How did the heavier elements form?

[Entropy]

Currenly scientists are stumped (atleast that I know of) on how elements heavier than iron formed. This is because stars can't produce elements heavier than iron through fusion. After iron, fusion switches from exothermic to endothermic. Some people think these elements are created with the immense forces in supernovae. But I have trouble imagining enough heavy elements being created in supernovae explosion to account for the abundance of heavy elements.

Anyone care to comment?

 

[turbo]

Scientists are not stumped on this one, and have a reasonable model for this:

Elements heavier than iron (up to and including bismuth) are created in the star's core by neutron capture. The heavier elements (up to and including uranium) are created in the heavy energy flux of supernovae.

 

[Nenad]

sorry to say, but turbo is right.

 

[Entropy]

sorry to say, but turbo is right.

Why are you sorry? Now I know more.

Just out of curiousity. Why don't elements heavier than uranium exist in nature? Are their half-lifes too short or something?

 

[turbo]

I'm not certain why the transuranic elements are all man-made. Perhaps the energies in a supernova are not sufficient to produce them, or maybe the time that those energies are produced is too short. Some Googling may help you find the answer (or at least a model you can trust). It's not a matter of half-life though, because if I recall, at least one isotope of Plutonium has a half-life in the tens of thousands of years.

 

[meteor]

Elements heavier than iron (up to and including bismuth) are created in the star's core by neutron capture. The heavier elements (up to and including uranium) are created in the heavy energy flux of supernovae.

I agree with turbo-1.Technically, neutron capture can be divided in r-process and s-process. s-process can produce elements up to bismuth, while r-process produces heavier elements

http://zebu.uoregon.edu/~js/ast223/lectures/lec21.html
"Neutron capture can happen by two methods, the s and r-processes, where s and r stand for slow and rapid. The s-process happens in the inert carbon core of a star, the slow capture of neutrons. The s-process works as long as the decay time for unstable isotopes is longer than the capture time. Up to the element bismuth (atomic number 83), the s-process works, but above this point the more massive nuclei that can be built from bismuth are unstable.

The second process, the r-process, is what is used to produce very heavy, neutron rich nuclei. Here the capture of neutrons happens in such a dense environment that the unstable isotopes do not have time to decay. The high density of neutrons needed is only found during a supernova explosion and, thus, all the heavy elements in the Universe (radium, uranium and plutonium) are produced this way."

 

[Nereid]

Just out of curiousity. Why don't elements heavier than uranium exist in nature? Are their half-lifes too short or something?

They do, in (core collapse) supernova remnants, and are a component of cosmic rays. However, as you correctly state, their half-lives are short compared with the age of the solar system (and any SN remnants that have impacted the Earth since have been too small, in mass, to count), so they've all long since decayed to way below the level of detectability.

IIRC, very heavy nuclei have been observed in cosmic rays, up to atomic number 126? The explanation is that they were created in an SN, and quickly accelerated to very near c, traveled thousands of pc, and hit a balloon-borne detector.

 

[Entropy]

They do, in (core collapse) supernova remnants, and are a component of cosmic rays. However, as you correctly state, their half-lives are short compared with the age of the solar system (and any SN remnants that have impacted the Earth since have been too small, in mass, to count), so they've all long since decayed to way below the level of detectability.

IIRC, very heavy nuclei have been observed in cosmic rays, up to atomic number 126? The explanation is that they were created in an SN, and quickly accelerated to very near c, traveled thousands of pc, and hit a balloon-borne detector.

Neat. Thanks for answering my questions.

Another thing I just thought of: its theorized that there is an island of stablity around the atomic number 160 or something, right? Is it reasonable to hypothesize that there isn't an island of stablity because we don't encounter these naturally, atoms that heavy can't be formed period or that supernovae lack the power to create them? If supernovae [hypothetically] can't create them, then do we have any hope of ever producing them in particle accelerators?

 

[Nereid]

Neat. Thanks for answering my questions.

Another thing I just thought of: its theorized that there is an island of stablity around the atomic number 160 or something, right?

I don't know - if you google up something on this, could you post a link here please?

Is it reasonable to hypothesize that there isn't an island of stablity because we don't encounter these naturally,

not necessarily; maybe we haven't yet looked in the right places, or carefully enough.

atoms that heavy can't be formed period

maybe

or that supernovae lack the power to create them?

Pretty unlikely; if they're not formed in SN, it's more likely that there aren't environments in an SN which could create them; to answer that, you'd need to have an idea of how such a heavy beast could be formed ... collisions of what with what, for example.

If supernovae [hypothetically] can't create them, then do we have any hope of ever producing them in particle accelerators?

Of course; it all depends on how they would be made! Who knows, perhaps they exist as minor constituents of the transient atmosphere of neutron stars, when a large lump of mass smashes onto the surface?

 

[Entropy]

Of course; it all depends on how they would be made! Who knows, perhaps they exist as minor constituents of the transient atmosphere of neutron stars, when a large lump of mass smashes onto the surface?

Very possible! Then again it would pretty hard for them to escape a neutron star's gravity and ego spead out into the universe where we might find them.

This was the initial source where I heard "island of stability": http://www.npl.washington.edu/AV/altvw17.html

 

[alpha_wolf]

Of course; it all depends on how they would be made!

For example, once we develop sufficiently powerfull accelerators, we could fuse plutonium ions to get something with an atomic number around 200... I think (we might get something different, like a few smaller atoms or something).

 

[Janus]

Neat. Thanks for answering my questions.

Another thing I just thought of: its theorized that there is an island of stablity around the atomic number 160 or something, right? Is it reasonable to hypothesize that there isn't an island of stablity because we don't encounter these naturally, atoms that heavy can't be formed period or that supernovae lack the power to create them? If supernovae [hypothetically] can't create them, then do we have any hope of ever producing them in particle accelerators?

Here's the thing about that "Island of Stability", it should be called an "Island of Relative Stability". While elements located there are predicted to have half-lives longer than those of the elements near them, they still will not be fully stable. While it is hard to predict the actual half-lives of these elements, they are generally believed to be measured in no longer than days.

 

[MaWM]

To add to what has been said already:

As has been mentioned, the element of the "island of stability" is still very unstable compared to common elements.

Plutonium has been found in nature. Though it has a short half life, it forms naturally from uranium under the right conditions. Heavier elements almost certainly form in supernovae, but do not last long enough to be detected.

 

[keinve]

back near the mass creation of heavy elements, there were enormous amounts of large stars. much larger than what is seen today. it would be possible for the heavy elements to be produced when they went supernova.