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Main content
Current time:0:00Total duration:12:54

Video transcript

in the last video we started with the star in the in its main sequence like the Sun and inside the core of that star you had hydrogen fusion going on so that is hydrogen fusion and then outside of the core you just hide hydrogen you just had hydrogen plasma and we say plasma it's the electrons and protons of the individual atoms have been disassociated because the temperatures and pressures are so high so they really just kind of it's like the soup of electrons and high electrons and protons as opposed to proper atoms that we associate with at lower temperatures so this is a main-sequence star right over here this is a main-sequence star right over here and we saw in the last video that this hydrogen is fusing into helium so we start having more and more helium here and as we have more and more helium the core becomes more and more dense because helium is a more massive atom and just be able to pack more mass in a smaller volume so this gets more and more dense so core becomes more dense more dense until at some point and so what while the core is becoming more and more dense that actually makes the fusion happen faster and faster because it's more dense more gravitational pressure more mass wanting to get to it more pressure on the on the hydrogen is fusing so it starts to fuse hotter so let me write this so the fusion so hydrogen fuses fuses faster and actually we even see this in our Sun our Sun today is brighter and hotter it's fusing faster than it was when it was born 4.5 or 4.6 billion years ago but eventually you're going to get the point so that the core you only have helium you only have helium so there's going to be some point where the entire core is all helium it's going to be way denser than this core over here all of that mass over there has now been turned into helium not all of it a lot of it has been turned into energy but most of it is now in helium it's going to be in a much much smaller volume and the whole time the temperature is increasing the fusion is getting faster and faster and now this in this dense volume of heat of helium that's not fusing you do have and we saw this video a shell around it a shell around it of hydrogen that is fusing so this right here is hydrogen fusion going on and then this over here is just hydrogen plasma now the unintuitive thing or at least this was unintuitive to me at first was is the stuff that's going at what's going on in the core is that the core is getting more and more dense it's it's fusing at a faster rate but and so it's getting hotter and hotter so the core is hotter fusing faster and getting more and more dense and I kind of imagine it starting to collapse every time it collapses it's getting hotter and more dense but at the same time that's happening the star itself is getting bigger the star itself is getting bigger and this is actually not drawn to scale red giants are much much larger than main sequence stars but the whole time that this is getting more dense the rest of the stars you can kind of view it as getting less dense and that's because this is generating so much energy that it's able to it's able to more than offset or better offset the gravitational pull in into it so even though this is hotter it's able to disperse the rest of the material in the Sun over a larger volume and so that volume is so big that the surface and we saw this in the last video the surface of the red giant is actually cooler is actually cooler let me write that a little neater is actually cooler than the surface of a main-sequence star this right here is hotter and just to put things in perspective if this when the Sun becomes a red giant and it will become a red giant its diameter will be a hundred times the diameter that it is today or another way to be put it it will be have the same diameter as the Earth's orbit around the current Sun or another way to do this where we are right now will be on the surface or near the surface or maybe even inside of that future Sun or another way to put it when the Sun becomes a red giant the earths going to be not even a speck out here and it will be liquefied and vaporized at point-in-time so this is super super huge and we've even thought about it just for light to reach the current Sun to our point in orbit it takes 8 minutes so that's how far that's how big one of these stars are to get from the to get from the one side of the star to another side of the star it'll take 16 minutes for light light to travel if it was traveling that diameter and even slightly longer if it was a travel it'd go in a circumference so these are huge huge huge stars and we'll talk about other stars in the future they're even bigger than this when they become super Giants but anyway we have the huge hydrogen in the center we've now become sorry we have the helium in the center let me write this down we have a helium core in the center helium core in the center we're fusing faster and faster and faster we're now a red giant the core is getting hotter and hotter and hotter until it gets to the temperature for ignition of helium so until it gets to 100 million Kelvin remember the ignition temperature for hydrogen was 10 million Kelvin so now we're at a hundred million Kelvin factor of 10 and now all of a sudden in the core you actually start to have helium fusion helium fusion and we touched on this in the last video but the helium is fusing into heavier elements and some of those heavier elements and predominantly it will be carbon and oxygen carbon and oxygen and you may suspect this is how heavier and heavier elements form in the universe they form literally due to fusion in the core of stars this but especially well especially when we're talking about elements up to iron but anyway we have the core now is now experiencing helium fusion it has a shell around it of helium that is not quite there does not quite have the pressures and temperatures to fuse yet so just regular helium but then outside of that we do have the pressures and temperatures for hydrogen for hydrogen to continue to fuse so out here you do have hydrogen fusion you do have hydrogen fusion and then over and then outside over here you just have the regular hydrogen plasma so what just happened here when you have helium fusion all set now this is once again providing some type of energetic support outward support for the core so you're going to it's going to counteract the the ever increasing contraction of the cores it gets more and more dense because now we have energy we have energy going outward energy pushing things outward but at the same time at the at the same time that that is happening more and more hydrogen in this layer is turning into helium is fusing into helium so it's making this inert part of the helium core even larger and larger denser and even larger and larger and putting even more pressure on this inside part and so what's actually going to happen within a few moments from I guess especially from a cosmological point of view this helium fusion is going to be it's going to be burning super I shouldn't use igniting or fusing in a super hot level but it's contained do all of this pressure but at some point that pressure won't be able to contain it and the core is going to explode but it's not going to be one of these catastrophic explosions where the star is going to be destroyed it's just going to release a lot of energy all of a sudden into the star and that's called a helium flash a helium flash but once that happens all of a sudden then now the star is going to be more stable and I use that in in and I'll use that in quotes without writing it down because red giants in general are already getting to be less stable than a main-sequence star but once that happens you now will have a slightly larger volume so it's not being contained in as small of a tight volume that helium flash kind of took care of that so now you have helium fusing into carbon and oxygen and there's all sorts of other combinations of things obviously there's many elements in between helium and carbon and oxygen but these are the ones that dominate and then outside of that you have the Healey you have you have you have helium forming you have helium that is not fusing and then outside of that you have your your fusing hydrogen you have over here you have hydrogen fusing into helium and then out here and the rest of the radius of our super huge red giant you just have you just have your hydrogen plasma out here now what's going to happen as the star ages well if we fast-forward this a bunch and remember as a star gets denser and denser in the core and the reactions happen faster and faster this core is expelling more and more energy out where the star keeps growing and the surface gets cooler and cooler so if we fast forward a bunch and this is what's going to happen to something the mass of our Sun if here's more massive then at some point the core of that of carbon and oxygen is forming can start to fuse into even heavier elements but in the case of the Sun it will never get to that 600 million Kelvin to actually fuse the carbon and the oxygen and so you'll have eventually you will have a core of carbon and oxygen or mainly carbon and oxygen surrounded by surrounded by fusing helium surrounded by fusing helium surrounded by non fusing helium surrounded by fusing hydrogen surrounded by fusing hydrogen which is surrounded by non fusing hydrogen or just a hydrogen plasma of the Sun but eventually all of this fuel will run out all of the hydrogen will run out in the stars all of this hydrogen all of this fusing hydrogen will run out all of this fusion helium will run out all of this is the olive this is the fusing hydrogen this is the inert helium which will run out will be used in kind of this core being fused into the carbon and oxygen until you get to a point where you literally where you literally just have a really hot core of carbon and oxygen and it's super dense this whole time it will be getting more and more dense as more heavier and heavier elements show up in the core so it gets denser and denser and denser but the super dense thing will not in the case of the Sun and it was a more massive star it would get there but in the case of the Sun it will not get hot enough for the carbon and the oxygen to form so it really will just be this dense ball of carbon super dense ball of carbon and oxygen and all of the other material in the Sun remember was super energetic it was releasing tons and tons of energy the more that we progressed down this the more was releasing outward and the larger the radius of the star game and the cooler the outside of the star became until the outside just becomes this kind of cloud of this huge cloud of gas and and around what once was the star and in the center so I could just draw it as this huge this is now way far away from the star much even bigger than the radius or the diameter of a red giant and all we'll have left is is a mass a super dense mass of I would call it inert inert carbon or oxygen this will is in the case of the Sun and at first when it's hot and it will be releasing radiation because it's so hot we'll call this a white dwarf this right here is called a white dwarf and it'll cool down over many many many many many many many years until it becomes when it's no longer was completely cooled down lost all of its energy it'll just be the super dense ball of carbon and oxygen at which point we would call it a we would call it a black dwarf a black dwarf and these are obviously very hard to observe because they're not emitting light and they don't have kind of the they don't have quite the mass of something like a black hole that isn't even emitting light but you can see how it's affecting things around it so that's what's going to happen to the Sun in the next few videos we're going to talk about what would happen to things less massive than the Sun and what would happen to things more massive the Sun although I think you can imagine the more massive these the carbon then this there would be so much pressure on these things because you have so much mass around it that these would begin to fuse into heavier and heavier elements until we get to iron