Where Does the Element Boron Come From?

Most recent answer: 08/11/2011

Direct quote from Wikipedia on the article for boron : "[...]boron is not produced by stellar nucleosynthesis[...]" Huh? I thought that stellar nucleosynthesis could produce every element (except for hydrogen) up to iron. How boron is made then?
- Anonymous
This is a very interesting question and possibly related to another one on elemental abundance, "Where is all the Lithium".    Wiki is correct; stellar nucleo-                       synthesis doesn't produce much Boron, but there are other mechanisms:  Big Bang nucleo-synthesis,  nuclear reactions such as Li7 + H3 --> B10 , or production in the earth's upper atmosphere by cosmic radiation. 
I'm not an expert in this but, fortunately, we have a local guru, Professor Fields who is.  Unfortunately, he is on vacation right  now.  I'll ask him when he returns and get back to you.


This is indeed a very interesting question.  In fact, the origin of boron was long a mystery to astronomers.  To give some context, one of the triumphs of 20th century astrophysics was the demonstration that the elements originate due to nuclear reactions taking place in a cosmic context--a process known as "nucleosynthesis."   It was shown that the most abundant elements in the universe, hydorgen and helium, were created in the first moments of the big bang (primordial nucleosynthesis).  Most of the other, heavier elements are created in nuclear reactions that go on in the enormously hot cores of stars (stellar nucleosynthesis). 

But a handful of elements arise from neither process.  Boron, as well as beryllium and some varieties of lithium, are not made in the big bang, nor are they made in stars.
In fact, stars *destroy* whatever lithium, beryllium, and boron they are born with.
So where are these elements made?  Wherever it is, their origin must involve nuclear reactions and thus high-energy particles. 

Interstellar space contains just such high-energy particles:  these are the cosmic rays,
which are mostly very high-speed, energetic protons.  Cosmic-ray protons move through interstellar space, where they unavoidably interact with interstellar gas.  That is, cosmic-ray protons collide at high speeds and energies with the nuclei of any and all interstellar atoms.
Of interest for us are collisions between cosmic-ray protons and interstellar carbon
and oxygen nuclei (which are observed in our our Galaxy and in other galaxes).
In these collisions, the carbon and oxygen are fragmented (a process called "spallation") and the debris include the lighter lithium, beryllium, and boron nuclei.  Thus boron is born in interstellar space, as a by-product of cosmic-ray irradiation!  This is  known as cosmic-ray nucleosynthesis.  The process is not very efficient, and so boron (and lithium and beryllium) have some of the smallest abundance of any stable elements!


(published on 08/11/2011)