When the universe came into existence some 10 to 15 billion years ago, the only elements were hydrogen, helium, and traces of lithium, beryllium, and boron. The heavier elements did not yet exist. They are the product of nuclear burning inside stars.

The burning of hydrogen produces helium, that of helium produces carbon and oxygen, they produce still heavier elements, and so forth. The ashes from the burning of one element become the fuel for successive nuclear burning.

When stars have used up their nuclear fuel, they die. Stars of modest mass eject planetary nebulae. The more massive stars explode as supernovae. Freshly synthesized heavy elements are ejected into space where they intermingle with the surrounding interstellar medium. This medium -- the gas and dust between the stars -- provides the raw material for the formation of new generations of stars.

The heavy elements synthesized by one generation of stars become the raw material for new generations of stars. They, in turn, produce still more heavy elements, which are eventually returned to the interstellar medium. The cycle of stellar death and birth continues.

The carbon in our bodies, the oxygen we breathe, the silicon in the Earth's crust, and the metals used in our industry were all produced inside stars. Star dust is the stuff we and our environment are made of.

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