Have you ever wondered how all the chemical elements are made? Then join me
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.