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If the heaviest elements that can be generated by a star is iron, how do we account for heaver element such as gold uranium and plutonium, etc?€¦

I am going to try to answer my own question but I would like everyone?€™s feedback.

All the matter that makes a planet is leftover trace elements of other stars the heaver elements such as gold, uranium and plutonium, etc?€¦ are created during the fusion process of a star, but these elements are trace element side effects from the massive fusion process in the core of a star. But then give Schwarzschild radius why isn?€™t all the heaver elements sucked into a black hole, or sitting in huge lumps of iron/carbon from the remains of dead stars? Doing some research into Wolf-Rayet type stars something struck me. Wolf-Rayet stars can generate so much outward pressure from heat of fusion that the star is literarily blowing itself apart.

Wolf-Rayet may be too massive to form a black hole. By the time the star gets to carbon fusion, the star has lost so much mass do to the pressure blowing itself apart, that it is under the Schwarzschild limit. However heaver elements were still created and ejected by the extreme fusion process. So when people look at iron asteroids, what they are really looking at is part of an iron shell with in a star that was ejected.

The elements heavier than iron usually are created in supernovas when they explode.

Selden is right, all elements heavier than iron are created in supernova explosions. I will explain it in some more detail to you.

You gain energy from nuclear fusion, because the binding energy per nucleon is bigger for the product of the reaction. (Take for example hydrogen and helium: The educts are four protons, i.e. hydrogen atomic cores. The product is one helium core consisting of two protons and two neutrons bound together. The binding energy is released in the fusion process.)
After all, this works only up to iron (precisely iron-56), which is the element with the highest binding energy. The binding energy per nucleon in the atomic core does then decrease again. This is why nuclear fission works: There you gain energy by splitting for example uranium to some lighter fragments which have a higher binding energy per nucleon than uranium.

Therefore, elements heavier than iron are normally not produced during the fusion processes in stars because you have to put in energy into this reaction and do not gain it. Actually, sometimes an iron nucleus in a stellar core would fusion to some heavier element by taking the energy from its environment - but very quickly it would be destroyed by hits from other nuclei or decay back to iron because this is an improved energetic state.

So where do the heavier elements come from? When a supernova explodes higher layer of the star fall to the center - the density becomes very high, temperature also. There is suddenly enough energy iron can take from to fusion. They start capturing neutrons - which were produced in this high density environment by pressing electron into protons. The problem is now to catch more neutrons very fast, because the so produced iron isotopes are radioactively instable and decay via beta radiation. The initial iron-56 must fetch at least three neutrons, then it will beta decay to cobalt, fetch another neutron, beta decay to nickel and so on. But this works only in the high-density stellar cores about to explode in a supernova, because most isotopes of the elements slightly heavier than iron are radioactively instable and it is very difficult for the atom "to climb the next step of the ladder".

You can distinguish then from so-called s- and r-process elements (s for slow and r for rapid) by the speed of the neutron catching. (When there is too much time between to catches, the atom would beta decay and become another element.) As a matter of fact, this is quite topical for astrophysicists and nuclear physicists.

One can calculate that this really works up to uranium, so it is true we're stardust

If you are interested, there is this book, "The alchemy of the heavens" by Ken Croswell which tells some more about it. For those with more knowledge in physics I would recommend Bernard Pagel's "Nucleosynthesis and chemical evolution of Calaxies"

I can not really follow some of your thoughts concerning the Wolf-Rayet- stars. First take it as a fact, no elements heavier than iron are build during the normal fusion processes in stars, no matter what kind of star. It's true, the heavier stars build up some sort of "shells" where different elements are produced at the end of their lives. (The sun would only make it up to carbon, heavier stars up to iron.) At the supernova explosion, these shells are destroyed. The outer layers (containing the lighter elements) are easily blown away, but also some parts of the inner shells and the core with the elements heavier than iron produced just before. In fact the relics of the supernova (mostly neutron stars) consist mostly of the former core of the star with the heavier elements. (For the neutron star this is also irrelevant, because the matter is converted almost completely into neutrons). Forget about the black hole, you must have a very, very massive star in the beginning (which is quite seldom) to form a black hole. Actually, Wolf-Rayet stars are quite massive but they loose much of their mass in stellar winds.

So when people look at iron asteroids, what they are really looking at is part of an iron shell with in a star that was ejected.

Quite apart from being completely on the wrong track with this (and Caro nicely explained how it actually works), iron asteroids are most likely the remnants of the exposed cores of large asteroids that differentiated into a crust/mantle/core, which were then broken up by impacts with other asteroidal bodies.

As Caro said, the thing you completely overlooked was that the process of going supernova is what creates the heavier elements (even up to the transuranic elements - there's indirect evidence from decay products in the Earth that we did actually have a small amount of plutonium contributing to our radiogenic heat output, but it's all decayed away now).

It's fairly sobering if you think about it - the stars that go supernova are the most massive ones, which are also the rarest stars (much less than 1% of all stars, IIRC?). When you consider just how small a proportion of the total normal matter of the universe must therefore consist of elements that are heavier than iron, it makes you feel kinda small .

Caro wrote:Forget about the black hole, you must have a very, very massive star in the beginning (which is quite seldom) to form a black hole. Actually, Wolf-Rayet stars are quite massive but they loose much of their mass in stellar winds.

First thanks for correction. You somewhat answered another question I had witch was that Wolf-Rayet are probably too hot to form back holes, by virtue that by the time the star gets to the final fusion stages, its ejected the mass required to from the black hole in the firs place. This would mean that there is a upper limit on how massive a star can be to from a back hole, unless you are going for a stellar back hole

On the topic of stellar back hole(s).

I was of the opinion that a ?€?true?€

Evil Dr Ganymede wrote:

So when people look at iron asteroids, what they are really looking at is part of an iron shell with in a star that was ejected.

Quite apart from being completely on the wrong track with this (and Caro nicely explained how it actually works), iron asteroids are most likely the remnants of the exposed cores of large asteroids that differentiated into a crust/mantle/core, which were then broken up by impacts with other asteroidal bodies.

As Caro said, the thing you completely overlooked was that the process of going supernova is what creates the heavier elements (even up to the transuranic elements - there's indirect evidence from decay products in the Earth that we did actually have a small amount of plutonium contributing to our radiogenic heat output, but it's all decayed away now).

It's fairly sobering if you think about it - the stars that go supernova are the most massive ones, which are also the rarest stars (much less than 1% of all stars, IIRC?). When you consider just how small a proportion of the total normal matter of the universe must therefore consist of elements that are heavier than iron, it makes you feel kinda small :).

Super nova explosions should have been much more common in the past, but yeah it does make you feel kinda small to know that we are the rejectect parts of a star. I will go craw into a hole now.

The problem with (stellar) black holes is that we still know very little about them. (The massive black holes in the cores of galaxies are better understood in the meantime...)
There's a very well-known mass limit for a white dwarf, when it's crossed the white dwarf will collapse to a neutron star, and therefore stars exceeding this limit will explode in supernova and become a neutron star instead of the white dwarf.
One can calculate, that there's also a mass limit for neutron stars, but here you can't give a precise number for the mass. (It depends on the pressure balancing gravity, which must be considered relativistic and depends in turn sensitively on particle composition and particle interaction)

The thing with a supernova explosion is that it's not the core that "explodes", it's the outer layers that were ejected. This is somehow more or less independent of the mass of the core, and supernovas producing neutron stars are considered similar those producing black holes. (After all, you still need a star of about 60 or 80 solar masses (initial) to get it.)

But indeed there's some consideration about black holes also forming "silently", i.e. mass infall up to the Schwarzschild limit without explosion of outer layers. But this must be a quite accelerated process, just like the supernova itself.

After all, I'm not working on black holes and the people who do also know not for sure.