernovae per time, leading to fast enrichment and then Pop II.
- dredmorbius 2y agoThat was one of my thoughts. Another is that p-p fusion is fairly common. Look up when the Big Room's bright and you'll see a ... stellar example yourself ;-) (p-p fusion dominates in stars < ~1.3 M, where M is a solar mass. CNO fusion is typical of more massive stars.)
- astro-throw 2y agoThe word "generation" isn't really a thing in astronomy jargon. "Population III" is a population, and it includes stars formed after some supernovae, up to the point where the metals % gets high enough to be Population II.