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From: john@frog.UUCP (John Woods)
Newsgroups: net.astro.expert
Subject: RE:Forming The Elements
Message-ID: <213@frog.UUCP>
Date: Sat, 29-Jun-85 16:41:38 EDT
Article-I.D.: frog.213
Posted: Sat Jun 29 16:41:38 1985
Date-Received: Wed, 3-Jul-85 08:01:56 EDT
References: <243@ihnet.UUCP> <776@inuxd.UUCP>
Distribution: net
Organization: Charles River Data Systems, Framingham MA
Lines: 36

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

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