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.