Just Stardust in Your Eyes

By number of atoms, humans and all other living creatures are roughly 63 percent hydrogen, 26 percent oxygen, 10 percent car­bon, and 1 percent nitrogen.36 Not only the iron in your blood, or the calcium in your teeth and bones, but every living organism is made from recycled stardust. John and Mary Gribbin emphasize that “the nitrogen in the air you breathe and in your DNA (along with most of the carbon in your body) had a previous existence as part of a planetary nebula, and was expelled from one or more red giant stars.” The lead in a pencil, potassium in bananas, zinc and selenium in vitamins were all forged in stars. Nickel in coins, oxy­gen in Earth’s atmosphere, mercury in a thermometer, as well as precious metals such as gold and silver are examples of the heavy elements forged as stars blow off their outer layers and die. “This is where neon in neon lighting, the sodium in common salt, and the magnesium used (appropriately) in fireworks comes from—carbon burning inside stars.”37 And, ultraviolet starlight irradiating the en­velope of gases surrounding nearby dying stars can produce large volumes of water, essential to the formation of Earth’s atmosphere and oceans and a key marker for life as we know it.38

The composition of a comet is the frozen record of the chemical environment of our region of the Milky Way as the Sun formed, the sum of generation upon generation of star birth and death. When a comet comes near the Sun, the energy reanimates chemi­cals that have been interred from 4.5 billion years. Fittingly, the comet lights to form a feast for our eyes, in a pale echo of the blinding stellar light in which those atoms formed, long before there were eyes to see them.