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13.3 Cosmic Abundance of Elements

An understanding of stellar evolution allows scientists to address another fundamental question — where did we come from? The Sun and other stars are mostly made of hydrogen and helium, but life depends on a number of heavier elements, including carbon, nitrogen, and oxygen. These elements were not present early in the universe; for millions of years after the big bang the universe only contained the two simplest elements: hydrogen and helium. Life-giving elements are produced deep inside stars and are then spat out into space during the late stages of a star’s life. We cannot understand our cosmic origins without understanding the cycle of star life and death.

Astronomers have measured the cosmic abundance of all the stable elements in the local universe. The abundance of hydrogen is almost a trillion (1012) times the abundance of gold! These values were derived from the Sun, but the relative proportions of different atoms are similar in all stars that astronomers have studied. Let’s use the analogy of a deck of cards. Imagine each card is an atom. In a typical sample of star matter, most particles are hydrogen atoms and only 1 in 12 is a helium atom. This would be like a deck where the aces represent helium and all the other cards represent hydrogen. The next most abundant element is oxygen, found at a level of 1 in 1500 atoms. We would need to search through 28 decks of cards to find a single oxygen "atom." All other cards are either hydrogen or helium. Gold is much rarer still. We would need to search through 10 billion decks of cards to find a single gold "atom!" Spectroscopy is such a powerful technique that astronomers can routinely identify elements at a concentration of less than one part in a billion.

The cosmic abundance of elements raises some interesting questions. Why are hydrogen and helium the most abundant elements? What explains the relatively high abundance of carbon and oxygen? Why is there a secondary peak in abundance at iron? What explains the "saw-tooth" pattern in the abundance distribution? Why are elements much heavier than nickel so fantastically rare? The creation of the materials of the everyday world takes place deep within stars. An explanation of the cosmic abundance of elements requires an understanding of the evolution of stars.