So we've had our a discussion of what 
happens to the sun in the future and that 
was pretty interesting and we talked 
about all kinds of options about a what 
can happen to the reminisce. 
I said that I would come back and say 
what happens with more massive stars and 
the time to do that is now. 
So we start with a star who's mass is 
bigger than eight solar masses, that 
turns out to be a good place to make the 
cutoff, between solar mass and eight 
solar masses there are some changes but 
these two extremes will give us a 
representation of what happens, start 
with. 
Massive star can finish its main sequence 
life in as little as ten million years. 
So these stars are ephemeral compared to 
the sun; a thousand times faster. 
And, as usual, main sequence was when it 
was fusing hydrogen in the core. 
When the core runs out of hydrogen the 
core contracts, the envelope expands and 
pools and you get a red giant except 
because it started out as a big star we 
might get something you would call a red 
super giant so. 
This happens within a million years of 
the core collapse. 
It's finished its red giant expansion 
stage. 
Its size can be five astronomical units. 
We have a hydrogen burning shell 
surrounding an inert helium core and a 
huge. 
extended envelope. 
There will be mass loss in this period. 
The only difference is that here the 
inert helium core is too big. 
It never actually becomes inert. 
It's a smaller fraction of the actual 
mass of the whole star. 
So it does not collapse because of 
degeneracy but the core becomes massive. 
Eventually, it's not its own gravity but 
it is compressed by the external layers 
the internal temperatures were already 
high enough and so 
this is the red supergiant phase. 
Notice that there's not a huge increase 
in luminosity. 
At this point the star is, the, outer 
layers are cooling rapidly as the star 
expands so that luminosity rises somewhat 
but not very much. 
this was already 100,000 solar luminosity 
object. 
It doesn't get that much more luminous as 
a red giant and as usual 
helium core ignites. 
It's not as flashy. 
But since we don't see the flash, that 
makes no difference. 
we now have the structure or we have 
helium fusion in the core, hydrogen 
fusion in a shell, the envelope, 
the the hydrogen fusing shell having 
expanded, reduces its rate. 
The envelope contracts and heats. 
We have the horizontal branch for 
this giant star it makes a blue super 
giant its moving over to the 
left of the diagram is becoming blue. 
It's 
lives on the horizontal branch. 
Number not 700,000,000 years but only 
1,000,000 years and again the helium 
fusion in the core is rapidly depositing 
a carbon oxygen core surrounded by a 
shell of inert helia fusing helium 
surrounded by inert helium, surrounded by 
hydrogen that is fusing surrounded by the 
inert hydrogen atmosphere. 
And here is where things become 
interesting. 
When the carbon oxygen core collapses 
here. 
It does reach a temperature, 600 million 
K, at which carbon nuclei can fuse and 
the carbon fusion process produces things 
like manganese and neon, magnesium and 
neon and oxygen and so we now have a 
collection of an inert a oxygen neon core 
surrounded by a carbon burning shell, 
surrounded by inert carbon. 
Surrounded by a helium burning shell, 
surrounded by inert helium. 
Surrounded by a hydrogen burning shell, 
surrounded by a red super giant envelope 
that is even bigger than before. 
And the carbon fusion phase is very 
inefficient energetically. 
Much of the energy is carried off by 
neutrinos. 
We have huge mass loss and stellar super 
winds and the whole core is. 
The converted from carbon to oxygen neon 
within 1,000 years. 
And then well, if the star is 
sufficiently massive, then we get neon, 
or oxygen fusing. 
And the process continues. 
We get neon fusion at one and a half 
billion degrees kelvin, one and a half. 
If the star is massive enough, the 
process continues. 
At 1.5 billion Kelvin, neon fuses to 
produce oxygen and magnesium. 
here the neutrino flux, remember the 
neutrinos just penetrate the star and 
leave, are carrying off a full solar 
luminocity just in neutrini. 
you get rid of the neon in just a few 
years. 
that slightly higher temperature, about 
two billion kelvin, oxygen fusion 
ignites, producing silicon and sulfur and 
phosphorus. 
Here neutrinos are carrying off a 100,000 
solar luminosities. 
You get through with oxygen in a year. 
The next state is silicon fusion at 3.5 
billion kelvin and silicon fusion 
produces nickel which decays to iron, and 
that's an important point. 
And here neutrinos are carrying off 
amazing amounts of energy. 
And the silicon fusions process is 
complete in about a day. 
The result of all of this story is that 
you have been building up an inert core 
of iron. 
The changes are going on very, very 
rapidly. 
The exterior 
atmosphere has far to much inertia to 
really respond to all these rapid changes 
and in the process the neutron capture 
slow nucleosynthesis process is producing 
nucleotides heavier than iron so this is 
where mut a lot of heavier elements get 
synthesized and at the end of the 
silicone day the day it took to fuse 
silicon you have a core. 
Radius of about the Earth maybe a mass of 
a few solar masses which contains the 
following onion structure, it's got an 
inert nickel iron core surrounded by a 
silicon fusing shell surrounded by inert 
silicon surrounded by an oxygen fusing 
shell surrounded by inert oxygen 
surrounded by a neon and you so on you 
get the idea and all of this surrounded 
by a five AU or larger 
hydrogen helium envelope. 
the temperatures are now. 
On the order of three, 5 billion K 
densities in the center are as high as 
ten to the eleven. 
Kilos per meter cubed. 
At these high temperatures the photons 
that are being emitted are gamma rays and 
they cause photodisintegration. 
So, in fact at this last stage, the star 
starts very rapidly undoing. 
Whereas it's been doing all these 
millennium. 
working to construct heavier and heavier 
elements. 
All these nuclei are broken apart. 
iron breaks down under bombardment to a 
collection of alpha particles and, and 
junk. 
And so on and even alpha particles 
decompose back into protons. 
And now, what happens when the combined 
mass of the star collapses this iron 
core? 
Well, you can't ignite new fusion. 
I mean, you might get iron nuclei to fuse 
if you try hard enough, but that doesn't 
produce energy that won't stabilize the 
collapse. 
Once this iron core decides to collapse, 
it's not really obvious what is going to 
stop it. 
we'll see what it is that stops it, in 
the next clip.