Becoming a Red Giant

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Becoming a Red Giant

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In the last video we had a large cloud of hydrogen atoms, eventually condensing into
a high pressure, high mass, I guess you could say, ball of hydrogen atoms, and when the pressure
when the pressure and the temperature got high enough, and so this is what we saw in the last video
when the pressure and the temperature got high enough. we were able to get the hydrogen protons
the hydrogen nucleuses close enough to each to other, or hydrgen nuclei
close enough to each other for the strong force to take over and fusion to happen and release energy
and then that energy begins to offset the actually gravitational force
so that the whole star, what is now a star, does not collapse on itself
and once we're there, we're now in the main sequence of a star
What I want to do in this video is to take off from that starting point and think about what happens
in the star next, so the main sequence, we have the core of the star
So this is the star's core.
and you have hydrogen fusing into helium.
and it's releasing just a ton of energy.
That energy is what keeps the core from imploding, it's kind-of the outward force to
offset the gravitational force that wants to implode everything, that wants to crush everything
So you have the core of a star
like the sun, and that energy then heats up all the other gas on the outside of the core
to create that really bright object that we see as a star, or in our case,
in our Sun's case, the Sun!
Now as this hydrogen is fusing into helium, you can imagine that more and more helium
is forming in the core, so I'll do the helium as green, so more and more helium
forms in the core, it will especially form
the closer you get to the center, the higher the pressures will be and the faster this fusion
this ignition, will happen. In fact the bigger the mass of the star, the more pressure,
the faster the fusion occurs.
And so you have this helium building up inside of the core as the hydrogen in the core gets fused.
Now what's going to happen there? Helium is a more dense atom.
It's packing more mass in a smaller space, so as more and more of this hydrogen here turns into helium
what you're going to have is the core itself is going to shrink.
so let me a draw a smaller core here, so the core itself is going to shrink
and now it has a lot more Helium in it, now let's just stick to the extreme point
where it's all helium, where it's depleted, but it's much denser
That same amount of mass that was in this sphere is now in a denser sphere
in a helium sphere, so it's going just as much attraction to it, gravitational attraction
but things can get even closer to it, and we know that the closer you are to a mass
the stronger the pull of gravity, so instead of having just the hydrogen
fusion occurring at the core, you're now going to have hydrogen fusion in a shell around the core
Hydrogen fusing in a shell around the core.
And let me just be clear, this doesn't just happen all-of-a-sudden
it is a gradual process, as we have more and more helium in the core
the core gets denser and denser and denser
and so the pressures become even larger and larger near the core because you are able to get closer
to a more massive core since it is now more dense
and as that pressure near the core increases even more and more the fusion reacion happens
faster and faster and faster, until you get to this point
so let me be clear, you have a helium core, all of the hydrogen in the core has been used up
and then you have the hydrogen right outside the core under enormous pressure
it's actually under more pressure than it was when it was just a pure hydrogen core
because there's so much mass on the outside here trying to, I guess you could say
exerting downward or gravitational force down, trying to get to that even denser helium core
because everything is able to get closer in, and so now you have fusion occurring even faster
even faster
and it's occurring over a larger radius, so this faster fusion over a larger radius
is then going to, the force is now going to expel...
The energy that's released from this fusion is now going to expel these outer
layers of the star even further.
so the whole time, this gradual process as the hydrogen turns into helium
or fuses into helium in the core the hydrogen right outside the core starts to burn faster and faster
I shouldn't say burn, it starts to fuse faster and faster
and over a larger and larger radius, so you might, the unintuitive thing is
the fusion is happening faster over a larger radius and the reason that is because you have even a denser core
that is causing even more gravitational pressure.
and as that's happening the star is getting brighter
and it's also, the fusion reactions, since they're happening in a more intense way
and over a larger radius are able to expel the material of the star even larger
so the radius of the star itself is getting bigger and bigger and bigger
so this star looked like this if this star, maybe I'll draw it in white
if this star looked like this, that's not white, this star looked like this
oh what's happening to my color changer?
there you go, ok that's
this star looked like this, right over here
now this star over here since the faster fusion reaction is happening over a larger radius
is going to be far larger, and I'm not even drawing it to scale
in the case of our sun, when it get's to this point it's going to be 100x the diameter
and at this point it is a red giant
and the reason that it's "redder"
than this one over here, is that even though the fusion is happeneing more furiously
that energy is being dissipated over a larger surface area, so the actual surface temperature of the
red giant at this point is actually going to be cooler
so it's going to emit light at a larger wavelength.
a redder wavelength than this over here
this thing, the core, was not burning as furiously as this thing over here
but that energy was being dissipated over a smaller volume, so this had a
higher surface temperature.
This over here, the core is no longer burning, the core is now helium that is not burning
it's getting denser and denser as the helium packs in on itself.
but the hydrogen fusion over here is occurring more intensely,
it's occurring in a hotter way but the surface here is less hot
because it's just a larger surface area so the increased heat is more than mitigated
by how large the star has become.
now this is going to keep happening, and keep happening
and the pressures keep intensifying because more helium is being produced
and this core keeps collapsing
and the temperature here keeps going up
so we said that the first ignition, the first fusion occurs at around 10mK
This thing will keep heating up until it gets to 100mK!
and now I'm talking about a star that is about as massive as the sun
some stars will never even be massive enough to condense it's core
so that the temperature reaches 100mK but let's just talk about the case in which it does
so eventually you'll get to a point, so we're still sitting in the red giant phase
we're this huge star over here we've got this helium core
that helium core keeps getting condensed and condensed, and then we have a shell of hydrogen
that keeps fusing into helium around it, so this is our hydrogen shell
hydrogen fusion is occurring in this yellow shell over here that's causing the radius of this star
to get bigger and bigger, to expand, but when the temperature gets sufficiently hot.
And now I think you're going to get a sense of how heavier and heavier elements form in the universe
and all of the heavy elements you see around you, including the ones that are in you
were formed this way from, initially, hydrogen.
When it gets hot enough, at 100mK in this core because of such enormous pressures
then the helium itself will start to fuse.
so then we're going to have a core in here where the helium itself will start to fuse
and now we're talking about a situation where we have helium and hydrogen
and all sorts of combinations will form but in general the helium is mainly going to fuse
into carbon and oxygen and it will form into other things and it becomes much more complicated
and I don't want to get into all of the details.
Let me just show you a Periodic Table
I didn't have this when I somehow lost it
Hydrogen here has one proton, it actually has no neutrons, it was getting fused in the main sequence
into helium
2 protons, 2 neutrons, we would need 4 of these to get one of those
because this actually has an atomic mass of 4, if we're talking about helium-4
And then the helium, once we get to 100mK can start being fused
if you get roughly 3 of them, and you know there are all these other things
coming and leaving the reactions
you can get to a carbon, you get 4 of them
you, 4 of them at least is the starting raw material and you get to an oxygen
so we're starting to fuse heavier and heavier elements, so what happens here
is this helium is fusing into carbon and oxygen so you start building a carbon
and oxygen core, so I'm going to leave you there, I realize I'm already past
my self imposed limit of 10 minutes
What I want you to think about is what is likely to happen here?
What is likely to happen here if this star will never have the mass to begin to fuse this carbon
and oxygen
if it does have the mass, if it is a super massive star, it will eventually
will be able to raise even this carbon and oxygen core to 600mK
and begin to fuse that into even heavier elements
but lets think about what's going to happen for something like the sun where it will
never have the mass, it will never have the pressure, to start to fuse carbon and oxygen
And that will be the topic of the next video