Hello. My name is James Connelly and I'm with the Center for Star and Planet Formation,
Natural History Museum of Denmark,
the University of Copenhagen.
The segment is discussing nucleosynthesis or the origin of elements in our solar system.
Nucleosynthesis models come from three separate lines of evidence.
The first, the composition of our solar system,
and by that I mean the elemental abundances that we have within our solar system.
The second, comes from
experiments on nuclear reactions under set conditions in the laboratory.
And the third is theoretical constraints on possible sites or
environments for these nucleosynthetic reactions that we imagine within the laboratory.
The most important of these constraints is possibly the composition of the solar system.
Let's just review for a second the atomic structure.
We have the nucleus of an atom which is made up of
two fundamental particles, neutrons and protons.
The elements character is defined by the number of protons in the nucleus,
and the number of protons is matched by the number of electrons in
the shell of electrons that are orbiting around the nucleus.
The second fundamental component of the nucleus are neutrons.
If the protons give the element its fundamental character,
the neutrons are adding mass to the nuclei
but don't actually determine any of the properties.
The other important thing to know about
the atomic structure is that an element then is defined by
the number of protons whereas the number of neutrons can vary for a given element,
and this gives us different isotopes of a set element.
In this example you see here,
we have beryllium which comprises four protons and it has five neutrons.
The neutron number can vary but the protons for beryllium is set.
This is a diagram where we have on the x-axis the number of
neutrons that exist within the nucleus and on the y-axis the number of protons.
You can see the black squares that form the center of this array,
those are the stable nuclei that we
have within the elements that we have within our solar system.
There are 80 elements that occur,
339 naturally occurring isotopes of these elements within our solar system.
This is a much more familiar presentation of the elements that we have.
You have all recognized the periodic table in which we have
a organized array of all the different elements where
we give some indication of
their chemical properties according to the way this is organized.
But this gives us no information about
the relative abundance of any of the elements that you see here,
or any information about the isotopes that might exist for each of these elements.
This is a more useful presentation for the elemental abundances of the solar system.
You can see here that you have along the x- axis you have the element number,
so you go from hydrogen and helium in the lower left-hand corner of the of the diagram,
all the way up to uranium as the heaviest element that we have,
and the relative abundance in a log scale along the y-axis.
You can see that hydrogen and helium by far
make up the majority of the elements that we have in our solar system.
In fact, they make up almost 99% of the elements that we have today and we go down to,
all the way down to uranium which is the least abundant.
The nucleosynthetic models that
we try and develop just to explain where the elements come from,
it has to explain this relative abundance.
How do we know the elemental abundance of the solar system as presented in this diagram?
That comes from several different sources
but one of the main sources that we use are meteorites.
Those stones that fall from space down onto earth,
we go out and we can collect those,
and there are certain type of meteorites that are called chondrites.
They are the most primitive,
they're like cosmic sediments and these meteorites are what we use to
represent the composition of
the solar system before we had the formation of planets and so on.
We use these as a proxy for the bulk composition of our solar system.
And they're very good for most of the elements,
but they're not especially good for noble gases and some of
the volatile elements which isn't critical for the discussion today.
Okay.
Now we move on to this issue of how are the elements formed.
There is two major hypothesis presented in about the late 1940s and
early 1950s which tried to explain
this distribution of the elements that we have within our solar system.
The first one is referred to as Big-Bang nucleosynthesis and it won
a Nobel Prize in 1967 in physics for its explanation of where all the elements came from.
It turned out to be incorrect as we'll see in a second.
A second model known as Stellar nucleosynthesis presented by Burbidge,
Burbidge, Fowler and Hoyle which became known as the B^2FH model.
This became the standard model which we use today for explaining nucleosynthesis.
This also won a Nobel Prize in 1983.
We're not going to consider any longer the Big Bang nucleosynthesis
but we'll instead investigate or discuss the Stellar nucleosynthesis,
also known as the B^2FH which we'll refer to later.
Okay. Within this model,
there are three categories of nuclide and when I say nuclide,
I mean these are just the elements and their isotopes,
so nuclide production, there are
three different categories of nuclide production in this model.
The first is Big Bang products,
hydrogen, helium, minor lithium, beryllium, and boron.
They were all formed, as we'll see in a second,
in the Big Bang.
The second, referred to as Stellar fusion products that goes from
lithium all the way up to the iron group elements including iron, cobalt, and nickel.
And then we have the third group which is
the products of nucleosynthesis related to stars and
supernovae and we'll explain that in a second but they generate
all the elements heavier than iron including some lighter elements like aluminum,
silicon, titanium, and vanadium.
Okay.
First to explore then the Big Bang nucleosynthesis.
In the simplest terms,
this is the elements that would have been formed at the instant that the solar,
sorry, the universe formed.
These are referred to as Big Bang nucleosynthesis.
This is what we inherited in
the very short period of time immediately after the Big Bang.
And as I said a minute ago,
that is mainly hydrogen and helium and so at the instant of
the Big Bang you would have had fundamental particles of only protons and neutrons,
and those protons and neutrons would have begun to have forming.
As the system cooled,
you would begin to have the protons and neutrons coming together in
the first step in this diagram to form hydrogen atoms.
These hydrogen atoms as time went on and things got--,
but were still on a very brief period of the Big Bang,
the hydrogen would have started to combine together to give you
helium-3 and then helium-3 would have
combined together with other hydrogen atoms to give you helium-4.
So you have hydrogen and helium in
the immediate moments after the Big Bang would have formed,
and cooling would have come down to the point where you could
have these nuclides existing.
But the original Big Bang nucleosynthetic model
suggested that all the elements were formed this way
in the instant after the Big Bang that you would,
as cooling progressed you could have put these fundamental particles of protons,
neutrons, hydrogen, helium together to get you
all the way up to the highest and/or heaviest elements.
But it turns out that there is
a fundamental barrier that exists at beryllium and lithium,
so that as you try to build heavier elements from beryllium
and lithium you can't get above
lithium-7 because as you add another proton to lithium you end up producing helium again.
So you come back down to these lighter isotopes.
You basically spin within this cycle and you can't get beyond
this limited array of light elements.
So we need another model which is where this Stellar nucleosynthesis came into
play to explain the existence of all the heavier elements.
Let's look again at a different type of diagram which
looks again with the x-axis with the mass number,
which is to say, lightest isotopes in
the left-hand side and heaviest elements on the right-hand side.
The mass number refers to as the number of protons and neutrons together.
So, hydrogen and helium again at the left-hand side,
uranium at the right-hand side of this diagram.
As you look up the y-axis,
the y-axis refers to the binding energy of the nucleus of these different elements.
And you'll see, a very important point in this diagram is
that you go from a low binding energy at hydrogen and helium,
up to a maximum at iron-56 and then you begin to
decrease again all the way up to uranium-238.
The important thing to take away from this diagram is,
there is a maximum binding energy at
iron-56 that is important for nucleosynthetic models.
Everything to the left of iron-56 would prefer to combine
together in reactions which we could call fusion reactions in which energy is emitted.
These atoms would prefer or nuclei would prefer to come together to form
more and more stable nuclei up until iron-56,
but fusion cannot proceed above iron-56
because you get lower and lower binding energies from that point forward.
This diagram is split into two regions.
The left-hand region which we refer to as
fusion reactions and the right-hand side of the diagram which we refer to
neutron capture reactions and they require
two different processes to give you all of the different elements in this diagram.
Let's first look at
the fusion reactions that you see on the left-hand side of this diagram.
