NARRATOR: Have you ever wondered how all the chemical elements
are made?
Then join me as we are lifting all this data
secrets to understand the cosmic origin of the chemical
elements.
ANNA FREBEL: We just talked about the fusion processes
and how elements are made in stars, mostly
for energy generation purposes.
Now let's look at how that actually
manifests in the star as a whole because, as astronomers, we
can observe stars but we can't really look inside of stars.
We can only see the surface.
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There are a number of ways we can get clues
from the surface of the star as to what's going on in the core.
And so overall this is really a nice example of how nuclear
physics and astrophysics-- nuclear and astrophysics--
come together because the nuclear physics governs what's
happening inside the core and then the astrophysics provides
what we can actually observe.
And both kind of need to come together and work out.
So over the last several decades a lot of progress
has been made to put these two together
and to understand what is now called stellar evolution.
That is actually governed by the nuclear physics
processes, specifically fusion, in the core.
I wanted to share this with you because it's very insightful.
So I'm going to draw what we call
a Hertzsprung-Russell diagram.
It basically shows how a star evolves during its lifetime.
We can use the sun as an example.
And what we're going to have is, we're
going to have hot stars here and cool stars here.
And then we have more luminous stars
up here and less luminous stars down here.
And there is a certain track that
looks like this, half of a Christmas tree.
And the sun actually sits right now about here.
One can put any star in this diagram.
And you will see in a moment how we can then
learn about the evolutionary phase of the star and hence
what's going on inside its core.
So the sun is sitting here.
And we know on this branch here, which
we call the main sequence, that stars burn hydrogen to helium.
How does this look?
If we draw a star here, in the core hydrogen
is burned into helium just like what
we had in the previous section.
The star, when it moves on--
oh, and I should say, every star will
start somewhere along the main sequence here
and it will stay there for about 90% of its lifetime.
Which means coming back to the old stars for a second,
90% of 15 billion years is about the age of the universe which
means the stars that started here when they were born
in the early universe, they are just
at the end of this hydrogen to helium process which really
means they haven't done anything else
but burning hydrogen into helium which
really is the key to why these stars don't show their age.
They are just like what they've always done.
And we can observe them today and infer
things about the early universe from them
today because they haven't changed.
That's the key.
But if we look at a star that has a shorter lifetime
and wants to evolve, it will move up here.
And it will move up here when the core--
let's see, this was hydrogen here
and it has been converted to helium--
when we indeed have just helium in the core and there
there's no hydrogen left in the core.
Then the star will get a little bit rumbly.
And so it's going to start moving along here.
And there are all sorts of things going on in the core
because the thermostat is out.
There is no energy being produced right now.
And so what happens is, the star actually
inflates to counteract that and it will move up here
and become very luminous.
And up here we have the red giants.
They're called red giants because they
are much bigger and more luminous
but they're also cooler because--
they're bigger and so they turn red.
And so they have just a helium core and what's happening
is, in an outer shell here they still
burn hydrogen to helium burning going on in the shell.
And that provides a little bit of substitute energy,
a little interim energy to the star as it moves up here.
And then up here, we have something
called the helium flash which means
the helium here in the core is now being converted to carbon.
How can I draw this?
I'll make this go away.
So we eventually get--
helium gets converted to carbon.
So eventually we're going to get to a carbon core.
And then we have helium burning further out
and hydrogen burning yet further out.
When helium burning starts here, by the time it reaches here
it has this carbon core.
So here it reaches a helium core, this region.
And then helium starts to burn.
And then by the time it gets here we have a carbon core,
and then it moves up here.
And this last part here, it really
depends on the mass of the star.
The sun is actually not going to do much.
It's going to just stick it out with a carbon oxygen core
here and then turn into white dwarf and just cool down.
So the sun is actually pretty boring star
that has a pretty boring fate.
If we make the sun much more massive,
let's say 10 times more massive, it would move up here
in this carbon burning phase.
And if we have variety of later burning stages
that lead to iron.
And then it would have an iron core up here.
And you already know what's going to happen.
If a star has an iron core it has lost its energy source
and it will explode as a supernova.
And so before it explodes, what's it going to look like?
We have a whole bunch of these so-called shells.
Sometimes they're referred to as onion shells.
In the center you have iron and then
there's silicon and all the other elements folding out
here--
oxygen, carbon, helium.
Let's draw another one.
Hydrogen.
There are a few other elements being
produced in minority processes.
So some of these shells are not pure in these elements.
But that's the basic idea that you go--
this is oxygen and silicon.
This is sulfur and others.
That you have a star that looks like that.
So what you see here is that whatever is happening
in the core has a direct impact of where the object sits
on this diagram here.
And so by measuring the luminosity of a star
as well as its temperature we can place it on this diagram
and then learn which evolutionary state
the star is in, which tells us what is going on in its core.
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