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.