Nucleosynthesis can be broken up into to parts: nucleo- which means the nucleus or center of the atom, and -synthesis which means to put together or to make. If we put those two together, we get "to make a nucleus." In other words, nucleosynthesis is how the center of atoms are made. Remember that the nucleus of an atom is what determines the atom's element, so nucleosynthesis is also how all of the elements in the universe were made.

Nucleosynthesis happens in stars now-a-days. Some of it also happened during the Big Bang, but most of it happens in stars. Stars, remember, are giant balls of hot gas and plasma. Because they are so big, they have a lot of gravity. All that gravity means that there is a lot of pressure inside the star. At the center of the star, that pressure is so great that it can force the nuclei of two atoms together. Even though most of these atoms just bounce off of each other, a lot of them get pushed together so hard that they fuse together to become a bigger nucleus. The bigger nucleus then gets pushed out to other parts of the star so new nuclei can be fused together. This process is called nuclear fusion. As long as the star is alive, the atoms will stay there, but when the star dies, the atoms get flung out across the universe. Eventually those atoms can collect and make planets, such as the Earth.

There are several different reactions that happen during nucleosynthesis. The most basic is the fusion of hydrogen atoms to make a helium atom. Stars will perform this reaction for most of their lifetime, but there are other reactions that can happen, too. Stars can fuse helium and hydrogen to make lithium. Two helium atoms can make beryllium and carbon can be made with three helium atoms. Carbon can fuse with hydrogen to make nitrogen, or with helium to make oxygen. Most of these processes only happen near the end of a star's life, however. As a star runs out of hydrogen, it begins to produce these kinds of elements. Stars continue to do this until they produce iron. Iron is special in that when you fuse two iron atoms together, you get less energy out of it than you put in. This means that the iron atoms just absorb the star's energy, killing it. It's when stars die that even heavier elements are produced.

When small stars die, they die a very quiet death and just slowly burn out. These stars don't have enough mass to even make iron and they die by slowly fizzling out. Medium-sized stars, like the sun, will expand into a red giant, but will then fall apart. The outer layers of the sun will fall off and become a nebula while the core of the sun will stay a very hot ember, slowly cooling. The sun might be able to produce iron at the end, but most of the iron really comes from large-sized stars. These stars will go nova when they die (or supernova if they are giants). The force of the explosion will have enough energy to fuse iron atoms together, creating the heavier elements like gold, silver, platinum, uranium, etc. All the elements on earth probably came from supernovae--explosions of giant stars--over 5 billion years ago.

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Nucelosynthesis is a process of making different elements. It does this by nuclear fusion: forcing the centers of two atoms so close together that they become a new atom. Nucleosynthesis happens in stars, mostly by the fusion of hydrogen into helium. As stars get older, they begin to make other elements from lithium to oxygen and even up to silicon. When stars die, smaller ones go softly, medium ones are a bit more exciting and produce elements all the way up to iron, and large and giant stars explode and make all the elements heavier than iron. The explosion of these stars pushes the elements out into space, where they can be recycled into new planets.