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Lifecycle of Massive Stars
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- We've already talked about the life cycles of stars, roughly the same mass as our sun, give or take a little bit
- what i want to do with this video is talk about more massive stars - massive stars.
- When i'm talking about massive stars, I'm talking about stars that have masses great than nine times the sun.
- So the general idea is exactly the same, you're going to start off with this huge cloud of mainly hydrogen,
- and now the cloud is going to have to be bigger than the clouds that condensed to form stars, like our Sun.
- We are going to start with that, and eventually gravity is going to pull it together, and the core of it is going to get hot and dense enough
- for hydrogen, to ignite - for hydrogen to start fusing.
- And it is now fusing. Hydrogen Fusion. You now have hydrogen fusion in the middle so it's ignited and around it you have
- the other material of the cloud, so the rest of the Hydrogen, and now it's so heated it's, really a plasma 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 happen around 10 million Kelvin
- and I want to make it very clear, since we are talking about more massive stars, even at this stage,
- there is going to be more gravitational pressure, even at this stage, during the main sequence of the star
- because it is more massive, so this is going to burn faster and hotter.
- So this will be faster and hotter than something the mass of our Sun. And so, even this stage is going to happen over a much
- shorter period of time than a star the mass of our sun. Our Sun's life is going to be 10 or 11 billion
- total years, here we are going to be talking about things in the 10s of millions of years, so the factor
- of 1000x shorter life span. But anyway, let's think about what happens
- and so far just the pattern of what happens is going to happen faster because we have more pressure,
- more gravity, more temperature, but its going to happen in pretty much the same way as what we saw in the star the mass of the sun.
- Eventually, that hydrogen is going to fuse into a helium core
- that is going to have a hydrogen shell around it - a hydrogen fusion shell - around it.
- 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 in the shell fuses,
- and this is in a star the mass of our sun,
- this is when it starts to become a red giant because this core is starting to get denser, and denser,
- and denser as more, and more helium is produced
- and as it gets denser, and denser, and denser,
- there is more and more gravitational pressure
- being put on this hydrogen shell out here where we have fusion still happening
- and that is going to release more outward energy 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 are going to have heavier and heavier elements forming in the core
- those heavier and heavier elements as the star gets denser and denser
- will eventually ignite kind of supporting the core,
- but because the core itself is getting denser and denser, and denser, the material is getting pushed further and further out with more and more energy.
- Although if the star is massive enough its not going to be pushed out as far
- as you would have with a red giant as you would have in a red giant, with 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
- its going to ignite and its going to fuse into carbon and your going to have a carbon core forming
- so that is carbon core, around that you have a helium core, and near the center of the helium core,
- you have a shell of helium fusion turning into carbon, making that carbon core denser and hotter
- and then around that you have hydrogen fusion, and then around that you have the rest of the star.
- And so this process is going to keep continuing
- eventually that carbon is going to start fusing,
- and you're going to have heavier and heavier elements formed of course
- 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 and heavier elements until eventually you get to iron
- and in particular we are talking about
- iron-56 - iron with an atomic mass of 56.
- Here on this periodic table, that 26 is its atomic number - its how many protons it has.
- 56 is viewed as a count of protons and neutrons
- although its 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 to fuse iron won't help support the core.
- So just to be very clear, this is how the heavy elements actually formed
- we started with hydrogen
- hydrogen fusing in to helium, helium fusing in to carbon, and then all of these things
- in various combinations, and i won't go into all the details, are fusing heavier and heavier elements:
- neon, oxygen, and you see it right over here -
- silicon - and these aren't the only elements forming, but these are kind of the main core elements 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 this is actually how you can form up to nickel 56 just to be exact.
- There will also be some nickel 56
- which has the same mass as iron56
- just has two fewer neutrons and two more protons
- so its nickel 56, will also form, can also be a nickel-iron core
- but that's about how far a start can get regardless of how massive it is
- at least by going thru traditional fusion, thru the traditional ignition mechanism.
- What i want to do is leave you there,
- just so you can think about what might happen next now that we can't fuse this
- star anymore and what we are actually going to see is it will Supernova.
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