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So we've had our a discussion of what 
happens to the sun in the future and that 

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was pretty interesting and we talked 
about all kinds of options about a what 

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can happen to the reminisce. 
I said that I would come back and say 

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what happens with more massive stars and 
the time to do that is now. 

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So we start with a star who's mass is 
bigger than eight solar masses, that 

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turns out to be a good place to make the 
cutoff, between solar mass and eight 

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solar masses there are some changes but 
these two extremes will give us a 

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representation of what happens, start 
with. 

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Massive star can finish its main sequence 
life in as little as ten million years. 

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So these stars are ephemeral compared to 
the sun; a thousand times faster. 

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And, as usual, main sequence was when it 
was fusing hydrogen in the core. 

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When the core runs out of hydrogen the 
core contracts, the envelope expands and 

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pools and you get a red giant except 
because it started out as a big star we 

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might get something you would call a red 
super giant so. 

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This happens within a million years of 
the core collapse. 

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It's finished its red giant expansion 
stage. 

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Its size can be five astronomical units. 
We have a hydrogen burning shell 

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surrounding an inert helium core and a 
huge. 

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extended envelope. 
There will be mass loss in this period. 

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The only difference is that here the 
inert helium core is too big. 

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It never actually becomes inert. 
It's a smaller fraction of the actual 

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mass of the whole star. 
So it does not collapse because of 

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degeneracy but the core becomes massive. 
Eventually, it's not its own gravity but 

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it is compressed by the external layers 
the internal temperatures were already 

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high enough and so 
this is the red supergiant phase. 

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Notice that there's not a huge increase 
in luminosity. 

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At this point the star is, the, outer 
layers are cooling rapidly as the star 

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expands so that luminosity rises somewhat 
but not very much. 

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this was already 100,000 solar luminosity 
object. 

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It doesn't get that much more luminous as 
a red giant and as usual 

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helium core ignites. 
It's not as flashy. 

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But since we don't see the flash, that 
makes no difference. 

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we now have the structure or we have 
helium fusion in the core, hydrogen 

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fusion in a shell, the envelope, 
the the hydrogen fusing shell having 

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expanded, reduces its rate. 
The envelope contracts and heats. 

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We have the horizontal branch for 
this giant star it makes a blue super 

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giant its moving over to the 
left of the diagram is becoming blue. 

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It's 
lives on the horizontal branch. 

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Number not 700,000,000 years but only 
1,000,000 years and again the helium 

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fusion in the core is rapidly depositing 
a carbon oxygen core surrounded by a 

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shell of inert helia fusing helium 
surrounded by inert helium, surrounded by 

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hydrogen that is fusing surrounded by the 
inert hydrogen atmosphere. 

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And here is where things become 
interesting. 

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When the carbon oxygen core collapses 
here. 

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It does reach a temperature, 600 million 
K, at which carbon nuclei can fuse and 

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the carbon fusion process produces things 
like manganese and neon, magnesium and 

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neon and oxygen and so we now have a 
collection of an inert a oxygen neon core 

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surrounded by a carbon burning shell, 
surrounded by inert carbon. 

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Surrounded by a helium burning shell, 
surrounded by inert helium. 

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Surrounded by a hydrogen burning shell, 
surrounded by a red super giant envelope 

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that is even bigger than before. 
And the carbon fusion phase is very 

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inefficient energetically. 
Much of the energy is carried off by 

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neutrinos. 
We have huge mass loss and stellar super 

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winds and the whole core is. 
The converted from carbon to oxygen neon 

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within 1,000 years. 
And then well, if the star is 

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sufficiently massive, then we get neon, 
or oxygen fusing. 

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And the process continues. 
We get neon fusion at one and a half 

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billion degrees kelvin, one and a half. 
If the star is massive enough, the 

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process continues. 
At 1.5 billion Kelvin, neon fuses to 

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produce oxygen and magnesium. 
here the neutrino flux, remember the 

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neutrinos just penetrate the star and 
leave, are carrying off a full solar 

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luminocity just in neutrini. 
you get rid of the neon in just a few 

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years. 
that slightly higher temperature, about 

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two billion kelvin, oxygen fusion 
ignites, producing silicon and sulfur and 

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phosphorus. 
Here neutrinos are carrying off a 100,000 

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solar luminosities. 
You get through with oxygen in a year. 

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The next state is silicon fusion at 3.5 
billion kelvin and silicon fusion 

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produces nickel which decays to iron, and 
that's an important point. 

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And here neutrinos are carrying off 
amazing amounts of energy. 

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And the silicon fusions process is 
complete in about a day. 

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The result of all of this story is that 
you have been building up an inert core 

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of iron. 
The changes are going on very, very 

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rapidly. 
The exterior 

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atmosphere has far to much inertia to 
really respond to all these rapid changes 

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and in the process the neutron capture 
slow nucleosynthesis process is producing 

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nucleotides heavier than iron so this is 
where mut a lot of heavier elements get 

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synthesized and at the end of the 
silicone day the day it took to fuse 

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silicon you have a core. 
Radius of about the Earth maybe a mass of 

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a few solar masses which contains the 
following onion structure, it's got an 

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inert nickel iron core surrounded by a 
silicon fusing shell surrounded by inert 

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silicon surrounded by an oxygen fusing 
shell surrounded by inert oxygen 

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surrounded by a neon and you so on you 
get the idea and all of this surrounded 

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by a five AU or larger 
hydrogen helium envelope. 

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the temperatures are now. 
On the order of three, 5 billion K 

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densities in the center are as high as 
ten to the eleven. 

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Kilos per meter cubed. 
At these high temperatures the photons 

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that are being emitted are gamma rays and 
they cause photodisintegration. 

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So, in fact at this last stage, the star 
starts very rapidly undoing. 

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Whereas it's been doing all these 
millennium. 

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working to construct heavier and heavier 
elements. 

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All these nuclei are broken apart. 
iron breaks down under bombardment to a 

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collection of alpha particles and, and 
junk. 

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And so on and even alpha particles 
decompose back into protons. 

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And now, what happens when the combined 
mass of the star collapses this iron 

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core? 
Well, you can't ignite new fusion. 

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I mean, you might get iron nuclei to fuse 
if you try hard enough, but that doesn't 

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produce energy that won't stabilize the 
collapse. 

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Once this iron core decides to collapse, 
it's not really obvious what is going to 

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stop it. 
we'll see what it is that stops it, in 

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the next clip.