all right we're back for part two of Star life cycles we've already done the red dwarf life cycles in part 1 now we're going to be doing the low-mass star life cycle this life cycle is important for us because our son is a low-mass star all right now as a reminder we don't need to do the first two stages again because the low-mass star life cycle starts with the protostar and main sequence stages just like every other star the only difference is because these low-mass stars are higher mass than the red dwarfs those stages take less time then the red dwarfs took to do them now after the main sequence stage remember the main sequence stage is when the core of the star is burning hydrogen when the core is fusing hydrogen we have a main sequence star that can only go on for so long because eventually this core will run out of hydrogen as soon as that happens we move into the next stage which I'll give you the name of in a moment the name comes from what happens to the star so let's first look at what happens to the star after the main sequence and then I'll explain what the name is now after the main sequence well during the main sequence the star is extremely stable not a lot happens to the star it tends to have the same size the same temperature for a very long amount of time now after that core runs out of hydrogen though that's going to go through some changes specifically the core size will change the core temperature will change the luminosity will change the surface size will change and the surface temperature will change now these things will change a lot over the next few stages of the star's life so I have made a chart here and you probably should try to make this chart as well it's a chart that's going to show whether these things are increasing or D creasing I'm going to use arrows in the chart to indicate increasing and decreasing an up arrow means an increase a down arrow means a decrease alright I'm also going to draw what's happening in the core region of the star now by core region I mean the core and the area around it alright so let's assume that the main sequence has ended now because fusion is happening in the core during the main sequence the pressure that's caused by that fusion fights the inward pull of gravity that's why the star stays very stable now when the fusion turns off in the core that means nothing is fighting gravity in the core if nothing's fighting gravity in the core gravity will crush the core now if gravity crushes the core what should happen to the surface size well it should be pretty simple to see that the surface size decreases now if we're crushing the core that means we're making it more well we're crushing a guess and if you remember if you crush a gas it gets hot so if the core is shrinking the core temperature will go up now at this point remember the reason that the the hydrogen around the core was not fusing before is because it wasn't hot enough out there but if we just crushed the core and made extra heat in the core then the area around the core can start fusing so that means during this next stage the core region is going to have the core itself is made out of helium because that's what you make out of hydrogen fusion and there's going to be a shell around it where hydrogen is fusing so whenever I draw these diagrams here I'm always going to write what the the region's made out of or what the area is made out of as an element hydrogen helium and if and region is fusing I'll put a little lowercase F so during this stage the helium in the core is not fusing but the hydrogen around it is fusing this is called hydrogen shell fusion now because our core temperature is now extra hot and because this shell is actually bigger than the core was we're actually fusing more now than we were during the main sequence so that means that the luminosity is also going to go up now if the luminosity goes up then that means more light is pushing its way out of the star that's gonna push the rest of the gas in the star out now what is that going to do to the overall size of the star the surface size well what's gonna make the star grow and if we let the rest of the star outside of the court spread out or grow then that means it's going to cool down so the surface temperature will actually go down so I want you to notice something here the SIRT of the core shrunk and got hot but the surface grew and got cold the surface did the opposite of what the core did now these changes are all very extreme the surface is going to grow to the point where our Sun will completely engulf both Mercury and Venus we won't quite be inside of the Sun at this point but that means the Sun is much bigger in this stage now because the surface cools down it's it's tipped the temperature determines the color so right now our Sun is yellow if it cools down it's going to become orange and then red so this is a very big red star that's why this stage is called the red giant stage red giant stays so so far the low-mass star lifecycle starts with the protostar stage then the main sequence stage and then the red giant stage now the red giant stage also won't last forever and that's because as this chord it's hotter and hotter and hotter eventually we're going to get to the point where the the core starts to fuse again it's way harder to fuse helium than it is hydrogen because helium is a bigger element the bigger the element the harder it is to fuse so that helium does not fuse in the red giant stage but as soon as it gets hot enough we move into the next stage where the helium in the core is fusing we still have the hydrogen shell around it fusing but the helium is now fusing too so we have two things fusing in this next stage which is called the horizontal branch horizontal branch I'll show you over here all right so low mass star lifecycle protostar main sequence red giant horizontal branch all right so the horizontal branch specifically is the stage of a star's life where its core is fusing helium all right now during the horizontal branch the core is actually going to grow again and the reason for that is that if you turn fusion back on in the core that means it's producing pressure again and it fights gravity it's going to beat gravity during this stage it pushes harder outward then gravity pushes inward so the core size will grow if we let the core grow its temperature will drop because the gas is spreading out horizontal branch the tip core temperature goes back down that means though that even though we now have two different things that are fusing because it's fusing at a much lower temperature it's actually going to produce less light than before that means that the luminosity will go down as well okay but if you remember the reason that the surface grew during the red giant stage is because the extra light coming out of the core if we just lowered the amount of light coming out of the core that means nothing's pushing the rest of the star out anymore and it's going to shrink back down almost as small as it was during the main-sequence stage now if the surface is shrinking that means it's getting tighter it's being squeezed by gravity and that means its surface temperature will go back up and the star will stop being red it will actually look like a slightly bigger version than a main-sequence star all right now the next stage of the star the helium in the core will not fuse forever eventually you'll run out of it when you do the thing you make out of fusing helium is carbon and oxygen so the core in this next stage will be made out of carbon and oxygen however you need an extremely high temperature to fuse carbon and oxygen so notice I did not put a little F in that area the carbon and oxygen is not fusing now because the carbon and oxygen is not fusing nothing is fighting gravity down there in that core so during this stage the core is going to shrink again so after the horizontal branch the core shrinks again just like in the red giant stage that's going to make the temperature go up but because this core is now shrinking more than ever before the core is going to get hot than ever before and therefore you're actually going to have two different shells the hydrogen fusion shell that we still have from the previous stages but because this inner region is now extra hot we get to have a helium shell too right next to the core so because we now have two shells fusing and the temperature is higher than ever before the luminosity is going to go up so let's write an up arrow in our luminosity column or row now if we're making more luminosity than ever before then that means the star is going to puff up larger than ever before so this star is going to get even bigger than when it was a red dwarf sorry I'm sorry yeah it's gonna get even bigger than when it was a red giant yeah okay now because the star is getting even bigger than ever before that means its surface is going to get colder than ever before whenever you let something it spread out it gets colder now that means if you notice this check out these arrows now check out the arrows in the red giant stage compare these arrows to the red giant stage they're exactly the same just more extreme so this stage is like the red giant stage but more extreme its name is the AGB giant stage AGB giant so going back over here the low mass star life cycle is protostar main sequence red giant horizontal branch AGB giant don't worry about what AGB stands for just think of the AGB giant stage as a more extreme red giant stage so here's the complete table that we drew and I want you to notice you do have to know all of these arrows but you really only have to remember one stages arrows and the red giant stages because if you look at the arrows from stage to stage everything that happened here in the red giant stage flipped when we got to the horizontals branch stage and then it flipped right back to the same direction when we got to the AGV giant stage if you can remember this one just remember it flips flips also in every single stage whatever the core is doing core size and temperature the surface always did the opposite thing so if the core is shrinking and getting hot the surface is growing and getting cold if the core is growing and getting in cold the surface is shrinking and getting hot finally if you have a hard time remembering what the luminosity is doing it always matches the core temperature if the core gets hot it's gonna make more light if the core gets cold it makes less light pretty simple complicated but as long as you know the rules you're okay now the AGB giant stage is going to mark be basically the end of this star's life the end of the AGB giant stage is where this type of star dies now the death of this star is a little bit complicated so you're gonna have to bear with me now let's look right here remember in the AGB giant stage there are two shells around the core now the helium shell will run out of fuel first because it's hotter the closer you are to the core so let's imagine that this helium shell runs out of fuel that means it's going to collapse and get hotter overall however remember when we fuse hydrogen out here we just make more helium so as soon as this runs out if the extra temperature will make the hydrogen shell make more helium when it makes more helium it refills the helium shell so it can turn back on when it turns back on it spreads back out and cools things down because it's spreading out and that makes it so the hydrogen shell can't make as much helium then the helium shell will keep going until it runs out again and this whole process repeats over and over and over long story short what that means is that at the end of the AGB giant stage these shells start to take turns turning on and off on and off on and off take turns first the hydrogen then the helium first the hydrogen then the helium first the hydrogen then the helium now if these shells turn off and on and off and on and off and on basically what's happening is that the luminosity will go up and down and up and down and up and down and up and down meaning that overall the star will start to puff in and out and in and out and in and out it in and out and in and out that means the star starts to get unstable now it gets so unstable that it starts to release the outer layers of itself and that's going to look like this so here the outer layers of the star have been released from the star this is how low mass stars die now if this low mass star has released its material into space that means it's released its gas back into basically the gas of the star has become a cloud again and we call a cloud of gas in space and nebula so this star has created a new nebula as it's died we call this type of nebula a planetary nebula so the stage after the AGB giant stage in a low mass stars life cycle is the planetary nebula stage now this name is just absolutely terrible a planetary nebula has nothing to do with planets at all it's this thing was this type of nebula was named before we knew what they were exactly so it has an out-of-date name don't be confused as nothing to do with planets now after a low-mass star dies this way the only thing that gets left behind is the core now that core is going to be very very very small and because it's very very very small we call it a dwarf that core also is going to be very very hot because it was the core of a Sun that's why it's gonna have a white color so we call that leftover dead star a white dwarf so this is the complete lifecycle of low mass stars protostar main sequence red giant horizontal branch AGB giant planetary nebula white dwarf now how big is a white dwarf not very big here we go so this is a picture of a white dwarf not this that's a white dwarf so white dwarf is literally a very small white star now how big are they in comparison to our Sun they're about a hundred times smaller than the Sun they're roughly the size of Earth so if you look in this picture right here we have earth and a white dwarf however a white dwarf is much much much denser than the earth so we'll talk about white dwarfs when we get back to class end of part 2