How do elements combine to form new elements

[rockinreel]

Reference Carl Sagan's (Cosmos) show last night, Live by the Stars

If hydrogen is the first element and once, the only element, and helium is formed by nuclear fusion inside a star, by pressing hydrogen together with so much energy it become helium, how did the other elements form?

For instance how does Oxygen exist? Oxygen has 8 protons and 8 electrons, seeking a parter with two electrons or one each, as in H20. Now it's outer shell is complete and the element is stable until energy forces it to seek more electrons to fill a new outer shell (18 - 32 electrons). I think I just answered my own question, but anyway... How did all these other elements, such as Uranium with 92 protons, get 92 protons squeezed into it's nucleus?

[Nowhere Man]

I must be quick, but if you look up "stellar nucleosynthesis" and "supernova nucleosynthesis" at www.wikipedia.org (http://www.wikipedia.org), you should get a good overview.

Fred

[Hamlet]

Reference Carl Sagan's (Cosmos) show last night, Live by the Stars

If hydrogen is the first element and once, the only element, and helium is formed by nuclear fusion inside a star, by pressing hydrogen together with so much energy it become helium, how did the other elements form?

For instance how does Oxygen exist? Oxygen has 6 protons and 6 electrons, seeking a parter with two electrons or one each, as in H20. Now it's outer shell is complete and the element is stable until energy forces it to seek more electrons to fill a new outer shell (18 - 32 electrons). I think I just answered my own question, but anyway... How did all these other elements, such as Uranium with 92 protons, get 92 protons squeezed into it's nucleus?

Elements from helium up through iron are formed as fusion products in the cores of stars. This process is called Stellar Nucleosynthesis (http://en.wikipedia.org/wiki/Stellar_nucleosynthesis). For elements heavier than iron, fusion no longer liberates energy but absorbs it instead. This requires a supply of energy which can come from supernova (http://en.wikipedia.org/wiki/Supernova_nucleosynthesis) explosions.

A star the mass of our Sun produces helium in its core by fusing hydrogen. When the hydrogen supply is used up the core will contract and eventually get hot enough to begin fusing helium into carbon via the triple-alpha process (http://csep10.phys.utk.edu/astr162/lect/energy/triplealph.html). Other reactions can form oxygen from some of the carbon.

Higher mass stars can continue to fuse elements beyond carbon and oxygen. This is why Sagan refers to us a being "made of star-stuff". We are quite literally made up of material cooked in stars or supernovae.

[Ken G]

Yes. An and interesting wrinkle to all this is, if the Big Bang started out hotter than the cores of stars, why did we not get carbon and oxygen, etc., from that? The answer is, as you mentioned, you need the triple-alpha process because two alphas (two helium nuclei) don't make anything stable. Thus you not only need high temperature, you need high pressure. When the Big Bang was hot enough, the pressure was actually lower than you find in the cores of stars! So we see the real function of stars-- to generate high pressure zones where you can fuse heavier elements.

[rockinreel]

Okay, fascinating! Sow now we have all these elements floating in space. So what was next? The iron starts attaching to each other (and nickel some other metal with magnetic properties), and attracting other things to it, creating a mass(es)? And how do non-iron core masses develop? There's nothing to magnetically attract into a tight ball.

[cran]

by chemistry (you mentioned H20, but other molecules also form); physics (eg, turbulence, pressure gradients); and good old gravity ... for details, I defer to the more competent among us ...

[Hamlet]

Okay, fascinating! Sow now we have all these elements floating in space. So what was next? The iron starts attaching to each other (and nickel some other metal with magnetic properties), and attracting other things to it, creating a mass(es)?

Some of the detritus flung into space from dead stars eventually makes its way into giant molecular clouds of gas which are enriched with the heavier elements. If that cloud then collapses into new stars and planets, the "seeded" material will be incorporated with them. If conditions are right some of these elements may react chemically with one another to form many of the compounds we are familiar with in our everyday life.



And how do non-iron core masses develop? There's nothing to magnetically attract into a tight ball.

It's gravity, not magnetism that causes new stars and planets to form.

[Ricimer]

and don't forget, the time after the BB when it's both hot and high pressure, you get "photo-dissociation" where the really high energy photons (created by the extremem temperatures) take any formed nucleus and break it into pieces again.

[TheBlackCat]

I have a question. Why isn't the iron found in oxide (rust) form? Considering how much more common oxygen is than iron, and how easily iron combines with oxygen, how do we explain meteorites that contain metallic iron? I can understand planet cores having metallic iron, since the heat and pressure would strip off oxygen, but asteroids were not subject to such heat and pressure as far as I know. The surface iron on rocky planets seems to be exclusively be in oxide form, and in fact for a long time meteorite iron was the only source of metallic iron for human use. With the exception of nitrogen and halogens pretty much every other common even moderatly reactive element seems to be found primarily in oxide form in nature (water, carbon dioxide, silicon dioxide, metallic oxides, etc.).

[Ken G]

I can understand planet cores having metallic iron, since the heat and pressure would strip off oxygen, but asteroids were not subject to such heat and pressure as far as I know. The surface iron on rocky planets seems to be exclusively be in oxide form, and in fact for a long time meteorite iron was the only source of metallic iron for human use.
Your insightful analysis is correct, you are just missing one point. If you find metallic iron in a meteor, then the parent body of that meteor was subjected to the kind of high heat and pressure that would strip off the oxygen. It must have been a large parent body (a canonical size appears to be about 100 km), to get that hot and have the dense metals sink to the core. Then something must have shattered the parent body all the way down to its iron core to get that irony meteorite. This is why irony meteorites are rare to see fall out of the sky. But if you find a meteor that has survived thousands of years of weathering on Earth, chances are it must have been one of those rare iron ones. And the presence of that meteor suggests that something pretty awful must have happened to something pretty big.

You might find this interesting: http://www.meteorlab.com/METEORLAB2001dev/albtxt1.htm

[TheBlackCat]

Ah, that would make sense. Thanks, Ken!