Well, I've had fun playing in our own 
backyard, the solar system, last week. 
we learned a lot of things. 
We saw ways to apply the physics we've 
learned. 
But we want to broaden our horizons 
beyond that. 
And if we look back at Athens and that 
dark field back in November. 
we were looking at the moon and at 
Jupiter. 
But most of what we saw there was the 
stars with their various colors and 
brightnesses. 
And we're going to spend this week indeed 
taking the first step beyond the solar 
system and that's going to involve trying 
to understand the stars. 
I think it was [INAUDIBLE] who said that 
we're very limited as regard to the 
stars. 
We can look at them. 
But we can't touch them, and we can't 
smell them, and we can't taste them. 
And we can't do any experiments with 
them, so how are we ever going to figure 
out what's going on. 
And what stars are made of or how they 
work. 
And there are two things that I would 
add. 
Two places where comp was wrong. 
One is, we've learned as we'll see, to 
look very closely and in ways that he 
couldn't have imagined. 
And the other thing we can do is we can 
think about stars, we can bring to bear 
all the physics we've learned, and 
perhaps even some new physics. 
And that's what we're going to spend this 
week doing, try to under, use, all of 
those things to see what we can say about 
stars, and so here's the plan. 
Of course a natural place to start, just 
as we started our discussion of planets 
with Earth, is to start our discussion 
by, of stars by looking at the Sun, our 
local star. 
We have a lot more observations of the 
Sun than we do of other stars. 
It's a good place to develop an 
understanding. 
And as we'll see, that will take us to 
some new physics. 
And then, once we want to look beyond the 
sun at other stars, 
we need to start figuring out how to 
measure things about the stars. 
What do you, what do we want to know? 
We want to know their luminosities, we 
want to know their temperatures. 
We want to know their sizes and masses. 
So that we can parametrize our 
understanding of the physics by 
matching it to the observed parameters of 
the models. 
And then, at the end of the week, we will 
take all of those things that we will 
learn. 
You can measure for all these stars. 
And look at the statistical distribution 
of such things. 
And the information from that, or review 
how people use the information from that, 
to, refine a set of stellar models. 
And give us a pretty deep understanding 
of what goes on inside stars. 
That's it, we move on, we start by 
talking about our sun and immediately you 
bump into a problem. 
What's the most salient thing about our 
sun, the most salient thing about our sun 
is that it shines it produces these four 
times ten to the twenty-sixth watts of 
energy it does that we know because it's 
a large hot object at 5800 degrees with 
that surface area, it radiates that much. 
So far so good. 
It's radiating out energy, it's been 
doing it for four and half billion years 
and the question is, why is it not cold 
yet, why is it still like 5800 degrees on 
the surface. 
And one can start assessing the ways we 
know of producing energy. 
And the first, the way we usually produce 
energy, is various chemical reactions. 
the common factor between all of these is 
that all chemical reactions involve 
rearranging the electrons in an atom. 
Rearranging the electrons in an atom, is 
essentially farming the electromagnetic 
potential energy atoms. 
because of the size of an atom and the 
size of the charges this produces on the 
order of ten to the minus nineteen joules 
per atom. 
And if the atom in question is a hydrogen 
atom, which is what the sun is made of. 
And not saying I can propose a way to 
chemically burn hydrogen in the absence 
of oxygen. 
But imagine if there were some a bizarre 
chemistry that takes place at solar 
temperatures or something. 
You would produce this much energy, 
figured out that there are six times ten 
to the 23 hydrogen atoms in a gram, and 
you find that a kilo of hydrogen would 
produce about 6 * 10 to the 7 joules. 
60,000,000 joules is a lot of energy, but 
the sun produces 10 to the 26 joules 
every second so you'd need to burn about 
six times 10 to the 18 kilos of, hydrogen 
per second to, power the sun by chemical 
forces. 
You'd run out of energy in about 10,000 
years. 
it was this idea that led Lord Kelvin to 
the idea of Kelvin-Helmholtz heating. 
The process that we've been seeing is 
very important in astrophysics. 
this involves converting to heat the 
gravitational potential energy from the 
collapse of the solar nebula that formed 
the sun. 
We did an estimate of this in the 
homework. 
there will be solutions, at some point, 
if you didn't do it. 
And indeed you produced a lot more energy 
that way than you ever would by burning, 
hydrogen or any other chemical reaction. 
But the gravitational potential energy 
that the sun, liberated by condensing to 
form a star, would only last about 
10,000,000 years. 
This is great. 
The Sun has been doing this for four and 
a half billion years. 
And our first question is going to be a 
very important one. 
How does the sun shine? 
Where does the energy come from? 
That of course will take us to more of 
new physics, and, we'll take it from 
there.