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# 13.16 Cycles of Star Life and Death

Most stars are less massive than the Sun, so they trap their heavy elements and take them to the grave. They become cooling embers called white dwarfs, with cores so dense that a teaspoonful brought to Earth would weigh as much as a mountain. Their carbon-rich material is a crystalline form of carbon; Pink Floyd gave a nod to white dwarfs in their 1975 song "Shine On You Crazy Diamond." High mass stars entomb heavy elements like carbon, neon, oxygen, and silicon in layers like an onion skin, with iron at the center.

Luckily, other stars do share their bounty with the rest of the universe. As in the line from an even earlier album by Joni Mitchell, "We are stardust, we are golden. We are billion year old carbon." After 10 billion years in a stable state, the Sun, like all other stars, will eventually exhaust its nuclear fuels. It will become unstable and shuck off a layer of gas containing helium and carbon, blowing a huge smoke bubble into space. Higher mass stars lose even more mass-they drop off heavy sweaters of gas into the interstellar medium. The most spectacular loss of mass occurs when the massive stars die. Massive stars digest their elements slowly and the steady absorption of neutrons ratchets elements slowly up the periodic table. At the end of their lives, titanic explosions hold the key to the rest of the story of the elements.

With no more energy to be gained from nuclear reactions, the balance between gravity pulling and fusion pushing is disrupted for the last time. The floor drops out and massive stars collapse in a spherical free fall. This implosion creates a fantastic density and leaves behind a core of dense-packed neutrons (a neutron star is a gigantic atomic nucleus) if the star is fairly massive (1.5 to 3 times the Sun's mass) or a black hole if the star is extremely massive (3 or more times the Sun's mass). But during the collapse too much material rushes into a confined space, so most of it bounces back out in a spherical blast wave. This gas meets gas still falling in and the collision instantly creates temperatures of billions of degrees, enough to break through the iron "wall." Supernova!

Literally within seconds, explosive fusion creates the elements heavier than iron, all the way up to lead. These elements surf the blast wave and mingle with the thin gruel of gas between stars. For a few days, as witnessed by Brahe and Kepler and civilizations throughout history, a supernova outshines an entire galaxy, it's dazzling enough to see in broad daylight.

If you pause to reflect on your origin, it's as amazing as any tall tale told around a camp fire. Our bodies — the iron in our blood, the oxygen in our lungs, the calcium in our bones, and the carbon and nitrogen that forms our genetic material-are formed from the detritus of generation upon generation of stars that lived and died before Earth was born. The atoms in the jewelry that we adorn ourselves with-gold and silver and platinum — are glittering relics of long-gone supernovas.

The Sun is a relative latecomer in the story of star birth and death. When the first stars formed, a few hundred million years after the big bang, they were pristine balls of hydrogen and helium. They had no planets because there wasn't anything to make them from. Over billions of years, stars forged heavy elements in their cores and ejected them into space where they could become part of the gas clouds that would go on to form new stars. Generations of stars have handed down gas, steadily enriching it with the silicon and oxygen to make rocks, and the carbon, nitrogen and oxygen to make life.

As carbon-based life forms, we borrow our carbon from the world around us as we grow in the womb; carbon that we'll return when we die. That same carbon is part of a much larger cosmic pageant of stellar life and death. Life is, indeed, a strange and wonderful journey.