Lifecycle of Massive Stars

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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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