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In the last video we had a large cloud of hydrogen atoms, eventually condensing into

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a high pressure, high mass, I guess you could say, ball of hydrogen atoms, and when the pressure

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when the pressure and the temperature got high enough, and so this is what we saw in the last video

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when the pressure and the temperature got high enough. we were able to get the hydrogen protons

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the hydrogen nucleuses close enough to each to other, or hydrgen nuclei

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close enough to each other for the strong force to take over and fusion to happen and release energy

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and then that energy begins to offset the actually gravitational force

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so that the whole star, what is now a star, does not collapse on itself

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and once we're there, we're now in the main sequence of a star

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What I want to do in this video is to take off from that starting point and think about what happens

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in the star next, so the main sequence, we have the core of the star

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So this is the star's core.

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and you have hydrogen fusing into helium.

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and it's releasing just a ton of energy.

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That energy is what keeps the core from imploding, it's kind-of the outward force to

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offset the gravitational force that wants to implode everything, that wants to crush everything

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So you have the core of a star

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like the sun, and that energy then heats up all the other gas on the outside of the core

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to create that really bright object that we see as a star, or in our case,

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in our Sun's case, the Sun!

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Now as this hydrogen is fusing into helium, you can imagine that more and more helium

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is forming in the core, so I'll do the helium as green, so more and more helium

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forms in the core, it will especially form

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the closer you get to the center, the higher the pressures will be and the faster this fusion

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this ignition, will happen. In fact the bigger the mass of the star, the more pressure,

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the faster the fusion occurs.

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And so you have this helium building up inside of the core as the hydrogen in the core gets fused.

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Now what's going to happen there? Helium is a more dense atom.

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It's packing more mass in a smaller space, so as more and more of this hydrogen here turns into helium

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what you're going to have is the core itself is going to shrink.

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so let me a draw a smaller core here, so the core itself is going to shrink

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and now it has a lot more Helium in it, now let's just stick to the extreme point

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where it's all helium, where it's depleted, but it's much denser

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That same amount of mass that was in this sphere is now in a denser sphere

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in a helium sphere, so it's going just as much attraction to it, gravitational attraction

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but things can get even closer to it, and we know that the closer you are to a mass

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the stronger the pull of gravity, so instead of having just the hydrogen

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fusion occurring at the core, you're now going to have hydrogen fusion in a shell around the core

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Hydrogen fusing in a shell around the core.

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And let me just be clear, this doesn't just happen all-of-a-sudden

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it is a gradual process, as we have more and more helium in the core

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the core gets denser and denser and denser

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and so the pressures become even larger and larger near the core because you are able to get closer

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to a more massive core since it is now more dense

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and as that pressure near the core increases even more and more the fusion reacion happens

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faster and faster and faster, until you get to this point

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so let me be clear, you have a helium core, all of the hydrogen in the core has been used up

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and then you have the hydrogen right outside the core under enormous pressure

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it's actually under more pressure than it was when it was just a pure hydrogen core

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because there's so much mass on the outside here trying to, I guess you could say

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exerting downward or gravitational force down, trying to get to that even denser helium core

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because everything is able to get closer in, and so now you have fusion occurring even faster

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even faster

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and it's occurring over a larger radius, so this faster fusion over a larger radius

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is then going to, the force is now going to expel...

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The energy that's released from this fusion is now going to expel these outer

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layers of the star even further.

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so the whole time, this gradual process as the hydrogen turns into helium

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or fuses into helium in the core the hydrogen right outside the core starts to burn faster and faster

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I shouldn't say burn, it starts to fuse faster and faster

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and over a larger and larger radius, so you might, the unintuitive thing is

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the fusion is happening faster over a larger radius and the reason that is because you have even a denser core

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that is causing even more gravitational pressure.

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and as that's happening the star is getting brighter

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and it's also, the fusion reactions, since they're happening in a more intense way

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and over a larger radius are able to expel the material of the star even larger

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so the radius of the star itself is getting bigger and bigger and bigger

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so this star looked like this if this star, maybe I'll draw it in white

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if this star looked like this, that's not white, this star looked like this

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oh what's happening to my color changer?

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there you go, ok that's

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this star looked like this, right over here

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now this star over here since the faster fusion reaction is happening over a larger radius

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is going to be far larger, and I'm not even drawing it to scale

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in the case of our sun, when it get's to this point it's going to be 100x the diameter

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and at this point it is a red giant

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and the reason that it's "redder"

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than this one over here, is that even though the fusion is happeneing more furiously

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that energy is being dissipated over a larger surface area, so the actual surface temperature of the

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red giant at this point is actually going to be cooler

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so it's going to emit light at a larger wavelength.

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a redder wavelength than this over here

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this thing, the core, was not burning as furiously as this thing over here

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but that energy was being dissipated over a smaller volume, so this had a

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higher surface temperature.

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This over here, the core is no longer burning, the core is now helium that is not burning

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it's getting denser and denser as the helium packs in on itself.

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but the hydrogen fusion over here is occurring more intensely,

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it's occurring in a hotter way but the surface here is less hot

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because it's just a larger surface area so the increased heat is more than mitigated

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by how large the star has become.

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now this is going to keep happening, and keep happening

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and the pressures keep intensifying because more helium is being produced

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and this core keeps collapsing

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and the temperature here keeps going up

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so we said that the first ignition, the first fusion occurs at around 10mK

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This thing will keep heating up until it gets to 100mK!

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and now I'm talking about a star that is about as massive as the sun

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some stars will never even be massive enough to condense it's core

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so that the temperature reaches 100mK but let's just talk about the case in which it does

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so eventually you'll get to a point, so we're still sitting in the red giant phase

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we're this huge star over here we've got this helium core

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that helium core keeps getting condensed and condensed, and then we have a shell of hydrogen

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that keeps fusing into helium around it, so this is our hydrogen shell

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hydrogen fusion is occurring in this yellow shell over here that's causing the radius of this star

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to get bigger and bigger, to expand, but when the temperature gets sufficiently hot.

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And now I think you're going to get a sense of how heavier and heavier elements form in the universe

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and all of the heavy elements you see around you, including the ones that are in you

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were formed this way from, initially, hydrogen.

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When it gets hot enough, at 100mK in this core because of such enormous pressures

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then the helium itself will start to fuse.

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so then we're going to have a core in here where the helium itself will start to fuse

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and now we're talking about a situation where we have helium and hydrogen

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and all sorts of combinations will form but in general the helium is mainly going to fuse

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into carbon and oxygen and it will form into other things and it becomes much more complicated

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and I don't want to get into all of the details.

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Let me just show you a Periodic Table

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I didn't have this when I somehow lost it

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Hydrogen here has one proton, it actually has no neutrons, it was getting fused in the main sequence

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into helium

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2 protons, 2 neutrons, we would need 4 of these to get one of those

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because this actually has an atomic mass of 4, if we're talking about helium-4

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And then the helium, once we get to 100mK can start being fused

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if you get roughly 3 of them, and you know there are all these other things

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coming and leaving the reactions

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you can get to a carbon, you get 4 of them

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you, 4 of them at least is the starting raw material and you get to an oxygen

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so we're starting to fuse heavier and heavier elements, so what happens here

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is this helium is fusing into carbon and oxygen so you start building a carbon

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and oxygen core, so I'm going to leave you there, I realize I'm already past

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my self imposed limit of 10 minutes

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What I want you to think about is what is likely to happen here?

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What is likely to happen here if this star will never have the mass to begin to fuse this carbon

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and oxygen

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if it does have the mass, if it is a super massive star, it will eventually

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will be able to raise even this carbon and oxygen core to 600mK

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and begin to fuse that into even heavier elements

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but lets think about what's going to happen for something like the sun where it will

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never have the mass, it will never have the pressure, to start to fuse carbon and oxygen

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And that will be the topic of the next video