大质量恒星的生命周期

we've already talked about the life cycle of stars roughly the same mass as our Sun give or take a little bit what I want to do in this video is talk about more massive stars massive stars and what I'm talking about massive stars I'm talking about stars that have masses greater than nine times greater than nine times the Sun so the general idea is exactly the same you're gonna start off with this huge cloud of mainly hydrogen and now this cloud is going to have to be bigger than the clouds that started that that that condensed to form stars like our Sun but we're gonna start with that and eventually gravity is going to pull it together it's going to put together and the core of it is going to get hot and dense enough for hydrogen to ignite for hydrogen to start fusing so this is hydrogen and it is now fusing let me write it it is now fusing hydrogen fusion let me write it like this you now have you now have hydrogen fusion in the middle so it's it's ignited and around it you have just the other material of the cloud so the rest of the hydrogen and now since it's so heated it's really a plasma it's really kind of a soup of electrons and nucleuses as opposed to well formed atoms especially close to the core so now you have hydrogen fusion we saw this happens to around 10 million Kelvin and I want to make it very clear since we're talking about more massive stars even at this stage this is going to there's going to be more more gravitational pressure even at this even at this at this stage during the main sequence of this star because it is more massive and so this is going to burn faster and hotter so this is going to be faster and hotter than something the mass of our Sun faster and hotter and so even this stage is going to happen much the evenin over a much shorter period of time than for a star the mass of our Sun our son's life is going to be 10 or 11 billion total years here we're gonna be talking about things and maybe the tens of millions of years so a factor of a thousand shorter life span but anyway let's think about what happens and so far just the pattern of what happens it's going to happen faster because we have more pressure more gravity more temperature but it's going to happen in pretty much the same way as what we saw in the stuck with us are the mass of the Sun eventually that helium sorry that hydrogen is going to fuse into a helium core that's going to have a hydrogen shell around it it's going to have a hydrogen shell around it hydrogen fusion shell around and then you have the rest of the star around that so let me label it this right here is our helium core and more and more helium is going to be built up as this hydrogen and the shell fuses and this is in a star the size of our Sun or the mass of our Sun this is when it starts to become a red giant because this core is getting denser and denser and denser as more and more helium is produced and as it gets denser and denser and denser there's more and more gravitational pressure being put on the hydrogen on this hydrogen shell out here where we have fusion still happening and so that's going to release more outward energy to push out to push out the radius of the actual star but then when you fast forward there so the general process and we're going to see this as the star gets more and more massive is we're gonna have heavier and heavier elements forming in the core those heavier elements as as the star gets denser and denser will eventually ignite kind of supporting the core but because the the core itself is getting denser and denser and denser material is getting pushed further and further out or with more and more energy although with the Stars massive enough it's not going to be able to be pushed out as far as you will have in a kind of a red giant was kind of a sun-like star but let's just think about how this pattern is going to continue so eventually that helium is going to once it gets dense enough it's going to ignite and it's going to fuse into carbon and you're going to have a carbon core forming so that is carbon core that's a carbon core around that you have a helium core you have a helium core and you're the set near the center of the helium core you have a shell of healy of helium fusion that's helium not hydrogen turning into carbon making that carbon core denser and hotter and then around that you have helium few you have hydrogen fusion have to be very careful you have hydrogen fusion and then around that you have the rest of the star you have the rest of the star and so this project is going to keep continuing eventually that carbons going to start fusing and you're going to have heavier and have your elements form the court and so this is a depiction off of Wikipedia of a fairly mature massive star and you keep forming these shells of heavier and heavier elements and cores of heavier heavier male elements until eventually you get to iron and in particular in particular we're talking about we're talking about iron 56 iron with an atomic mass of 56 here on this periodic table that 26 is it's atomic number it's how many protons it has 56 is you can kind of view it as a count of the protons and neutrons although it's not exact but at this point the reason why you stop here is that you cannot get energy by fusing iron fusing iron into heavier elements beyond iron actually requires energy so it would actually be an endothermic process so the fuse iron actually won't won't help support the core so what I want to do in this will and so just just to be very clear this is how the heavy elements actually formed we started with hydrogen hydrogen fusing into helium helium fusing into carbon and then all of these things in various combinations and I won't go into all of the details they're fusing heavier and heavier elements neon oxygen and you see it right over here silicon and these aren't the only elements they're forming but these are kind of the main core elements they're forming but along the way you have all this other stuff lithium beryllium boron all of this other stuff is also forming so this is how you form elements up to iron 56 and also actually this is actually how you can form up to nickel-56 just to be per tick to be exact there will also be some nickel 56 which has the same mass as iron 56 just has two fewer neutrons and two more protons so it's nickel-56 will also form can also kind of be it'll be a nickel iron core but that's about how far a star can get regardless of how how massive it is at least by going through traditional fusion through the traditional ignition mechanism what I want to do is leave you there and think of it just so you can think about what might happen next now that we can't fuse this star anymore and what we're actually going to see is it will supernova