With Earth Day 2008 fast approaching on April 22, we Earthlings are busy congratulating ourselves on our sustainability practices and our green lifestyles. To hear us tell it, you’d think we had invented recycling. But recycling is nearly as old as time itself. And thereby hangs a tale.
Once upon a time in the early universe (and when I say “early,” I mean around 200 million years after the Big Bang), a prolific period of star formation began.
Prior to that time, the expanding universe had been a dark wasteland. Only hydrogen and helium atoms--the two simplest chemical elements--were abundant. But now, in the vast primordial clouds of those simple gasses, gravity was about to liven things up.
The gasses began to condense under the force of gravity, and the clouds fragmented. The cloud fragments collapsed further and heated up, eventually becoming giant spinning spheres of hot gas. When a sphere’s internal pressure and temperature grew high enough, its primary fuel--hydrogen--began burning into helium in a nuclear reaction. The sphere began to glow, heat and light were released, and a star was born.
These first-generation stars were massive and short lived, quickly using up their hydrogen fuel and then exploding. However, their remains enriched the universe with the extra helium from their nuclear reactions and may also have seeded the universe with some heavier elements. The next-generation stars, created from this enriched matter, were a bit smaller and more stable. In their nuclear furnaces, helium could now burn into significant amounts of carbon, an element essential to life. In turn, carbon could burn into heavier elements, and those elements into progressively heavier and more complex elements, and so on.
It is in the death throes of the element-rich, next-generation stars that our story--the human story--begins. Modest-sized stars reached the end when internal pressure from nuclear burning exceeded the gravity that kept them intact. They gently disintegrated, puffing off layers of life-supporting elements like carbon, oxygen, and nitrogen. Massive stars, however, met with a violent end.
The Dumbbell Nebula - a disintegrating star
These stellar giants burned increasingly heavier elements in their cores until their cores burned into iron. The iron cores collapsed and rebounded violently in cataclysmic explosions called supernovas. These explosions blasted the complex elements created in the stars into space. In addition, new elements were forged in the extreme environments of the supernova blasts themselves.
The Crab Nebula - a supernova remnant
The peculiar life forms known as human beings are able to exist because of stars that died billions of years ago. Those stars seeded the cosmic cloud that became our solar system with the elements needed for life--not to mention the elements needed for rocky planets like Earth to form. Most of the elements found today in the universe formed from nuclear reactions in stars and during supernova explosions. Many of these elements are found in trace quantities in our bodies.
Today, in galaxies (gravitationally-bound star systems) like our own Milky Way, giant clouds of gas and dust called nebulas are collapsing, fragmenting, and heating. These nebulas contain the complex chemical legacy of the early stellar generations. With the ignition of nuclear burning in ultra-dense, ultra-hot clumps inside these hydrogen-rich nebulas, new stars are forming--just as stars have formed throughout the history of the universe. Some of these stars are Sun-like, some may have rocky planets in tow, and some may one day support life forms. And like our Sun and its attendant planets and inhabitants, one day these new star systems will complete their life cycles and, unraveling, will return all their raw materials to the cosmos for reclamation.
The cycle continues, because the universe is the ultimate recycler. Happy Earth Day.