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Cosmic Origin of the Chemical Elements, Ep. 9: Formation of the Heaviest Elements

Have you ever wondered how all the chemical elements are made?

Then join me as we are lifting all the star dust secrets to understand the cosmic origin of the chemical elements.

We talked a lot about the lighter elements that are made in fusion processes

in the cores of stars but what about all the other elements

from the bottom half of the periodic table?

We haven't really talked about those yet -- Let's do that!

What we need are so-called seed nuclei. We have an iron nucleus, here, and if we are

in a situation where there is a strong neutron flux available (we'll talk about

where that happens in second) then if we have little neutrons here and this seed

nucleus is getting bombarded with these neutrons then it's going to swell and

turn into a much larger nucleus that is radioactive and neutron-rich and it's an

isotope that has lots of neutrons in it. What's happening then, because it was

radioactive, it doesn't like to stay in this way, it will what we call

beta-decay which is just a fancy word for saying that all these neutrons here are

being converted, or good fraction of them, into protons, and so we end up with a

stable element that's much larger than this original one,

the iron. Or it could also have been a carbon atom. This is the basic idea how all the

other heavy elements are made. An example will be barium here or uranium. Uranium-238

is technically not a stable element but it's half-life is 4.7

billion years so for us humans that's pretty stable but on cosmic

timescales it is not. But it if we want to consider it as stable it would be the

heaviest element that we have on Earth that's long-lived. They're all

made by this so-called neutron-capture process -- neutron-capture process. Now

there are a few details that we should consider, mostly that there are

actually two different ways where this neutron-capture

process can happen. One is in a slow way -- slow neutron-capture -- and

the other way is rapid. And that refers to how fast and over

what timescale this neutron bombardment is occurring. In the case of the slow

neutron-capture, the timescale is about 10,000 years, and what happens is that in

evolved red giant stars -- evolve red giants -- in the inner layers in

some of the shell layers where the nuclear fusion is going on, and there

are secondary nucleosynthesis fusion processes operating, and as a result of

that free neutrons are produced. They provide a steady flux of neutrons

that then get essentially shot onto these seed nuclei, and so over the

timescale of something like 10,000 years, heavy elements are successively

build up. A neutron is added, it turns into radioactive isotope, it decays and then

you have a steady one. You add another neutron, it will decay again, and so you

kind of it build up, one by one by one, all the way up to lead. It's the heaviest

stable-stable element, if you take away thorium and uranium because they *are*

radioactive, as I just mentioned. Now in the case of the rapid neutron-capture

that really requires much more energetic and extreme conditions, and what recent

research has shown is that rapid neutron flux only operates in two locations. One

is perhaps in supernovae. When the iron core collapses at the end of

star's life, it actually implodes and forms a neutron star. There's a

really dense neutron star in the middle -- that's a contact remnant left over after

the supernova -- and in the process of making this neutron star, there are of

course lots of neutrons floating around. They can provide this kind of flux,

operating on a 1 to 2 second timescale. So a huge neutron bombardment

within a few seconds and that can lead to a very, very fast buildup of

giant radioactive nuclei here that then decay. So if you have enough seed

nuclei and all of them would make "whoom" [become huge] and then slowly decay back to the

different elements that make up the entire bottom of the periodic table.

Another option is merging neutron stars. If you take two of these neutron

stars, and you have them in a binary system where they orbit each other, and

if this system eventually, or the two stars in the system, eventually coalesce

and merge, then you also have some kind of firework of neutrons, and that can

also have this rapid neutron-capture going on. We have to add here, so

either in supernovae or the proto-neutron star if you have like that

or in neutron star mergers. That are all the options and what

we now want to really figure out is how can we put all this theory to the test,

right, how can we observe this? That's when our old stars come

back into play. Imagine that in the very beginning of the universe, when

the first stars emerged and maybe the second generation of stars, so not too

much of all the heavy elements was present at that time. Let's

say, you have a neutron star merger go off at this very early time, the rapid

neutron-capture process will occur, and all these new heavy elements gets spilled

into the surrounding, and then you form a next-generation star from this enriched

material. And because the universe was not too much enriched in all the

other elements, we have this opportunity to observe a clean nucleosynthesis

process of this r-process (it's sometimes abbreviated' rapid' with r) so 'r-process'.

And actually, this here is s-process,

you could have guessed that. So at the earliest times, it is possible

to observe the signature of the r-process, a clean signature, as well as the s-process.

That is not possible anymore today. The universe has experienced

13 billion years of chemical evolution, so it's a pretty messy place

out there. If one more event goes off, that signature just gets

diluted into whatever else is out there. But at the earliest times, we have this

chance to find these clean signatures.



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Have you ever wondered how all the chemical elements are made?

Then join me as we are lifting all the star dust secrets to understand the cosmic origin of the chemical elements.

We talked a lot about the lighter elements that are made in fusion processes

in the cores of stars but what about all the other elements

from the bottom half of the periodic table?

We haven't really talked about those yet -- Let's do that!

What we need are so-called seed nuclei. We have an iron nucleus, here, and if we are

in a situation where there is a strong neutron flux available (we'll talk about

where that happens in second) then if we have little neutrons here and this seed

nucleus is getting bombarded with these neutrons then it's going to swell and

turn into a much larger nucleus that is radioactive and neutron-rich and it's an

isotope that has lots of neutrons in it. What's happening then, because it was

radioactive, it doesn't like to stay in this way, it will what we call

beta-decay which is just a fancy word for saying that all these neutrons here are

being converted, or good fraction of them, into protons, and so we end up with a

stable element that's much larger than this original one,

the iron. Or it could also have been a carbon atom. This is the basic idea how all the

other heavy elements are made. An example will be barium here or uranium. Uranium-238

is technically not a stable element but it's half-life is 4.7

billion years so for us humans that's pretty stable but on cosmic

timescales it is not. But it if we want to consider it as stable it would be the

heaviest element that we have on Earth that's long-lived. They're all

made by this so-called neutron-capture process -- neutron-capture process. Now

there are a few details that we should consider, mostly that there are

actually two different ways where this neutron-capture

process can happen. One is in a slow way -- slow neutron-capture -- and

the other way is rapid. And that refers to how fast and over

what timescale this neutron bombardment is occurring. In the case of the slow

neutron-capture, the timescale is about 10,000 years, and what happens is that in

evolved red giant stars -- evolve red giants -- in the inner layers in

some of the shell layers where the nuclear fusion is going on, and there

are secondary nucleosynthesis fusion processes operating, and as a result of

that free neutrons are produced. They provide a steady flux of neutrons

that then get essentially shot onto these seed nuclei, and so over the

timescale of something like 10,000 years, heavy elements are successively

build up. A neutron is added, it turns into radioactive isotope, it decays and then

you have a steady one. You add another neutron, it will decay again, and so you

kind of it build up, one by one by one, all the way up to lead. It's the heaviest

stable-stable element, if you take away thorium and uranium because they *are*

radioactive, as I just mentioned. Now in the case of the rapid neutron-capture

that really requires much more energetic and extreme conditions, and what recent

research has shown is that rapid neutron flux only operates in two locations. One

is perhaps in supernovae. When the iron core collapses at the end of

star's life, it actually implodes and forms a neutron star. There's a

really dense neutron star in the middle -- that's a contact remnant left over after

the supernova -- and in the process of making this neutron star, there are of

course lots of neutrons floating around. They can provide this kind of flux,

operating on a 1 to 2 second timescale. So a huge neutron bombardment

within a few seconds and that can lead to a very, very fast buildup of

giant radioactive nuclei here that then decay. So if you have enough seed

nuclei and all of them would make "whoom" [become huge] and then slowly decay back to the

different elements that make up the entire bottom of the periodic table.

Another option is merging neutron stars. If you take two of these neutron

stars, and you have them in a binary system where they orbit each other, and

if this system eventually, or the two stars in the system, eventually coalesce

and merge, then you also have some kind of firework of neutrons, and that can

also have this rapid neutron-capture going on. We have to add here, so

either in supernovae or the proto-neutron star if you have like that

or in neutron star mergers. That are all the options and what

we now want to really figure out is how can we put all this theory to the test,

right, how can we observe this? That's when our old stars come

back into play. Imagine that in the very beginning of the universe, when

the first stars emerged and maybe the second generation of stars, so not too

much of all the heavy elements was present at that time. Let's

say, you have a neutron star merger go off at this very early time, the rapid

neutron-capture process will occur, and all these new heavy elements gets spilled

into the surrounding, and then you form a next-generation star from this enriched

material. And because the universe was not too much enriched in all the

other elements, we have this opportunity to observe a clean nucleosynthesis

process of this r-process (it's sometimes abbreviated' rapid' with r) so 'r-process'.

And actually, this here is s-process,

you could have guessed that. So at the earliest times, it is possible

to observe the signature of the r-process, a clean signature, as well as the s-process.

That is not possible anymore today. The universe has experienced

13 billion years of chemical evolution, so it's a pretty messy place

out there. If one more event goes off, that signature just gets

diluted into whatever else is out there. But at the earliest times, we have this

chance to find these clean signatures.


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