as we are lifting all the star dust secrets to understand the cosmic origin of the

chemical elements. We just talked about fusion processes and how elements are

made in stars, mostly for energy generation purposes.

Now let's look at how that actually manifests in a star as a whole because

as astronomers, we can observe stars but we can't really look inside of stars.

We can only see the surface.

There are a number of ways we can get clues from the surface of the star as to

what's going on in the core. Of all that, a really nice example of how

nuclear physics and astrophysics -- nuclear and astrophysics -- come together because

the nuclear physics governs what's happening inside the core, and then the

astrophysics provides what we can actually observe and both kind of need

to come together and work out. So over the last several decades, a lot of

progress has been made to put these two together and to understand what is now

called stellar evolution. That is actually governed by the nuclear physics

processes, specifically fusion, in the core. I wanted to share this with you because

it's very insightful. I'm going to draw what we call a Hertzsprung-Russell-

Diagram. It basically shows how a star evolves during its lifetime. We can

use the Sun as an example, and what we're going to have is hot stars here

and cool stars here, and then we have more luminous stuff up here and

less luminous stuff down here. And there is a certain track that looks like this

-- half of a Christmas tree -- and the Sun actually sits right now about here. One

can put any star in this diagram. You will see in a moment how we can

then learn about the evolutionary phase of the star and hence what's going on

inside of the core. The Sun is sitting here and we know on this

branch here, which we call the main sequence, that south burn hydrogen to

helium. How does this look? If we draw a star here, in the core

hydrogen is burned into helium, just like what we had in the previous section. The

star, when it moves on, and I should say that every star will kind

of start somewhere along the main sequence here, will stay there for

about 90% of its lifetime which means, coming back to the old stars for a

second, -- 90% of 15 billion years is about the age of the universe -- which means the

stars that started here when they were born in the early universe, they are just at the end of

this hydrogen to helium process which really means they haven't done anything

else but burning hydrogen to helium which really is the key to why these

stars don't show their age. They are just like what they've always been and we can

observe them today and infer things about the early universe from them today

because they haven't changed. That's the key here.

But if we look at the star that has a shorter lifetime and wants to

evolve, it will move up here, and it will move up here when the core -- let's see, this

was hydrogen here, and it has been converted to helium -- when we indeed have

just helium in the core, and there's no hydrogen left in the core. Then the star

will get a little bit rumbly and so it's going to start moving along here

and all sorts of things going on in the call because the thermostat is out,

there is no energy being produced right now, and so it what happens is the

star actually inflates to counter act that and it will move up here to become

very luminous and up here, we have the red giants.

They're called red giants because they're much bigger and more luminous

but they also cooler because they are bigger and so they are turn red and so

they have just a helium core, and what's happening is in an outer shell here they

is still hydrogen to helium burning going on in the shell.

And that provides a little bit of substitute energy, a little interim inner energy, to

the star as a moves up here. Then up here, we have something called the helium flash

which means the helium here in the inner core is now being converted to

carbon. How can I draw this? I'll make this go away...

So we eventually, helium gets converted to carbon, so eventually

we're going to get to a carbon core. And then we have helium burning further out

and hydrogen burning yet further out. When the helium burning

starts here, by the time it reaches here, it has this carbon core, so here it

reaches a helium core, this region. Then helium starts to burn and then

by the time you come here, we have the carbon core and then it moves up here

and this last part here it really depends on the mass of the star. The Sun

is actually not going to do much, it's going to just stick it out with a carbon-

oxygen core, here, and then turn into white dwarf and just cool down. so the

Sun actually a pretty boring star that has a pretty boring fate. If we

make the Sun much more massive, let's say 10 x more massive, it would move up

here in this carbon burning phase and a variety of later burning stages that

lead to iron. And then it would have an iron core up here

and you already know what's going to happen if a star has an iron core -- it

has lost its energy source and it will explode as a supernova. So before it

explodes, what's it going to look like? We have a whole bunch of these so called

shells, sometimes they refer to as onion shells. In the center, we have iron and

then they're silicon and all the other elements following out here, oxygen, carbon,

helium, let's drawn another one, hydrogen. There are a few more other elements being

produced in in minority processes so some of these shells are not pure in

these elements but that's kind of the basic idea (that is oxygen,

silicon, sulfur and others...) that you have a star that looks like that. So what

you see here is that whatever is happening in the core has a direct

impact of where the object sits on this diagram here. By measuring the

luminosity of the star as well as its temperature, we can place it on this

diagram and then learn in which evolutionary state the stars is currently in

which tells us what is going on in its core.