The Cosmic Classroom - Evolution of Low Mass Stars


Uploaded by vmargoniner on 30.10.2009

Transcript:
Hi, there. Welcome back to the
Cosmic Classroom. We'll now talk about
the lifes of stars with low mass, and
then we'll give you, and then I'll make an another
video to talk about the life of stars with high mass.
Alright? So let's start with low mass.
One very important thing to understand before we start
talking about how stars change during their lifetime,
is to understand something that's called hydostratic,
Hydrostatic Equilibrium. Or we sometimes
call, call it,a Solar Thermostat.
Ok? It's a process that allows
the star to self regulate. So if you please
look at my first slide. What I'm showing here
is that, at every point inside the star
there has to be a balance between the
pressure due to... sometimes the pressure
is due to the heat, sometimes the pressure is due to
the degeneracy pressure. Whatever kind of
pressure, but there has to be a balance
between the pressure that wants to make the
star expand and gravity, that always wants to
make everything come together and everything
collapse into one thing. So at every point in
this star there's a balance between
pressure and gravity. Pressure stops
gravity, and gravity stops pressure and
it's happening everywhere in the star.
When this is happening we say that the star
is in equilibrium. Ok?
So having understood that let me start talking
to you about what happens as the star
reaches the main sequence. So the main sequence is
a phase on which the star is in Hydrostatic Equilibrium.
It's not, it's not collapsing or it's, or getting bigger.
And what's keeping this Hydrostatic Equilibrium is
the fact that there is fusion of Hydrogen into Helium
in the core of the star. There is Hydrogen all
around the star, everywhere. The star is made
mostly of Hydrogen. Fusion happens only in
the core, because that's where it's hot enough
for those atoms of Hydrogen to bounce
into each other to get close enough to each
other to be able to fuse together and form
a heavier element. So when this fusion
happens there is energy that's released
and this energy creates a thermal pressure that
then balances gravity. So the star will live
in the main sequence for most of its life.
Our sun is in the main sequence now,it has
been for the last four and a half billion years.
It will remain there for about another five billion years.
While the star has Hydrogen at the right temperature
to fuse into Helium, it will continue to dissolve and the
star will be pretty calm it won't be changing very much.
Now eventually this star will, will run
out of Hydrogen at the right temperature.
It's not that it will run out of Hydrogen, it has
a lot of Hydrogen around it to run how out of
Hydrogen in the core at the right temperature.
So then what you have is a core that has
Helium, all that Helium that was formed before.
Right? And more and more Helium
keeps being added to this core as, as there's fusion
of Hydrogen into Helium. The core as it becomes
more massive it actually becomes smaller and smaller
this is because the core is being held by the degeneracy
pressure, which has to be the topic of another video.
But just think about it as some kind of pressure.
Alright? So that the Helium core
is, is becoming smaller and smaller and smaller
and the Helium is accumulating in there.
Eventually the core collapses so much
that it brings together some Hydrogen that
was around that shell. That Hydrogen is
then hot enough to start fusing in a
shell around the core. So if you look at this
slide you'll see that in this case there is this,
inert Helium core, the Helium that is
there is not hot enough to start fusing yet.
And there is fusion of Hydrogen into helium
in this core, in this shell around the core.
And the prop the process continues
more and more Helium is continuously
added to the core. Until, you add enough Helium.
Alright? So the core becomes
smaller, denser,hotter and so then you have enough
Helium at the right temperature to start fusing.
Right? So the Helium, finally
when the core is very small the Helium starts
fusing into heavier elements. The Helium starts fusing
into Carbon, in the core. And while this is
happening there is Hydrogen fusing
into Helium in a shell around the core.
Well now you have two processes of a, that
two processes that are generating energy.
And this star is growing because there's so much
energy that's being generated. And one thing to
remember is that the Helium fusion doesn't
start slowly, it starts very fast because the
core is so compact that when the
temperature is achieved in the core it's achieved
everywhere in the core and the nuclear fusion of
Helium just skyrockets and becomes really, really high.
So the star expands enormously it becomes
more luminous and it becomes bigger.
As you can see here the star is pretty big, right?
So, with time the core because all the,because
so much energy. Alright.
So because so much energy was generated in
this core of Helium, the core finally starts to expand.
There's so much heat that the core finally starts to expand.
As the core does that, starts to expand the
rate of fusion decreases. Alright?
The rate of fusion decreases and the star
finds equilibrium again. It finds an equilibrium, in
which it's fusing high, helium in the core
and fusing Hydrogen in a shell around it.
So you see that as the core expands, the fusion
rate decreases, the star shrinks because there
isn't so much thermal pressure and the star
achieves equilibrium again. They technically won't last
very long, but while there is fusion happening in
the core it is, there is this equilibrium.
So slowly what happens is that we viewed a
Carbon core, a core that contains only Carbon.
And in low mass stars that temperature
never increases enough for Carbon to fuse
into heavier things. So that's already getting
to the end of the life of the star because that
core won't be able to fuse anything else.
So for a while you have a double shell burning.
You'll have a core of, of Carbon that nothing
has happened, a shell around that has
Helium fusing into Carbon and another core that
has, and another shell, excuse me, that has
Hydrogen fusing into Helium. This double shell stage
never reaches equilibrium. Ok?
So the star starts to pulsate sometimes it
creates too much fusion and it then becomes
very hot and it expands. And then because it,
cools down gravity wins and then comes back again.
So it keeps pulsating and pulsating until it
finally ejects and, until it finally starts to eject
its outer layers. So it gets rid of its outer layers.
So this picture that you see here is from a star
that has just died, it the areas that you see around
here are shells that have been ejected from the
star as it was pulsating. And what you see right
here in the center is what's left from that star.
It's the very core of the star that's why it's,why
it's very hot, it's the very core of the star.
Star lost everything that was around and what's left
is this core, this core is called a White Dwarf.
It's the, it's the, it's very dense and very hot.
With time the White Dwarf will cool down and
will become a Black Dwarf and you won't be
able to see it anymore. And with time the, this
nebulosity around the star would just move
away and continue moving away until it
becomes so thin that you cannot see it anymore.
So you can only see it for a while.
And that's the end. For a little mass
star, like our sun. So that's it for that.
Then let's talk about high mass stars next time.