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The Science Behind DRAGON
The Origin of the Elements
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Without the elements, we would be nothing. Whether it be the oxygen we breathe, the
carbon in our bones, or the gold in our precious jewelry, elements make up every part
of our world. But where did these elements come from? When the universe began,
the only matter that existed was hydrogen, the smallest of the elements. Since the
universe was still fairly hot at this time, some hydrogen was able to fuse (or 'burn')
into helium and small quantities of heavier atoms. Still, these processes do not account
for the majority of elements we encounter in our day to day lives.
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To discover where the rest of the elements came from, we must look to the heavens.
As the universe cooled, the only place where temperatures were high enough for fusion to occur
were the high pressure centers of stars. Less massive stars, like our sun, primarily burn
their hydrogen fuel through a cycle known as the
proton-proton-chain to produce helium. If carbon is present, they can also burn their
fuel through the CNO-cycle
(CNO stands for Carbon, Nitrogen and Oxygen). This cycle creates
some of the slightly heavier element such as nitrogen and oxygen, but the end result is
still helium.
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Once the hydrogen supply in the core of these stars is depleted, the star will begin to
collapse. When the star becomes hot enough, helium will then start to burn into carbon.
When the supply of helium is depleted, the carbon will then start to burn, and so
forth. Eventually, the star will expell its outer gas layers into space, leaving behind a
core of carbon. These events do not account the carbon found in our world, however. The
carbon created by these stars is left trapped in a stellar remnant known as a white
dwarf. The carbon we're familiar with has a much more explosive background.
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Most of the heavy elements found on the Earth were created in explosive stellar events known as
novae, supernovae, and x-ray bursts. These events happen through the death of massive stars, or
through interactions between red giants and white dwarfs. The heavy elements can be created through
one of several different processes. They can be be created through a series of rapid proton captures
(the rp-process), a series of rapid neutron captures (the r-process), slow neutron captures
(the s-process), or through the alpha capture process (alpha particles are the nuclei of helium).
Through these processes, stars are able to produce elements as heavy as iron.
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Explosive envionments at the end of a star's life have very high energies and thus,
reactions can happen quite rapidly. This changes the usual cycles for element creation,
since it is now possible for radioactive isotopes (radioactive versions of the elements)
to be present. The goal of DRAGON is to measure the rates of these particular reactions.
These explosions distribute stellar material throughout the universe. Over time, the
elements made their way to the cloud of gas that eventually made up our solar system.
As Carl Sagan rightly said, "We are all stardust."
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