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Author Topic: Why is O the next most abundant element in the universe?  (Read 585 times)

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Morlaf

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Why is O the next most abundant element in the universe?
« on: May 15, 2018, 02:00:04 AM »

Hydrogen? No1 cares about hydrogen. It's like saying I have rice in my risotto. Helium? yeah that's all stars do: turn H into He. Why, oh why is O the next most abundant element in the universe? Where is Be (Z=4) (following the principal of fuse 2 H--> He, fuse 2 He --> Be?) is there some magical stability regarding Z=8 over Z = 4? please help an amateur understand..
« Last Edit: May 16, 2018, 09:25:58 AM by Mitch »
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mjc123

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Re: Oxygen?
« Reply #1 on: May 15, 2018, 03:05:51 AM »

Answer is yes - see attached graph of binding energy per nucleon. Higher value means more stable; peaks for 4He, 12C and 16O. 56Fe is highest. Why that is - ask a nuclear physicist.
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Borek

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Re: Oxygen?
« Reply #2 on: May 15, 2018, 03:31:59 AM »

If you want to delve deeper https://en.wikipedia.org/wiki/B2FH_paper is a landmark starting point.
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Morlaf

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Re: Oxygen?
« Reply #3 on: May 15, 2018, 09:20:01 PM »

thanks.... :-)
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Galaxy

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Re: Oxygen?
« Reply #4 on: May 16, 2018, 06:59:58 AM »

See The Alchemy of the Heavens by Ken Croswell, who interviewed all four authors of the classic B2FH (Burbidge, Burbidge, Fowler, and Hoyle) paper:

[A] few astronomers questioned B2FH's motives, thinking their work a veiled attempt to bolster the steady state [as opposed to the big bang].  Said Geoffrey Burbidge, "Willy [Fowler] used to give talks and come back and say, `Why do they always blame me and say I'm doing this for the steady state?'  And I used to say to Willy that some of the antipathy towards you is that they think you are using this as a way of pushing the steady state."

According to Geoffrey Burbidge, some astronomers failed to recognize the big bang's inability to produce heavy elements.  "Astronomers never understood the difficulty that the big bang people had with nucleosynthesis, in that you can't get over mass five," said Burbidge.  "There is no stable mass five.  But the astronomers couldn't understand that, so some of them were quite convinced that the only reason for our developing this theory was that you required it in the framework of the steady state.  But it was clear to any good physicist that the mass five was a real problem to get over."

- The Alchemy of the Heavens by Ken Croswell, pages 114-115.

Just as there is no stable mass 5, there is also no stable mass 8.  You have to leap over both to make carbon-12, then this gets converted into oxygen-16.  Most oxygen arises in massive stars (stars born with more than 8 solar masses), which release the oxygen into the Galaxy at large when they explode.
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Enthalpy

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Re: Why is O the next most abundant element in the universe?
« Reply #5 on: May 16, 2018, 01:29:56 PM »

Also, the nuclides result from sets of nuclear reactions in the stars, which can only produce some of them. The temperature far to cool to produce a set of nuclides according to equilibria. It's similar to chemical reactions at a reasonable temperature: the products depend on the reactants, the possible reactions, the catalysts and so on. Only a flame, thanks to its hot temperature, can completely reorganize the atoms and reach a simple equilibrium.

The situation is a bit different in a supernova with hotter conditions. Few years ago, astronomers told supernova made the elements heavier than iron, while fusion in the main sequence stars had to stop at iron or before. But such concepts may have changed meanwhile.
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Borek

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Re: Why is O the next most abundant element in the universe?
« Reply #6 on: May 16, 2018, 10:34:03 PM »

Few years ago, astronomers told supernova made the elements heavier than iron, while fusion in the main sequence stars had to stop at iron or before. But such concepts may have changed meanwhile.

Yes, after https://en.wikipedia.org/wiki/GW170817 there is a general consensus that heavier elements are produced by https://en.wikipedia.org/wiki/R-process in https://en.wikipedia.org/wiki/Kilonova events.
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Morlaf

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Re: Why is O the next most abundant element in the universe?
« Reply #7 on: May 18, 2018, 12:21:33 AM »

thank you all, again! :-)
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Enthalpy

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Re: Why is O the next most abundant element in the universe?
« Reply #8 on: May 21, 2018, 10:11:18 AM »

I just wonder why the model of fast neutron absorption is necessary to explain the abundance of neutron-rich isotopes of the elements.

The very same process of neutron stars merging, that releases enough neutrons for multiple absorptions faster than decays, couldn't release neutron matter that decays in neutron-rich isotopes of the elements? If the merge spats and splashes some matter away, the droplets have to decay since only gravity made neutron matter stable.
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