Okay, so we've solved the fundamental 
problem that the sun posed. 
We now know what the sun's source of 
energy is. 
Now we trust our models and we try to use 
this understanding to figure out what we 
can about how sun, how the Sun is 
constructed. 
What its structure is. 
How do we study the structure of the Sun. 
Well, lots of numerical modeling. 
We measure reaction rates and properties 
of hydrogen here on Earth, and then try 
to apply them. 
But it's hard to simulate the conditions 
of the Sun here on Earth. 
another important. 
Measurement two is helioseismology. 
Just as we infer the interior structure 
of the Earth from the propagation of 
sound waves through the Earth. 
Sound waves propagate through the very 
compressed hydrogen that is the Sun. 
And the properties of that propagation 
can be used to infer interior properties 
of the sun. 
How do we generate sunquakes? 
Well, we don't need to. 
The Sun oscillates. 
The Sun is a fluid system. 
It oscillates. 
And what we do is we measure the Doppler 
shift, 
of radiation from, spectral lines 
actually from particular points on the 
sun and we notice that, that 
when we plot this, that the sun is 
wobbling in particular ways. 
And studying the frequency and, spatial 
structure of these wobbles, these 
resonances. 
The sun is basically ringing like a bell. 
And, you can try to hear the shape of a 
bell. 
And that's what we're trying to do. 
And that's one of the main tools we have 
for, verifying our modelling predictions 
for the structure of the interior of the 
sun. 
And so this is what, how we, we, this 
plus modeling is how we understand 
everything that happens between the core, 
that inner, densest part, and hottest 
part, where fusion is ongoing, and the 
photosphere, which is the name we give 
the outside of the sun, the part that we 
see. 
So the part that we see is the 
photo-sphere it makes the photons and now 
you know, we have our understanding of a 
spherical object. 
It's a fluid object, it's held together 
by gravity and it's held up against 
gravity. 
By the intense heat and pressure and 
radiation pressure generated by all that 
energy being released, in the interior in 
the core. 
And, like any object in equilibrium, 
density and pressure and likewise 
temperature increase as you move from the 
surface of the sun, and the core is the 
hottest and densest, and as you move out, 
it becomes the sun is cooler and less 
dense, as one expects for hydro static 
equilibrium. 
This is what holds the sun up against 
collapse. 
That due to gravity is the heat coming in 
from the core, and here is what we 
understand in terms of the structure of 
the Sun. 
So the internal core of the Sun occupies 
the regions to within a quarter of the 
solar radius. 
Solar radius is the radius of the 
Photosphere by convection, and that's 
about 696,000 kilometers. 
And to with, and to, from the center, 
about a quarter of a solar radius out, 
that is the core. 
That is the region where 99% of the 
fusion power is generated. 
Temperatures at the center of the core 
reach about 15,000,000 or 16,000,000 
kelvin. 
And at the outside of the core, there's 
7,000,000. 
And by that time, fusion is becoming 
completely inefficient. 
The density two decreases at the center 
of the sun. 
The density of hydrogen is 150 times the 
density of water. 
And by the time you get to the outside of 
the core it's only twenty times the 
density of water and this region, the 
inner quarter solar radius of the sun 
contains 40% of the mass of the sun. 
This might not sound impressive until you 
remember that volume scales by radius Q. 
So we're talking about a 64th of the 
volume of the sun containing almost half 
the mass. 
This is indeed far denser than the rest 
of the sun, this is the region where 
hydrogen is most extremely compressed. 
And, importantly, the core is in a state 
of equilibrium in the following sets. 
If fusion rates in the core decrease, for 
example. 
Then, the core is producing less energy. 
There is less radiation pressure and less 
thermal pressure. 
The core cools a little. 
What this does under the. 
Is that, then, the force of gravity. 
The weight of the outer layers of the sun 
compresses the cores a little bit. 
And when the core compresses, densities 
and temperatures increase. 
Densities, because it's compressing. 
temperatures, because it's now heated by 
Kelvin Helmholtz heat, 
it's contracting. 
This in turn, increasing the temperature 
and the density, increases the rate of 
fusion. 
So there, there's this self-regulating 
thing. 
If the, on the other hand, the rate of 
fusion were to increase, the core would 
heat up, expand a little the expansion 
would cool it down, and this would 
decrease the rate of fusion. so the rate 
of fusion is in fact controlled by the 
weight of the outer atmosphere of the 
sun. 
In other words, the luminosity is 
determined by mass of the outer layers of 
the sun pushing down on the core, 
determining how much of the sun is going 
to be compressed. 
When we talk about other stars, this is 
going to be very important, 
the luminosity of the star is determined 
essentially by it's total mass, that 
determines how much of the hydrogen is 
compressed enough in the core and hot 
enough to undergo fusion. 
This is the internal, this is sort of the 
engine of the sun. 
This is where the energy gets produced. 
And then there is what you might call the 
analog of the mantle of the Sun, the next 
layer labeled number two here. 
It's called the radiation zone. 
It extends from a quarter of the, solar 
radius out up to about 7.. 
Temperatures in the, in the radiation 
zone decrease from seven million at the 
outer boundary of the core to only two 
million kelvin at the 
outside of the radiation zone, densities 
to decrease, down by the, by the, the 
outside of the convection zone, the sun 
has thinned out to nearly the density of 
water. 
It's merely, 
but remember, this is, hydrogen gas at 
that density. 
throughout this region, temperatures are 
in the millions of Kelvin. 
There are no atoms here. 
The interior of the sun is a plasma. 
In other words, nuc-, nuclei, mostly 
hydrogen nuclei. 
And, electrons move completely 
independent of each other. 
There's no coherent atomic structure that 
survives at these temperatures. 
The thermal energies of things are much 
higher than the binding energies of 
atoms. 
And 
this region is characterized by the fact 
that the way energy is transferred, 
so of course, there's a net transfer. 
Energy is produced in the core, and it 
flows out, and then is radiated out to 
heat us. 
And the mode of heat transfer in this 
part of the sun is radiation. 
Now, this does not mean that this part of 
the sun is transparent, it's radiation 
diffusion. 
In other words, high energy photons 
produce gamma rays produced in the core. 
Are admitted they are immediately 
absorbed by interaction with the charged 
particles, 
in the plasma. 
And then readmitted and reabsorbed and 
readmitted and reabsorbed, 
an average photon, or the energy of an 
average photon takes 170,000 years to 
travel the distance from the core to the 
outer edge of the radiation zone. 
So this is not moving at the speed of 
light. 
This is some very slow diffusion. 
When you have charged particles in a 
plasma, then there's a, a strong 
interaction between the charges and the 
radiation. 
And radiation does not penetrate a plasma 
the way it penetrates a neutral gas like 
the light in this room. 
But, the mode of energy transfer is 
radiation, diffusion, and, eh, that means 
it that, that energy moves up, but the 
plasma itself, is not in any, under going 
any macroscopic, global motion. 
This changes in the outer mantle of the 
sun, that's the convection zone, between. 
.7 of the solar radius all the way out to 
the photosphere and here temperatures 
decrease very rapidly from two million 
degrees in the interior down to the 
surface temperature of the sun that we 
measure, 5,777 Kelvin. 
densities decrease from about the density 
of water to 
a fifth of that density, still pretty 
densed for hydrogen gas its still 
compressed by gravity and the mode of 
heat transfer in this region is 
convection. 
In other words in this region the plasma 
becomes opaque essentially and the most 
efficient mode of heat transfer becomes, 
hydrogen that is heated at the inner, 
bottom of the convection zone by the 
incoming radiation from the radiation 
zone rises, and then cools when it 
reaches the surface, and then sinks again 
so you get these convection cells. 
This is responsible for the sort of 
granular appearance of the photosphere 
which is very evident in the following 
movie. 
Here we see very high resolution image of 
the solar surface. 
Details as small as 500 kilometers across 
can be measured and for proportion. 
We've indicated here the size of the 
earth so that you know the size of the 
object we're looking at. 
We're looking near a sunspot and you see 
the sort of granular shape of the surface 
of the sun. 
This characterizes the way the 
photosphere looks. 
In the center of a granule you imagine is 
where warm gas is welling up and then on 
the edges, between the granules is where 
cooler hydrogen is seeping back down and 
because it's cooler we observe a lower 
temperature. 
Take t to the fourth, from Stephan 
Boltzman, means those region radiates 
less which is why they look dark. 
They are still very warm and they are 
radiating but they are radiating less. 
Then the surroundings. 
And it's interesting to watch the 
dynamics of this process. 
We see sort of, constantly changing 
convection patterns. 
There's not a stable convection pattern 
on the sun, but there's these oozing 
changing patterns. 
So we've gotten all the way out to the 
outside of the sun. 
But of course, like the earth, the sun 
extends be, beyond its surface. 
There is more of the sun outside the 
photosphere the sun. 
What you might call the, the sun's 
atmosphere. 
There's several layers. 
The density of gas decreases 
dramatically, but it turns out that. 
So there's not much there in the solar 
atmosphere. 
The mass of the sun, is below the 
photosphere. 
But 
The temperature it turns out because of 
various processes some of which we'll 
discuss increases with altitude the first 
lower layer of the atmosphere, it's 
called the chromosphere. 
That's the layer up to an altitude of 
2000 kilometers above the photosphere. 
And in the chromosphere the temperature 
rises to about 50,000 kelvin but the 
density decreases by seven orders of 
magnitude. 
So there's not much gas there. 
It's very hot. 
But we don't see the chromosphere. 
We're blinded by the photosphere even 
thought the chromosphere is hotter, 
because the T to the fourth is outweighed 
by the fact that there's basically 
nothing there compared to that. 
To observe the chromosphere we use the 
fact that in the chromosphere there are 
hydrogen atoms. 
hydrogen atoms emit a characteristic 
frequency spectral line at 656.28 
nanometers. 
It's the H alpha line. 
We've talked about it before and will 
again and so when you take pictures of 
the sun using the filter that allows only 
that wavelength through, you're 
essentially taking a picture of the. 
photosphere and this movie is indeed such 
a picture. 
Those dark finger like shapes are called 
spicules. 
They reach thousands of kilometers up out 
of the atmosphere of the sun and their 
precise dynamics in structure are not 
well understood. 
But again we get this image of broiling 
atmosphere of the sun and we'll see more 
of that. 
Above the Chromosphere the temperature 
jumps very rapidly, as you enter the 
corona. 
And the temperature jumps from the 50,000 
in the, at the top of the Chromosphere to 
2,000,000 Kelvin. 
This is not a fusion going region. 
The density is down to ten to the minus 
twelve. 
there is again virtually nothing there 
and this region extends as you can see in 
this beautiful coronagraph out to one or 
two solar radii. 
So the blacked out region, a coronagraph 
is a telescope that blacks out the 
blinding illumination of the photosphere 
so that you can see the rest. 
another way to see the corona is wait for 
the moon to play the role of a 
coronagraph and black out. 
The central disc of the sun and during 
the total eclipse indeed, people take 
beautiful pictures of the corona, you can 
find many of them online. 
We see here a few spicules, these redish 
the two grids are spicules in the 
chromosphere. 
I told you that the dynamics are not 
understood, but the red color is the 
region, the reason that part of the 
atmosphere is called the chromosphere. 
because of the high temperatures we 
absorb the corona ideally in ultraviolet 
or X Ray wave lengths where it competes 
with the sun and where it emits most of 
its light. 
The fact that the temperatures are 
millions of Kelvin allows many of the 
particles as we did in our, little helium 
calculation for Earth, the fastest 
particles in the corona where they're all 
ionized because the temperature is 
millions of degrees can in fact escape 
sun, achieve escape velocity, and stream 
away from the Sun. 
In what we call the stream of charged 
particles. 
which achieve escape velocity and move 
away from the sun is the solar wind. 
We've talked about it's effects, this is 
where it actually comes from. 
And the solar corona, as we can see in 
this movie, is an active and interesting 
place. 
the white disc here is the photosphere, 
the corona graph blocks more of that. 
We see 
coronal mass ejections and flares 
bursting out from the sun. 
We'll talk about the mechanism. 
And we, interestingly see, the detector, 
being attacked by streams of these 
charged particles that's that sort of 
staticy stuff. 
Those are the streams of charged 
particles hitting the detector. 
And this is an image from 2001, and I 
believe that if we wait long enough, or 
if you watch that image till the end, you 
will actually see something transit 
behind the sun. 
But, you can look at that at your own, 
leisure later.