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I heard somewhere that the sun is considered to be a 3rd generation star. Why is that and how do 'they' know?

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From the abundances of the elements in the Sun, and also its age and location in the galactic disk.

To simplify greatly a very complex topic, the first ("population three") stars were Hydrogen, Helium and a touch of Lithium, nothing else. Second generation ("population two") stars have some heavier elements, but few, and are found in the galactic halo, not the disk. They're generally very old, and low metallicity. Finally stars like the Sun are in the galactic disk, and contain notable amounts of heavier elements formed in earlier generations of stars.

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Thanks Ben. What kind of heavier elements are we talking about? (I forget what comes next on the periodic table!) Carbon?

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In Astronomy, people talk about Hydrogen, Helium and "Metals", i.e. everything else, but from the point of view of working out the ancestry of a star a few are important. Carbon, Nitrogen and Oxygen are all enriched in third-generation stars, but Iron abundance is possibly the most relevant.

The initial, population-III stars are still poorly understood (they are all long gone so we can't directly observe them). Theory tends to suggest they were very large and lived briefly without ever entering the later stages of the life of stars where the heavier elements are formed efficiently. The second generation stars are therefore referred to as 'metal-poor', i.e. they started life with very limited quanities of the elements heavier than Helium. However, these stars were less massive than the Pop-III stars and processed Helium to heavier elements more efficiently, in particular Iron and the related 'iron group' elements near it in the periodic table. So third generation stars - which form from the remains of the second generation when they explode as supernovae - have significantly higher metal abundances than the older, second generation stars.

We see the process continuing, so if you look at young clusters of stars today that have just formed, the stars there contain more metals than the sun does.

Hope this explains a bit, it's a complex topic...!

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I listened to an old astronomycast (www.astronomycast.com) episode the other day about this very topic. You want show 75, the description of which is:

"After the big bang, all we had was hydrogen, a little bit of helium, and a few other trace elements. Today, we’ve a whole periodic table of elements to enjoy, from oxygen we breathe to the aluminium cans we drink from to the uranium that powers some people’s homes. How did we get from plain old hydrogen to our current diversity? It came from stars, in fact successive generations of stars.8wDfBziOEJw"

Well worth a lsiten.

Cheers

Andy

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