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Narrator: In this demonstration
you'll use balls of clay to explore how stars fuse elements inside their
cores. Each different color represents a different element. Here,
pink represents hydrogen, blue is helium,
purple is carbon, orange
is oxygen, and this tennis ball is neon.
We'll start with hydrogen just like you've in the center of a 
young star. The inside of a star is hot and dense and all of these
hydrogen atoms keep bumping together. Often, some of them will stick 
together, this is called fusion.
So we squish together four pink balls representing
hydrogen and these form a new element--helium--represented by the blue clay.
The sun's been doing this for nearly five billion years.
Eventually the hydrogen in the center of the star runs low and we've mostly got
a lot of helium jostling together instead. When hydrogen fusion
stops the star is no longer in equilibrium and the core shrinks down.
But this makes things even hotter so the star can start to fuse helium into 
carbon. So we squish together three blue helium atoms to make one
purple carbon atom. In the sun this
is where things will end once the helium runs low. But if a star is 
much bigger than the sun, the core will shrink again and it will get even hotter at the center.
Now the star can fuse together a carbon and a helium atom creating
oxygen. So we'll squish a blue helium atom and a purple carbon
atom and that turns into one orange oxygen atom. You can 
imagine what happens when carbon runs low and fusion stops once more.
The core shrinks, things get hotter, a new fusion can begin again,
helium an oxygen start to fuse into neon. So we squish
a blue helium atom and an orange oxygen atom, and swap that out for a 
tennis ball that represents neon. Depending on the size of the star
this process can continue through the periodic table up to the point where iron is formed.
But to fuse iron you need to input energy. With energy going
into fusion instead of coming out of it, the star can never be in equilibrium 
again and its day are numbered.
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