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How old could population I stars theoretically be? What's the earliest they could have formed?

Curious Layman
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  • Are you referring to the third generation of stars that formed, or Population III stars, which formed first? – HDE 226868 Nov 07 '18 at 15:17
  • No, generation three. I believe third generation stars (I believe pop I stars) are necessary for the formation of life. I wanted a theoretical limit to earliest life could form in universe. A rough estimate obviously. In addition to the earliest, what time in general did these highly metallic stars start to form? – Curious Layman Nov 07 '18 at 17:25
  • I'm not familiar with this "generation" terminology. As the wiki article I reference states, it's very plausible that there was a habitable epoch, between about 10 to 17 million years after the Big Bang, during which life could plausibly arise. This is argued by Loeb at Harvard. See here, https://en.wikipedia.org/wiki/Chronology_of_the_universe#Speculative_%22habitable_epoch%22 – Daddy Kropotkin Nov 07 '18 at 18:35
  • The Sun can indeed be termed a "third generation star". It contains elements that have been in at least two other stars. See https://astronomy.stackexchange.com/questions/16311/how-can-there-be-1-000-stellar-ancestors-before-our-sun/16313#16313 – ProfRob Nov 14 '18 at 14:03
  • Population I though is not quite the same thing. It refers to a set of chemical and kinematic characteristics whose definition is not particularly rigid. – ProfRob Nov 14 '18 at 14:04
  • @RobJeffries I know they're not synonymous. But, how would you tell a third generation star from a fourth or fifth? The point of my question is to get a feel for the period of time it takes for hydrogen to form, clump, star, supernova, reform, star, supernova, and finally Pop I star. – Curious Layman Nov 14 '18 at 16:53
  • @JimmyG. There is no 4th or 5th generation. What you describe in your comment is not what a Population I star is, illustrating the point of my comment exactly. However, your question is easy enough to interpret and an answer should point out the complexities of definition. – ProfRob Nov 14 '18 at 18:20
  • @RobJeffries So, I was looking into it more. The Populations are categorized by where they are found in our galaxy, how they are moving through our galaxy, and age... and metallicity just happens to correlate. I.e. older=less metal? Am I close? – Curious Layman Nov 14 '18 at 20:10
  • @RobJeffries I missed your earlier comment suggesting I read that other question. Thank you. That did clarify a lot. But this mixing of the stellar material of billions stars over billions of years, how long before we have Pop I stars? And I am pretty sure that other answer answered my question about Pop III stars going straight to Pop I. No, because Pop III stars don't use all of the methods of nucleosynthesis needed to fuse all of the elements we have??? – Curious Layman Nov 14 '18 at 20:27
  • @JimmyG. -- Astronomers tend not to use the terms "Population I" and "Population II" much any more (though "Population III" is still very common), because the distinction -- while generally valid for the Milky Way and M31 -- turned out to be not very clear. The central regions of massive galaxies contain stars that are high in metallicity and very old. Conversely, some low-mass dwarf galaxies have young stars that are metal-poor. And so forth. – Peter Erwin Nov 15 '18 at 08:19
  • @PeterErwin I did just read something that concluded by saying rather bluntly, a 2 or 3 category system was doomed to failure. – Curious Layman Nov 17 '18 at 07:22

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Since Pop III stars are the oldest stars, then they could have theoretically began to form in the Cosmic Dark Age (CDA), see section 5, since the oldest galaxy we've observed so far (in infrared) is about half way through the CDA (~$480$ million years). As the article states,

Stars and galaxies are formed when dense regions of gas form due to the action of gravity, and this takes a long time within a near-uniform density of gas and on the scale required, so it is estimated that stars did not exist for perhaps hundreds of millions of years after recombination.

So not only must the temperature of the universe be sufficiently cool, gravity must have acted for a long enough amount of time for baryonic structure to begin to form.

Daddy Kropotkin
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