Becoming a Red Giant Becoming a Red Giant
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
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At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
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