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Current time:0:00Total duration:11:30

Video transcript

in the last video we had a large cloud of hydrogen atoms eventually condensing into a high pressure high mass I guess we could say ball of hydrogen atoms and when the pressure and and when the pressure and the temperature got high enough and so this is what we saw in the last video when the pressure and temperature got high enough we were able to get the hydrogen protons the hydrogen nucleus is close enough to each other or hydrogen nuclei close enough to each other for the strong force to take over in fusion to happen and release energy and then that energy begins to off begins to offset the actual gravitational force so the whole star what's 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 in the main sequence we have the core of the star so this is the star's core and you have you have hydrogen hydrogen fusing fusing into hydrogen fusing into helium and it's releasing just a ton of energy it's releasing just a ton of energy and 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 and so you have the core of a star the star like the Sun and that energy then heats up all of 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 Suns case the Sun now as the 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 more and more helium forms in the core it'll especially form the closer you get to the center the higher the pressures will be in the faster that this fusion this ignition will happen in fact the bigger the mass of the star the more the pressure the faster the fusion occurs and so you have this this helium building up inside of the core as this hydrogen in the core gets fused now what's going to happen there helium helium is a more dense atom it's packing more mass in a smaller space so as you get as more and more of this hydrogen here turns into helium what you're going to have is the core itself the core itself is going to shrink the core itself is going to shrink so let me draw a smaller core here so the core itself the core itself is going to shrink and now it has a lot more it has a lot more a lot more helium in it and let's just stick it to the extreme point where it's all helium where it's depleted but it's been tricky it's much denser that same amount of mass that was in this sphere is now in a denser sphere and a helium sphere so it's going to have 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 to a mass the stronger the pull of gravity so then instead of having just the fusion the heat just the hydrogen of fuge just the hydrogen fusion occurring at the core you're now going to have hydrogen fusion in a shell around the core so now you're going to have hydrogen hydrogen fusing fusing in a shell around the core around core let me just be clear this isn't just happens all of a sudden it is a gradual process as we get as we have more and more helium in the core the core gets denser and denser and denser and so the the pressures become even larger and larger near the core because you have a you're able to get closer to a more massive core since it is now more dense and is that pressure near the core increases even more and more the fusion reaction happens faster and faster and faster until you get to this point so here 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 of the core is now under enormous pressure it's actually under more pressure than it was when it was just a pure hydrogen core because it's it's there's so much mass on the outside here 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 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 even faster and it's occurring over a larger radius it's occurring over a larger radius so this faster fusion over a larger radius is then going to X the the force is not going to expel the energy that's released from this fusion is not going to expel these outer layers of the star even further so the whole time this gradual process as the hydrogen turns into helium refuses into helium in the core what's the hydrogen right outside of the core right outside of that area starts to burn faster and faster it starts to ignore I shouldn't say burn it starts to fuse faster and faster and it over a larger and larger radius so you might you know the unintuitive thing is the fusion is happening faster over larger radius and the reason that is 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 they're 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 if this star would look like this if this star maybe let me draw it in white this star look like this that's not white this star looked like what's happening to my color changer there you go okay this star looked like looked like this right over here now this star over here since the a faster fusion reaction is happening over a larger radius is going to be far larger now I'm not even drawing it to scale in the case of our Sun when it gets to this point it's going to be a hundred times the diameter and at this point it is a red it is a red giant and the reason why it's redder than this one over here is that even though the fusion is happening more furiously that energy is being dissipated over a larger surface area so the actual surface temperature the actual surface temperature of the red giant at this point is actually going to be is actually going to be cooler so it's going to emit a light at a at a at a at a larger wavelength a red or wavelength than this thing over here this thing the core was not burning as furiously as this thing over here but that energy was being dissipated over a lot of our smaller volume so this had a higher surface temperature higher surface higher surface temperature this over here the core is burning more sorry the core is no longer burning the core is now helium that's not burning it's getting denser and denser as the helium as the helium packs in on in 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 it doesn't make the increased heat is more than mitigated by how large the star has become now this is going to keep happening keep happening and this core is keep the pressures keep intensifying because more and more helium is getting produced and this core keeps getting keeps collapsing and the temperature the temperature here the temperature here keeps going up so we said that the first ignition the first fusion occurs at around 10 million at around 10 million Kelvin this thing will keep heating up until it gets to a hundred million Kelvin and now I'm talking about a star that's about as massive as the Sun some stars will never even be massive enough to condense the core so that as temperature reaches a hundred million 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 so where this huge where this huge star over here we have this helium core that helium core keeps getting condensed and can condensed and condensed and then we have a shell 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 expelling that's allowed that's causing the radius of the star to a 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 that you see around us including the ones that are in you were formed this way from initially hydrogen when it gets hot enough at 100 million Kelvin 100 million Kelvin 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 the helium will start to fuse and it now we're talking about a situation you have you know helium and yet hydrogen in 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 but I don't want to go into all of the details but let me just show you a periodic table I didn't have this in the last one I somehow had lost it but we see hydrogen here has one proton it actually has no neutrons it was getting fused in the main sequence into helium two protons two neutrons we would need four of these to get one of those because this actually has an atomic mass of four if we're talking about helium four and then the helium once we get to 100 million Kelvin can start being fused if you get roughly three of them and you know there's all these other things that are coming and leaving the reactions you can get to a carbon you get four of them you four of them at least a starting raw material you get to an oxygen so we're starting to fuse heavier and heavier elements so what happens here is this helium is 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 stride my self-imposed limit of ten minutes but what I want you to think about is what is likely to happen what is likely to happen here if this star net will never have the mass to begin to fuse this carbon and oxygen if it does have the mass is a supermassive star it will eventually we'll be able to raise even this carbon and oxygen core to 600 million Kelvin and begin to fuse that into even heavier elements but let's think about what's going to happen for something like the Sun where it'll never have the mass it'll never have the pressure just start to fuse carbon and oxygen and that'll be the topic of the next video