Stellar Nucleosynthesis

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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 and nickel atoms were made in the cores of stars (the process that creates iron also creates a smaller amount of nickel too). The lowest mass stars can only synthesize helium. Stars around the mass of our Sun can synthesize helium, carbon, and oxygen. Massive stars (M* > 8 solar masses) can synthesize helium, carbon, oxygen, neon, magnesium, silicon, sulfur, argon, calcium, titanium, chromium, and iron (and nickel). Elements heavier than iron are made in supernova explosions from the rapid combination of the abundant neutrons with heavy nuclei. Massive red giants are also able to make small amounts of elements heavier than iron (up to mercury and lead) through a slower combination of neutrons with heavy nuclei, but supernova probably generate the majority of elements heavier than iron and nickel (and certainly those heavier than lead up to uranium). The synthesized elements are dispersed into the interstellar medium during the planetary nebula or supernova stage (with supernova being the best way to distribute the heavy elements far and wide). These elements will be later incorporated into giant molecular clouds and eventually become part of future stars and planets (and life forms?)

Although the particulars of various nucleosynthesis processes are beyond the scope of this website, it is important to note a couple of things:

  1. The stellar nucleosynthesis theory correctly predicts the observed abundances of all of the naturally-occuring heavy elements seen on the Earth, meteorites, Sun, other stars, interstellar clouds---everywhere in the universe. (In the cosmology chapter you will see where the hydrogen and most of the helium came from.) We understand now why some elements like carbon, oxygen, silicon, and iron are common and the heaviest elements like gold, mercury, and uranium are so rare.
  2. In order to create a terrestrial planet like the Earth (and life on such a planet), enough of the heavy elements have to be created in previous generations of stars and then concentrated in the interstellar clouds to collect into sizable chunks around forming stars. There is necessarily a "lag" between the beginning of the universe and the beginning of life.

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last updated: June 8, 2010

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel