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Formation of the Elements July 20, 2009

Posted by jmegenhardt in Astronomy, stars, supernova.
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James Megenhard has his blog at:      http://eastrichlandchemistry.wordpress.com/

Stardust, the Building Block of Everything?

At the birth of our universe the only elements formed in any substantial amount was helium and hydrogen.  There were some heavier elements like lithium and beryllium, but these were so minor that they are not even considered.  So where did the carbon that all life is made of, the oxygen that all animals need to breathe, or the iron that makes up some of our strongest buildings come from?  Hydrogen and helium were formed during the Big Bang, while all of the other elements come from small bangs; the death of stars.

At this time, there are 118 known elements.  The simplest element is hydrogen which has only 1 proton and 1 electron.

hydrogen

In order to make hydrogen into another element, protons need to be added; which in turn requires the addition of electrons and neutrons.  For example, add a proton, an electron, and two neutrons, and hydrogen has become helium.

HeliumAtom2

Adding a proton, electron, and two neutrons creates lithium.  Adding yet another proton, electron, and neutron gives beryllium.  By simply adding more protons, electrons, and neutrons, heavier and heavier elements can be formed.  It seems reasonable for helium to form, after all, what else is to be done with the neutrons that hydrogen did not use from the Big Bang?  The question is, why would more protons, neutron, and electrons come together to make any elements past helium?

The Big Bang roughly states that everything that would one day form the physical universe began as a super hot, super condensed mass.  This mass reached a critical point which resulted in the mass exploding.  As the material from this mass cooled hydrogen and helium were formed.  The hydrogen and helium started pooling together into various gas clouds.  These gas clouds were pulled in towards their center, resulting in an increase of mass at center, which created more gravity, which resulted in more hydrogen and helium being pulled in.

In the Center of Star

More mass in center → More gravity in center

↑                                        ↓

More gravity in center ← More mass in center

The increasing density of that material resulted in more gas atoms colliding.  Each time there was a collision, some of that energy was converted to heat.  As more gas was pulled in, there were more collisions resulting in heat increasing, until the temperature reaches around 10 million degrees Kelvin at which point nuclear fusion of hydrogen begins; a star is born.  To sum it up…

More gravity in center → More density in center → More collisions in center → More heat in center until  nuclear fusion is reached.

Nuclear fusion is the process by which two atoms are combined to form a new atom.  When two hydrogen atoms fuse they produces helium and energy.  Click here to see how hydrogen becomes helium. It is the energy produced by nuclear fusion that runs a star.  Since hydrogen is the simplest element with only a proton and electron, the star begins the process of fusion with it, but as hydrogen is used up and temperature increases, the helium produced can undergo nuclear fusion of its own to produce carbon, oxygen, or neon.  Carbon can further fuse to form metals like sodium or magnesium, until nuclear fusion produces iron.

Layers of Fusion in a Star

fusion layers

But why did these elements not form during the Big Bang?  The answer is repulsion.  Hydrogen is a proton and electron, so its formation is easy since opposites attract.  Helium, on the other hand, was a little tougherto form since it needs two positive protons in its center; but like charges repel.  The only reason the two protons in helium did not repel away from each other was the pressure within the expanding material from the Big Bang was higher than the repulsive force.  But this force was not great enough for more than two protons to come together.  The only place in our universe where the force is so great that multiple protons cannot repel from each other is in the heart of a star.

The first 26 elements on the periodic table are formed by stars as they produce energy.  The remaining elements are formed from a star dying.  When a star dies, the gravitational pull upon that star causes the iron center to collapse.  As the center collapses, it reaches a point where the energy build-up causes the collapse to stop and reverse just like a rubber ball will collapse so far before it rebounds.  In other words, the center implodes and then explodes out.  As the center is blown back outwards, it collides with the outer material surrounding the star; which was also being pulled in to the center.  Just like in the birth of a star, this increase of collisions results in even more heat and pressure, which means even more nuclear fusion.  The net result being that as the star is being blown apart, further nuclear fusion is occurring resulting in elements even heavier then iron.  Not only does the stars death form these heavy elements, but it also causes those elements to be blasted out into space, where they can collect and form other astronomical bodies like planets and asteroids.

External resources:

The Universe Adventure

Questia

Think Space

NASA

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