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Comment by vlovich123 | Hacker News Reader

But our sun is quite young afaik (middle aged) and earth is not much younger so it’s weird to hear the reasoning that metal-poor systems implies young system…

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I’m assuming the poster misspoke and meant that metal-poor stars have formed when the universe was relatively young, not that the stars are young. It’s also my understanding that a planet such as ours could only have formed a bit later, after enough supernovae had formed all the carbon and iron and so on that we need to live.

Making matters confusing is the fact that the phrase "a young galaxy" is commonly used to refer to a galaxy formed when the universe was young – if it’s far away (high redshift, z >> 1) and as such we see it the way it was when the universe was young!

This specific study, however, looked for more nearby galaxies (z < 1, we see the light that was emitted a few billion years ago) so these galaxies (as we see them) are in fact "old".

I think essentially all existing galaxies either formed >10 Gy ago or are the results of galaxy mergers – so in that sense there are no "young", recently-formed galaxies in the universe at all as the raw material was long ago used up. As such in astro-talk "young galaxies" can fairly unambiguously mean "far-away galaxies". But formally they’d be called high-z or high-redshift galaxies.

> after enough supernovae had formed all the carbon and iron and so on that we need to live.

And let’s not forget some of the atoms we see around us have been through even more violent events than supernovae.

Population II systems must be very boring.

Here, “young” means measured from the Big Bang.

Our sun is made from the stuff of previous stars.

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