Okay. 
We figured out when it happened. 
Now what? 
we're going to go back four and a half 
billion years ago. 
The atoms that currently comprise the 
solar system are already around in this 
part of the cosmos. 
How those atoms were created, where they 
came from, we'll think later. 
The question now is, in what form were 
they likely to be found? 
And, over the past few decades we have a 
pretty good idea of what that form is. 
the material that currently forms the 
solar system was probably found in a 
large molecular cloud. 
What's a molecular cloud? 
Oh, it's a great big collection of, as we 
know, mostly hydrogen with some 
contamination, traces of other elements. 
it's called molecular if it's cool enough 
that hydrogen atoms actually form stable 
bonds and we have hydrogen molecules in 
it. 
And inside this molecular, wait a minute. 
This is already a problem. 
And it's a problem that I want to pay 
attention to, because this is a problem 
that will haunt us through the rest of 
the class. 
The problem is gravity. 
And the problem is that the idea of a 
large cloud of hydrogen floating around 
in space, on the face of it, makes no 
sense. 
Because if you're an atom in the center 
of a cloud of hydrogen, then of course 
you're attracted to all other atoms in 
the cloud. 
And the net attraction on you if the 
cloud is reasonably symmetric might be 
zero. 
Fair enough. 
But if you're an atom near the edge of 
the cloud then clearly the net effect of 
gravity is to attract you towards the 
center. 
Gravity applies a pressure, a contracting 
pressure to every object. 
And whenever we see something in the 
universe that is not collapsing and 
crunching down to a point, something must 
be holding it up. 
What holds up a molecular cloud? 
Well, it turns out that a molecular cloud 
is held up simply by thermal pressures. 
This molecule near the edge of the cloud 
may indeed be accelerated towards the 
center, but on its way there, it 
encounters other molecules which have 
random thermal velocities, it scatters 
off of them, and eventually never gets 
anywhere. 
The cloud, the general shape of the cloud 
can sustain itself, and we can do a 
really nice order of magnitude 
calculation to understand what the 
conditions in a cloud must be in order 
for it to sustain itself. 
And the way we think about it is that if 
we have a cloud, then we know that the 
average kinetic energy if a molecule say, 
so m would be the mass of a hydrogen 
molecule and the cloud is on the order, 
there may be factors of two or three that 
I'm skipping, of k Boltzmann times the 
temperature. 
And on the other hand, the typical 
gravitational velocity associated to this 
cloud can be estimated from the escape 
velocity, if a molecule is moving with 
the escape velocity then it's not really 
bound to the cloud it can escape. 
And the escape velocity we computed is 
again on the order of newton's constant 
time the mass of the cloud. 
Divided by, the year distance from the 
effective center. 
So R might be. 
The radius of the cloud would be a 
typical number to enter. 
And now if this velocity and this 
velocity are of similar orders of 
magnitude then one can imagine that the 
thermal fluctuations in the cloud are 
about the same order as the gravitational 
pressure. 
This will tell us that the cloud can 
sustain itself under pressure. 
This is how molecular clouds survive. 
The denser a cloud, the higher its 
temperature must be in order to prevent 
the gas from collapsing. 
So now we understand that you can have a 
molecular cloud sitting there. 
There is still a problem with gravity and 
this is a problem that is unique to 
gravity in contrast to other forces. 
And the problem that occurs with gravity 
is that imagine that through some reason 
and we'll talk about it later, this 
region of the cloud becomes denser. 
If this region becomes a little bit 
denser than other regions, then of course 
the net gravitational attraction 
everywhere is slightly more directed 
toward this region of the cloud. 
If the net fluctuation is large enough 
compared to the temperature, that in fact 
some significant infall occurs, well the 
net, then more mass will fall onto this 
region. 
This region will become denser still. 
Becoming denser still, the collapse will 
become more significant. 
In other words, once you have a 
fluctuation or some reason, an additional 
den- extra dense region in a cloud. 
Because unlike, say, electromagnetism, 
where if you charge some region with 
positive charge, it'll attract negative 
charge and neutralize itself. 
And enhanced mass density attracts mass, 
which increases the enhanced mass 
density. 
Gravitational collapse is a runaway 
process. 
Even if the cloud is holding itself under 
thermal pressure, the situation is 
unstable. 
If you make a sufficiently large 
perturbation, the cloud will collapse. 
And this is what happened to our 
molecular cloud that led to the formation 
of the solar system. 
So what caused one region of the cloud to 
be dense? 
Well, we think it was the shock wave 
created by the explosion of a supernova. 
In fact, probably more than one nearby 
and we'll see how that might have 
happened, again, later in the class. 
This causes the great big cloud to break 
up into fragments and triggers the 
collapse of these fragments. 
The fragment that created our solar 
system was probably on the order of 
magnitude of 20, 2 to 20,000 astronomical 
units in size abd with a mass of about 
3,000 solar masses. 
this did not create the solar system, in 
fact, it created a cluster between 1 and 
10,000 stars. 
So, the condition that we see in the 
solar system suggest that the sun was 
formed as part of an open cluster like 
the Pleiades. 
It's now not bound gravitationally to 
nearby stars and so it's not part of a 
cluster. 
This cluster has dispersed and in fact, 
this is the, this leaves a reasonably 
recent insight that the sun forms as part 
of a cluster and the search is on in the 
neighboring part of the Milky Way to find 
the stars that were once members of our 
cluster. 
I do not know that there has been success 
so far. 
And again the shock wave triggers. 
Remember, it's a sound wave. 
There's a region of high pressure in the 
cloud. 
That region becomes sufficiently dense 
that gravity overcomes thermal pressure. 
This is called the Jeans Instability. 
It's the fact that if you have a 
fluctuation, it will grow, and as we saw, 
If you have a fluctuation whos mass 
relative to the is related to the 
temperature by this equation where big M 
is the mass of the cloud say, or the 
region that's collapsing and little M the 
mass of the constituent particles. 
If you satisfy this condition, 
then the region will collapse. 
And this is the Jeans condition. 
And any object that is not collapsing 
that's made of gas, will have to obey 
stability condition against this 
collapse. 
And larger objects, the condition might 
change, but everything in the universe 
that is not collapsing is held up against 
gravity by something. 
That's the light motif that I want you to 
take away from this. 
So. 
we have this, large molecular cloud and 
it initiates a collapse. 
In this simulation, 
what we're going to see is how the 
collapse of a molecular cloud looks. 
On the left, we see the density. 
On the right, the temperature. 
There's some region of increased density. 
The cloud is collapsing towards the 
region of increased density. 
localized regions of high temperature are 
forming. 
The timescale here is tens or hundreds of 
millions of years. 
We see the beginning of stars forming. 
We see these explosives, explosive 
supernovae, and we see that after a 
supernova, there's local, 
flux of sudden star formation. 
Look at that big super nova and then the 
sudden concentration of star formation 
near it. 
So this is presumably what happened to 
us. 
The sun was formed as one of these 
clusters that were generated or triggered 
by an explosion of a nearby supernova. 
We'll get back to the star part of it. 
But first, we have our cloud, it's about 
200, 2000 astronomical units to aside, 
and it starts to collapse. 
But now, let us focus our attention on 
the region immediately near us in the 
cloud, immediately near where we are now. 
So somewhere there, we will zoom in, and 
look at a region of maybe 200 
astronomical units or 1,000 astronomical 
units around where we are now, that 
region begins to collapse under the 
weight of it's own gravity because the 
fluctuation makes it too dense for 
temperature to sustain it. 
What happens when a cloud starts to 
collapse? 
Well, first thing we understand is that 
as the size of the cloud gets smaller, 
any random rotation that the thermal 
motion, or whatever, might have included 
is going to get sped up by the 
conservation of angular momentum as this 
demo brilliantly shows. 
So what the demo showed us, is that 
whatever random rotation was part of the 
thermal motion of this huge 20,000 
astronomical unit sized cloud. 
as the cloud collapses, is accelerated. 
And what we find is that the whole cloud 
is spinning rapidly. 
Now it's not spinning rigidly, because 
it's not a piece of plastic hooked up. 
These are independent particles of gas. 
But the net result is the same. 
The rotation is enhanced. This is the 
rotation that makes the planet's orbit, 
the sun in the same direction as the sun 
rotates there was an overall rotation 
present and it was enhanced by collapse. 
This is another important result which is 
that objects that are collapsing as they 
come closer to the center and that such a 
direction that they're coming closer to 
the axis of rotation by the conservation 
of angular momentum, NVR being conserved 
means that as R shrinks their velocity, 
their speed has to increase, these 
objects have to move faster. 
This creates a barrier that prevents the 
cloud from collapsing into its axis. 
On the other hand, collapse in the 
direction along the axis, the other 
direction is uninhibited by this. 
And so, the cloud collapses far more 
readily along the axis than perpendicular 
to the axis. 
The net result is, that what was once 
probably a roughly spherical cloud is 
flattened into a pancake shape. 
This is the reason that we find the 
planets all orbiting the Sun in roughly a 
plane. The plane of the ecliptic is the 
plane perpendicular to the original axis 
that was randomly the axis of rotation, 
the neck rotation of a cloud. 
And then within about 100,000 years of 
the initial super nova trigger, the cloud 
is contracted to a radius of about 200 
astronomical units. 
And it's now flattened into what we 
called a proto planetary disc. 
A cloud of dust and gas 
Here is a nice infrared image of such a 
protoplanetary disk in the Orion nebula. 
And here's a sequence of nice Hubble 
images that show us how this takes place. 
So here we see an image of Orion's dagger 
with the Orion nebula in the center. 
And as I click, we will go through 
increasing magnifications. 
we magnify. 
Here's a magnified view of the nebula. 
In the center of the nebula we find all 
these dust clouds. 
And in them we find these cocoons in 
which stars are formed. 
And upon magnification the infrared image 
shows us very nicely the nascent star in 
the center surrounded by a protoplanetary 
disk. 
It's happening there. 
physics is universal, this is the way it 
happened here. 
In addition to spinning up, another 
process that begins to take it place as 
the solar nebula collapses, is a process 
called Kelvin-Helmholtz heating. 
This involves the fact that some of the 
gravitational potential energy, 
liberating by all this stuff falling in 
towards the center is indeed converted 
into rotational kinetic energy. 
But most of it is distributed by 
collisions into heat and into the random 
motion of the components around them. 
So as an object contracts under the form 
of gravity, it's gravitational potential 
energy's being converted into heat, and 
the temperature therefore rises. 
this sets up a profile, a distribution 
around the disc where of course the 
density is largest at the center of the 
disk and tapers off to the edges. 
And likewise the temperature, the most 
compression has taken place in the 
center, so the temperature near the 
center of the disk eventually reaches 
2,000 Kelvin. 
And when it does, we'll see that a 
surface, next week we'll see that a 
surface with a temperature of about 2,000 
kelvin sort of stabilizes the material 
inside it is the protosun. 
Most of that will eventually be the Sun, 
what's left outside is the remnants that 
will become the rest of the solar system. 
And so in the center the temperatures 
very high and the densities are rising 
and then both density and pressure 
decrease as you move outside. 
And so you have this disk with hydrogen, 
helium and some of 'em, some other 
materials. 
in this region where both temperature and 
density. 
Decrease with R with distance from the 
center, and this will have some serious 
impact on the state in which we might 
find different components of the solar 
nebula. 
Farther out we have essentially very 
small density, very small pressure, and 
very cold temperatures. 
Near the middle it is hot and relatively 
denser. 
Let's see 
interesting demonstration, of how this 
effects the situation. 
One of the important ingredients in the 
solar nebula is water. 
Hydrogen and oxygen find to form water. 
And remembering that it's despite, the 
increased density from collapse. 
Under very low pressure compared to the 
atmospheric pressure we feel on earth. 
We need to understand how water behaves 
in space and so what we've done here is 
we've taken some water colored blue so we 
can see it and place it in a vacuum 
chamber and essentially we're simulating 
the conditions in space. 
The temperature is room temperature and 
the water is boiling. 
We know, anybody that cooks knows, that 
as you go to higher altitude, it's lower 
pressure. 
Water boils at a lower temperature. 
Here the water is boiling at room 
temperature. 
Boiling water is converting liquid to 
water vapor. 
That requires an input of latent heat. 
When we boil water on the stove, that 
heat comes from the stove. 
Here, as when our sweat evaporates from 
our bodies, it's coming from the water 
itself and that's cooling the water down. 
We see that the boiling is slowing down, 
because, as more water evaporates, the 
water is in fact, becoming quite cold. 
And when it becomes cold enough, what we 
observe is that the boiling completely 
ceases, the water has frozen. 
what we've produced, when we take it out, 
is an ice pack. 
Essentially what we're learning, is that, 
at the low pressures obtaining in space, 
water can exist as ice, or as water vapor 
but not as liquid water. 
The answer to the question is there water 
on Mars or on the moon or in space in a 
silly sense is no. 
You can have ice and you can have water 
vapor, you can not have liquid water at 
the low pressures obtaining in space. 
This is, something we're familiar with 
for example carbon dioxide, exists on 
earth as vapor, and is solid, but not as 
a liquid. 
It's true for any substance that's 
sufficiently low pressure. 
And if the pressure's in space, it's true 
for all substances. 
Cannot resist the temptation to 
demonstrate that with nitrogen, a major 
component of air, so here we have liquid 
nitrogen. 
It is boiling because the boiling point 
of nitrogen is negative 100 degrees 
centigrade. 
We put it in the vacuum chamber. 
We evacuate. 
And again, we see that the nitrogen boils 
even more vigorously and as it boils even 
more vigorously at the low pressures, 
it's cooling itself down and like the 
water, eventually, nitrogen freezes. 
The freezing point is only a few degrees 
lower than the boiling point, but what we 
have there is solid nitrogen. 
Where we see in this simulation, 
is the impact that the temperature 
gradient through the nebula, will have on 
the state of various materials or various 
of the metals that are present in the 
nebula. 
We see that, as we said, the temperature 
near the center near the, the outside of 
the proto sun, reaches close to 2,000 
degrees. 
On the other hand, as one moves further 
out, the temperature decreases. 
And what we see is that as the 
temperature decreases some materials over 
here on the right, transition from gas to 
solid, notice there are no liquids, many 
oxides and metallic iron for example are 
solid at temperatures below 1300 degrees 
and so that is the temperature that 
obtains near the current position of 
mercury. 
How farther out where Earth is located. 
other minerals 
made of, iron and, sulfur, for example, 
are, solids, but water is still a gas. 
Water becomes a solid at a temperature of 
175 degrees K, and the line at which 
water, a much more plentiful material 
than, say, iron or nickel, because 
remember that, graph of abundance is, was 
declining on a logarithmic scale. 
Water, made up of hydrogen and oxygen, is 
relatively light elements. 
They're relatively plentiful. 
water becomes solid just inside Jupiter's 
orbit. 
Perhaps not a coincidence, and then 
farther out, ammonia and methane, become 
solid somewhere between Saturn's orbit 
and Uranus' orbit, and argon and neon 
noble gases are solid farther out. 
What we see is that as you go farther out 
into the nebula, there are more solids, 
and this will have an important impact 
on, planet formation. 
What we've seen is that, the fact that 
temperature and density depend on 
distance out in the nebula, will mean 
that the form in which various 
materials or constituents of the nebula 
are likely to be found will change closer 
to the center we only find gases. 
As you move farther and farther out, more 
and more of the materials are going to be 
solids and we saw that in the center 
we're forming this protosun. 
The protosun will eventually become the 
sun the region enclosed within this 
boundary of temperature 2000 Kelvin. 
Most of that will turn into the sun and 
that in addition to providing us life 
further on, 
such as ticking time clock for processes 
going on in this disc, full of, gas and 
in the outer reaches will be called dust, 
microscopic particles of solids. 
the ticking clock is, that within about 
ten million years, the sun will ignite as 
a star. 
When that happens, the stellar wind, the 
solar wind that we discussed undergoes a 
very intense period where, for a few 
million years, the solar wind is 
extremely intense, a few tens of millions 
of years. 
This is called the T-Tauri winds. 
Star in this stage are called T-Tauri 
stars. 
The sun was once a T-Tauri star. 
And, the stellar wind, the pressure of 
this stream of charge particles is so 
large that any remaining gas and dust is 
completely blown away from the system. 
And so, that gas and dust is to form 
planets and whatever else it wants to 
form. 
The timing is 10,000,000 years. 
What hasn't formed in 10,000,000 years is 
going to be blown away. 
Gas and dust cannot survive the solar 
wind although more heavier more robust 
objects are able to survive. 
Here's a nice demonstration. 
Of how things look 
from the point of view of producing the 
star, what we are going to do again is 
right density here, density is common 
density, so it is density throughout the 
depth of the nebula. 
We're seeing a two dimensional image and 
as time increases we see both density and 
temperature increase. 
And we see in the middle the formation of 
the protosun the red line indicates a 
temperature of 2000 degrees. 
This is where the protosun move forward 
and it's still surrounded by the proto, 
planetary disc where soon our planets and 
us will come into being. 
We'll start that process in the next 
clip.