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Hydrogen and helium and some lithium, boron, and beryllium were created when the universe was created. All of the rest of the elements of the universe were produced by the stars in nuclear fusion reactions. These reactions created the heavier elements from fusing together lighter elements in the central regions of the stars. When the outer layers of a star are thrown back into space, the processed material can be incorporated into gas clouds that will later form stars and planets. The material that formed our solar system incorporated some of the remains of previous stars. All of the atoms on the Earth except hydrogen and most of the helium are recycled material---they were not created on the Earth. They were created in the stars.
The use of the word ``created'' here is different than what is normally meant by scientists. In chemical reactions, different atoms or combinations of atoms are said to be produced or created when a reaction takes place. For example, in the Earth section of the planets chapter, I said that oxygen was produced in the photosynthesis process of plants. That oxygen then goes into the air and you breathe it in. To be more correct I should have said that the oxygen atoms were moved or broken off from one set of compounds [carbon dioxide (CO2) and water (H2O)] to form a molecule of two oxygen atoms bound together (O2) and a molecule of carbohydrate made of carbon atoms, hydrogen atoms, and oxygen atoms (C6H12O6). Each atom is rearranged or re-used. It was much simpler to say that oxygen was ``created'' as a by-product of the photosynthesis process. I hope you did not mind. In defense I want you to know that practically everyone, except for the astronomer researching stellar evolution, uses this loose meaning of ``creation''.
However, now that you know about stellar nucleosynthesis, I need to be more careful about what is being created from scratch and what is being re-used. Except for the hydrogen and most of the helium atoms, all of the materials around you, in the food you eat and drink, in the air you breathe, in your muscles and bones, in the paper and ink or toner of this book (or computer screen you are reading), everything (!) are made of atoms that were created in the stars. Those atoms are rearranged to produce the vast variety of things around and in you. In the cores of stars and in supernova explosions, new atoms are manufactured from nuclear fusion reactions. You will find out where the hydrogen and most of the helium atoms came from in the cosmology chapter.
The atoms heavier than helium up to the iron atoms were made in the cores of stars. The lowest mass stars can only synthesize helium. Stars around the mass of our Sun can synthesize helium, carbon, and oxygen. Massive stars (M* > 5 solar masses) can synthesize helium, carbon, oxygen, neon, magnesium, silicon, sulfur, argon, calcium, titanium, chromium, and iron. Elements heavier than iron are made in supernova explosions from the combination of the abundant neutrons with heavy nuclei. The synthesized elements are dispersed into the interstellar medium by the supernova explosion. These elements will be later incorporated into giant molecular clouds and eventually become part of future stars and planets (and life forms?)
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last updated: 25 May 2001