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Roger Weller, geology instructor

by Brian Barandi
Physical Geology
Fall 2012

Stellar Synthesis of the Elements



            When taking an introductory geology class, it is not surprising to learn about rocks, minerals, and the processes which work to create, change, and destroy these objects.  Judging by the definition of geology, the study of Earthís physical properties and the processes that work on them, having those concepts studied in class makes sense.  With any introduction science class, there is at least one small lecture about atoms, the elements and maybe even a small mention of the beginning of the universe.  These lectures are normally the first ones you have and then afterwards, you start on the important topics of the class, and that introduction fades away.  It is understandable that there is only so much time, and glazing over the material is better than not addressing it all.  In my opinion though, you will never truly understand any science class (or the beauty inherent within), without understanding the beginning of the universe and the events that unfolded to seed our universe with the fundamental elements that allow us to exist in the first place.  I wish I could go through all the details about our universe and the path it has taken, but for this paper I will focus on one topic.  Where did all atomic elements in our universe come from?   There is no better place to start, then in the beginning.

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In the beginning:


            Most of us have heard of the Big Bang, and are vaguely familiar with some of the details, but because this topic is by no means simple, even people interested in science donít get very far before it becomes oversaturated with complex concepts.  Because of the complexity, I can only go so far into explaining the big bang, but I will hit the concepts that are important to our topic.  In the beginning, there was nothing, no space, no matter, in fact there was no time, so even talking about what happened the second before the big bang has no meaning.  Cosmology has no definitive answers as to what caused the big bang, but in an instant, matter, energy, space, and time came into existence.  

The details after this point are beyond what I can explain in this paper so we will have to jump too roughly 100 seconds later, which doesnít sound like that much time, but there is an entire branch of science devoted to understanding what happened in the first nanoseconds, so 100 seconds later isnít a large gap.  At 100 seconds, the energy density of the universe was low enough to let 2 elements form, hydrogen and helium.  Now the energy density is so low at this time that there isnít enough energy left to change these ratios.  So 100 seconds later after that big bang, we are left with 75% hydrogen and 25% helium spread throughout an expanding space.  So we have 2 elements on our periodic table at this point and if things were to remain constant, that is all we would have had, and I wouldnít be writing this paper, and you wouldnít be around to be reading this paper.  So 2 elements is a good start, but where do we go from here.  As much as I would love to just jump straight into the next steps in our universes evolution, we have to cover one very important nuclear reaction, nuclear fusion. 

Nuclear fusion:

            If you want to change one element to another element, then some sort of nuclear reaction must occur.  There are multiple nuclear pathways to change elements, but in our case, we want to go from a lower atomic numbered element to a higher numbered element, and the only way to do this on a grand scale is nuclear fusion.  The basics of nuclear fusion are pretty straight forward.  You take two lighter elements and smash them together (fuse them) to get one heavier element and some energy is released.  Now I want to stress just how basic this explanation is, so I will take you step by step through the process. 

So letís start with the simplest element, hydrogen, and we will get rid of its electron, leaving us with a proton.   Now the goal is to get two protons close enough so that the strong nuclear force can grab them.  The strong nuclear force requires these to protons to touch each other, but we have a problem.  Protons are positively charged and objects with the same charge repel each other, so we must somehow overcome this repulsion force, also known as the Coulomb barrier, to get these protons to fuse.  So to accomplish this, we heat these two protons up to extreme temperatures, which means giving them extremely high speeds, and we put them in a container that is so small that ultimately they will collide.  As soon as they come in contact with each other, the strong force binds them, but two protons arenít stable, so one of the protons decays into a neutron and releases energy and a neutrino.  We still havenít really accomplished our goal though, because even though our element is heavier, it is still just an isotope of hydrogen.  So now we take 1 of these hydrogen isotopes, called deuterium, and another proton and we fuse those.  Now we have 2 protons and a neutron (Helium 3) and again energy is released.  Finally we have now fused Hydrogen into a higher element Helium. 



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There are a couple of things that are important to understand about this process.  The path I chose to get to helium isnít the only path that will get you to helium from hydrogen.  Another important thing is to realize is that this process can keep repeating with each element created and continue on to larger and larger elements, though iron and all elements after undergo a different process.  The distinctions between how much energy is required and how much energy is released for each element to fuse going up the periodic table will be explained in more detail as we move on to the next part of our story.

Stellar Formation:

            So now that we have covered the basics of nuclear fusion, letís go back to 100 seconds after the big bang.  We have hydrogen and helium floating around but because the energy is still high, though not enough to fuse any elements, the gases are sort of stuck in this high energy state, but no permanent changes can really occur.  There is hope though, the universe is slowly expanding and thus cooling down this energized hydrogen and helium.  From about 100 to 400 million years after the big bang the energy levels are low enough that gravity starts to come into play.  We have all this hydrogen and helium floating around the universe in a slightly uneven distribution.  This slight unevenness allows gravity to start clumping gas atoms little by little to form bigger clumps.  These bigger clumps start merging with more and more clumps.  Until these clumps become so large that gravity starts compressing them into more and more dense clumps. 


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As gravity starts compressing the gases, the atoms start colliding and they begin to heat up.  The more mass of the gas, the more gravity can compress and the hotter this gas starts to get.  Now knowing the basics of nuclear fusion, we know that if the gas gets hot enough, protons will have enough energy to fuse.  Creating a star though requires that the gas not only get hot enough, but is under large amounts of pressure and has a high density.  Gravity can accomplish all 3 of these conditions, but it is all dependent on the mass of the gas cloud that is being compressed.  If the cloud has 0.08 solar mass, then these conditions can be met.  For our purposes though, we are going to set our gas cloud at 20 solar masses.  This might seem a bit extreme, but there is evidence that there was enough pockets of gas to achieve this mass.  So our 20 solar mass cloud is now quickly being compressed by gravity.  As this happens, the core gets so hot and is under so much pressure that finally there is enough energy to breach the coulomb barrier and all at once, the protons in the core start fusing, a star is born.  Remember that as these ionized hydrogen atoms fuse, they release energy, and this energy starts pushing outwards against gravity.  So now there is a battle going on between the crushing forces of gravity and the internal pressure being produced in the core of the new born star. 

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As long as hydrogen keeps fusing in the core, the star will stabilize to a semi constant size and remain that way until the next step in stellar evolution.  So now that we are fusing hydrogen, we are losing hydrogen and producing helium. 

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Helium has two protons, so in order to fuse helium you need much more energy to overcome the repulsive force, so at this time, the helium is just accumulating in the core of the star as it is being produced.  Eventually though, the hydrogen will start running out and the internal pressure of the star will start to decrease.  When the hydrogen fusion is no longer producing enough pressure, gravity will start crushing the star once again.  This time it will crush it even further and the star will get even hotter.  As soon as it is hot enough, helium begins fusing.  Hydrogen still isnít completely exhausted yet, so there is a shell of hydrogen still fusing around the now fusing Helium. 

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Helium fusion halts the collapse of the core by creating energy to counter gravity.  Helium fusion is different from hydrogen fusion though, because the core is hotter so helium fuses at a much faster rate into Carbon.  Because we picked a star with 20 solar masses, each time one of the elements begins to run out, the mass of the star allows gravity to compress it to even hotter temperatures and start fusion of heavier elements and create onion like layers of each consecutive fusion product.   Each one of these fusion products fuse at an even faster rate than the previous. This keeps on going until you get to iron. 

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 Our 20 solar mass star would have enough mass fuse iron, but something different happens with iron.  Iron takes a lot of energy to fuse and as the star collapses down iron begins fusing, but instead of releasing energy, it takes more energy to get iron to fuse then the amount of energy it puts out.  This poses a very large problem.  Gravity wants to squeeze all this material to the most dense it can, and because we have a  20 solar mass star, there is plenty of gravity to pull this material in at extremely high speeds.  To give you an idea on just how fast, the star collapses in a tenth of a second, and the outer layer can reach speeds of 23% the speed of light.  All these different layers of the star start collapsing at different rates and as each one accelerates towards the iron core, it then rebounds off the iron core and starts colliding with all the rest of the material collapsing into the center.  This event is known as a supernova, and the energy that is released at this point can be more than all the stars combined in a galaxy (100 billion).  This collapse and release of huge amounts of energy create all the elements of the periodic table beyond Iron.  Because the supernova is so energetic, it forces all these elements into the universe as an explosive bubble seeding the rest of the universe with all these new elements.


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            So whenever you are taking any science class, or whenever you are doing anything in your daily life, keep in mind that all the material you are looking at is made up of material that once fused inside a star, including all the molecules in your body.



                A couple of great links to the full story of stellar evolution.

            Some links for details on the big bang.  Yes I know its Wikipedia, but itís a good starting point.

            Nuclear fusion and stellar fusion links.