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.