So we left our star 12.233 billion years old and an interesting situation the core
collapsed. Had stopped because the core had become
degenerate. the outside of the star was still
ballooning out. It was becoming a red giant.
And they said at 12.233 this growth would stop.
What is it that stops it? Well, what's happening is, we still have
hydrogen fusion in the shell. More and more helium being deposited onto
this degenerate core. The degenerate core is becoming hotter
and hotter. And eventually it's temperature hits 100
million Kelvin. When the temperature reaches 100 million
Kelvin something qualitatively new happens
at 100,000,000 kelvin. Two alpha particles can overcome their
electrostatic repulsion and you can get fusion of two alpha particles to form
beryllium. In fact once that happens, pretty rapidly
beryllium fuses with another alpha particle to form carbon twelve.
So this is called the triple alpha process.
This is the fusion of helium that produced predominantly the very stable
carbon twelve nucleus and. Because the helium in the core was
degenerate, this is a very different ignition of fusion that what we've been
used to because as fusion begins. The core starts to heat up, but as it
heats up it does not expand. Remember degeneracy pressure is
temperature independent. The core maintains degenerate density
until it gets hot enough the thermal pressure overtakes degeneracy.
During this time, fusion because it doesn't immediately expand, the fusion
and the heating associated with it, spreads explosively throughout the core.
And for a few seconds, this core produces illuminosities of 100,000 solar
illuminosities. Not for very long.
But there's a huge outcore of energy very dramatic helium flash is what it's
called. this doesn't make it to the outside.
You don't see the star suddenly flare up there is too much star between the core
and the outside the stellar envelope happily absorbs this energy it expands.
What really happens is that this, extra output of energy from the core expands
the hydrogen fusing shell, and so, in fact, the star's luminosity is going to
decrease. Momentarily, when the great pulse is
coming out, the atmosphere might balloon out and, in fact, some of it might get
blown away. You might get a second mass loss, but
expanding the hydrogen fusing shell, which is where most of the star's energy
is coming from, actually decreases the luminosity, in response to which.
A response to ignition of helium infusion is that the star in fact contracts and as
it contracts it heats up so The structure that we're seeing as the
star stops growing bigger and cooler when it hits the helium flash, and, in fact,
it starts heating and contracting. So you see the beginning of the next
phase of the sun's trajectory. And what happens is that.
As the, the envelope. Contracts and heats the deep convection
zone that was, I acted.
Because of the opacity the, the outside of the envelope ceases that rises up, we
get shallow convection. We get convection in the core where now
helium is fusing producing carbon and oxygen.
So we have a helium burning core surrounded by a shell where hydrogen is
fusing. And in fact they're separated by layer
not depicted here of inert helium that's been fused out of the hydrogen, but is
not yet compacted or heated enough to produce to, to be .
Eligible for fusing so we still have a hydrogen fusing shell surrounding a
helium fusing core this a. version of the sun after the envelope has
contracted and the star continues to heat at a pretty much constant luminosity is
what's called a horizontal branch star and notice that the, the horizontal
branch lasts all of a million years it's like the main sequence.
Except for helium burning and much, much faster.
the sun would have contracted to all of ten solar radii.
Its luminosity is down to 41 times its current value.
There is the solar system with mercury long gone, but the sun contracted.
Notice that the organs of the planets have expanded slightly due to the 30%
mass loss in the previous expansion. And when we plotted on the HR diagram,
this is what we're seeing. At the helium flash, the sun begins to
contract and heat up. And this horizontal branch is the analog
for helium burners of the mean sequence there's more variation to start contracts
and heats at almost constant luminosity and the whole process lasts about a
million years. What happens after a million years?
Well, you can imagine what's going on. Over these million years helium has been
very rapidly fusing in the core. The core is becoming now depleted in
helium. We actually have a core that is inert
carbon and oxygen. And, around that, we will acquire a
helium fusing shell surrounded by inert helium, surrounded by a hydrogen fusing
shell. We have this, onion version of a star.
Eventually the inert CO core can no longer support the outside of the star,
and that collapses to degeneracy. We now have,
It collapsing inner core again we have helium fusion in the shell accelerated by
the collapse. This increases the luminosity of the star
that expands again and cools the envelope.
The motion to the left when the star was heating and, contracting is reversed.
The star now starts to move over to the right to lower temperatures and up to
higher luminosities. Again the envelope cools.
We get convective envelope. And, because of the high temperature
dependence of the triple alpha process, we also have convection in the interior
of the star. In big stars, these two regions hook up,
and we get a second dredge-up where carbon and oxygen, the products of helium
fusion, are dredged up. Of the surface of the star we can see on
the absorption lines, this is what happened.
At A mill- 12.365 so a million years have
gone by, my times are not going to shift. Everything now starts happening so fast
that we won't be able to keep track of it.
The sun now balloons even larger than it was and even cooler, and luminosity is
even larger. Notice, if you will, that this diagram,
aside from omitting the inert helium shell in between the hydrogen fusing
shell and the helium fusing shell, is also greatly exaggerating the size of the
core. the entire fusing region, including the
hydrogen fusing shell, is probably about a tenth of a percent of the size of the
star. And so the radius of the star.
So this is a very tiny core on the inside of this hugely bloated envelope.
And because it's so hugely bloated the gas on the outside of the atmosphere is
very. Weakly gravitationally bound.
And, so again, every time the star gets large it loses the top level of its
atmosphere. There's a lot of mass lost in this early
asymptotic giant branch. Here's the picture of the solar system
when the sun has gotten this big. And on the H-R Diagram, what has
happened, as we said, is the leftward motion has been reversed.
We're now moving to the right. And we're on what is called the
asymptotic giant branch because it asymptotes to that old Hioshi Track where
temperature's controlled by negatively ionized hydrogen.
And as the star cools and expands. what's going to happen is that the
hydrogen fusing shell which was temporarily inactive begins to be active
it deposits more and more helium so that we have this structure now that is a
complete onion I don't know if I can draw it but imagine somewhere out there is the
star's envelope and inside it is. A shell where hydrogen is fusing and
inside that is a shell of inert helium that's not doing anything and inside that
is a shell whoops that was helium inside that is a shell where helium is fusing.
And inside all of that is the inert carbon oxygen
core and what's going on is because Hydrogen is fusing.
it is constantly depositing Helium into this inert Helium shell.
And Helium is making deposits into the carbon oxygen.
The entire thing because a number of particles is still decreasing is
crunching down. And the density and temperature of this
inert Helium rise. And over here the boundary.
Between fusing helium and inert helium we have large concentrations of helium which
suddenly become degenerate and hot enough and so there are flashes in the helium
burning shell where the helium burning shell suddenly becomes
fusing and produces a large amount of, large spike in helium luminosity.
What does this do? You got the drill by now.
The extra spike in helium luminosity expands the hydrogen shelf that decreases
the output of hydrogen fusion. Decreasing the output in hydrogen fusion
will call, cause the envelope to contract and to heat.
When this happens, eventually the contraction will reheat the hydrogen.
Hydrogen fusion rates will go back up, the helium rate will have settled down,
the new increased luminosity will cause the atmosphere to balloon back up.
And this happens with a period of like 100,000 years, one, two, three, four,
five times. And of course each time the star balloons
out, it loses a lot of its mass to this now stellar super wind is what it's
called, its luminosity reaches a maximum of 5,000.
Its radius is as large as it's going to get, 213 times the current radius of the
sun out to today's Earth orbit. Again when the atmosphere cools we get
these deep convection. In the atmosphere, we get convection in
between the shells, and, in a star slightly more massive than the sun, say,
two solar masses and above, we find what we call carbon stars, where, as I said,
the carbon, the, and other products of fusion, are being brought up to the
surface and ejected, and we actually see the star ejecting as they get away what
coalesces as actual soot. So we see, carbon solids in the stellar
wind, an example of, Asyntotic giant bran star, is Mira notice
gets harder and harder, to find stars in these existant stages, because these
stages last, less and less. that is why most stars are on the main
sequence, because any star spends most of its time, on the main sequence, mass also
in a carbon star, by the way, can be on the order of, ten thousandth of a solar
mass per year, so in 10,000 years, a star can lose, a complete solar mass,
obviously these are slightly more massive stars and The thermal pulse, asymptotic
giant phase lasts only about half a million years and so
The star does not completely evaporate, this is what the solar system will look
like. And what we are seeing in terms of the
evolutionary track is this part, the latter part of the asymptotic giant
phase. And then we see something really bizarre,
we see the star appearing to heat up at constant luminosity.
What is going on there. Well.
There is no more fusion. Carbon will not fuse in the sun.
There's not another phase of fusion that would cause us to move to the left like
hydrogen did initially and helium later. What's really going on is that with these
successive pulses, the star is losing its envelope.
And we are seeing what we call the star. Is deeper and deeper until eventually
what we're looking at is the bare carbon oxygen degenerate core.
And, of course, that makes it look like we're seeing things that are hotter and
hotter. So, the motion here is a little bit
sporadic and not regular. But this, what makes this
motion work to higher temperature is basically peering deeper and deeper into
the star. the last observation about the ABG phase
is that during this asintonic giant phase,
We have other, sort of processes that are enough neutrons floating around the core
of the star that, there are neutron capture processes that allow the star to
synthesize elements heavier than, carbon or oxygen, so we get, things, like
technetium being formed in stars with only slightly more than solar mass
because of this sort of slow process of neutron capture.
We'll see what happens at this very end in our last, clip about the sun's
torturous history.