🚀 go-pugleaf

Someone:
>A recent scientific American article described the explosion of a supernova.
>This fascinating article gave a simplified scenario of stellar evolution,
>burning hydrogen, helium, carbon, nitrogen, oxygen, silicon, finally
>producing iron. Paraphrasing: "Once a central core of iron is produced,
>reactions cannot continue.  Larger nuclei exist, but their formation
>is not favored.  Iron is the lowest energy nucleus." My question is, how did
>the other elements form, especially those above iron?
SOMEONE ELSE:
> 	I HAVE A FURTHER QUESTION!!WHERE DID THE; HELIUM,CARBON,NITROGEN,
> OXYGEN AND SILICON COME FROM LET ALONE THE GOLD?????????
> -- 
> 
First, He, C, N, O, and Si are formed "recursively" (:-) by fusing lighter
nuclei (starting with hydrogen) to yield heavier nuclei and free energy.

The problem with creating anything heavier than iron is that you don't get
energy out of it, it costs energy (as a chemist would say, the reaction is
endothermic, not exothermic).  Thus, it doesn't happen spontaneously (at
least, not frequently).

However, in a supernova (surprise!) there is scads of energy floating around,
which is more than sufficient to overcome the energy hump of slamming even
heavy nuclei together to fuse them; so much energy, in fact, that the heavy
fusion reactions do not noticably cool things down.

It is exactly analogous to standard chemistry, where if you want to drive
a reaction in reverse (from the low energy state to the high energy state),
all you need do is provide lots of reactants and some energy (plus a way to
extract the results before it breaks up).

--
John Woods, Charles River Data Systems, Framingham MA, (617) 626-1101
...!decvax!frog!john, ...!mit-eddie!jfw, jfw%mit-ccc@MIT-XX.ARPA

This has been a public disservice announcement.

> >My question is, how did
> >the other elements form, especially those above iron?
>
> > 	I HAVE A FURTHER QUESTION!!WHERE DID THE; HELIUM,CARBON,NITROGEN,
> > OXYGEN AND SILICON COME FROM LET ALONE THE GOLD?????????
> > -- 
> First, He, C, N, O, and Si are formed "recursively" (:-) by fusing lighter
> nuclei (starting with hydrogen) to yield heavier nuclei and free energy.

Pretty much right.  To be monotonously complete about it, here's more.
Hydrogen, helium, and some lithium were produced in the Big Bang.  (Helium
is also made in the "common" hydrogen fusion reactions that power the stars
most of their lives, but there's a neat calculation first done by Hoyle which
shows that most helium atoms are primordial -- i.e., from the Big Bang.)
Li, Be, and B are thought to arise principally in the interstellar medium
by spallation processes with cosmic rays; that is, cosmic ray particles
smack into other nuclei, shattering them and producing small amounts of
these light-element nuclei (the whole process is very much like a collision
in a terrestrial particle accelerator).

Things from carbon to nickel are made principally by fusion of helium, and
things produced by fusing helium.  The first step is the "famous" triple-
alpha process: three He's -> one C.  Another He nucleus onto this gives one
oxygen nucleus.  Fuse two oxygen nuclei and you can get lots of things
(the reaction chains get messy at this kind of energy), but often it's one
silicon or a magnesium plus other a helium, etc.  This works up to about
iron and nickel.

> The problem with creating anything heavier than iron is that you don't get
> energy out of it, it costs energy (as a chemist would say, the reaction is
> endothermic, not exothermic).  Thus, it doesn't happen spontaneously (at
> least, not frequently).
> 
> However, in a supernova (surprise!) there is scads of energy floating around,
> which is more than sufficient to overcome the energy hump of slamming even
> heavy nuclei together to fuse them; so much energy, in fact, that the heavy
> fusion reactions do not noticably cool things down.
> 

Sort of.  Heavy element synthesis is a fascinating web of sites and processes,
but not all of it can be blamed on supernovae.  Even there, though, you
don't continue to make things by standard fusion of regular nuclei (a side
note is that "regular nuclei" get scarce in a pre-supernova core -- it's
a pretty messed-up little volume).

Past the "iron peak" (Fe, Co, Ni) things are produced by neutron capture,
for the most part.  There are a couple of reactions in the chain which
produce lots of neutrons.  Neutron capture can go on at lower temperatures
than other things (neutrons, being neutral, don't have to overcome the
charge barrier that works against regular fusion reactions), and many
heavy nuclei are made by feeding iron nuclei one neutron at a time for
a few million years.  (This is the slow neutron capture process, or
"s-process" in the business.)  If you generate huge numbers of neutrons
in one shot -- and this is where supernovae enter the picture, since
doing this is very tough on the local architecture -- you can dump scads
of neutrons on one nucleus in one shot (the rapid or "r-process") and make
nuclei that way.  This is where the actinides (uranium and its daughters)
come from.  Details of nuclear physics determine whether a nucleus (I don't
say "element" here because different isotopes of the same element can have
different origin!) is of s- or r- (or other! there's a lot more to it)
process origin.  To get specific, gold has one stable isotope which is
generally considered to be an r-product, so the gold in your teeth (or
mine, if you've taken better care of yours than I have) did come from
some obliging supernova.