Expert: James Gort - 12/14/2006

Question
hey James,Can you help me out with the quotes "we are stardust" and "we are nuclear waste" in terms of the evolution of a star and its planetary system from a molecular gas cloud and the source of that cloud?

I know its a big question, my study group is stuck on this question.It would help us out a lot if you could answer it or if you know a website that could help us with it.THanks in advance, Mike

Answer
Hi Mike,

According to the currently accepted theory of stellar evolution, we are both "stardust" and "nuclear waste" because all elements heavier than hydrogen were made by nuclear fusion in the interiors of stars.  In the early universe, it is presumed that only hydrogen gas existed, formed from the big bang primordial soup (this soup was pre-elementary particles).  As the gas dispersed, clouds formed by random motion and by gravitational attraction of hydrogen molecules.  If the mass was large enough (enough gravity) and the temperature low enough (high temperatures would tear apart the hydrogen cloud), then the cloud could eventually collapse into a more compact object, collect more hydrogen along the way, and form a protostar.  As the protostar collapsed into a full-fledged nuclear-fueled star, heavier elements were formed in the interior.  Hydrogen was fused into helium, and depending on the mass of the star, heavier elements elements could be formed.  The most massive objects produced a range of elements, including carbon, nitrogen, oxygen, silicon, and iron.  No heavier elements than iron could form in normal stars, since fusion of heavier elements would no longer provide energy to the star.  At the end of the star's lifetime, the outer atmosphere of the star would be blown off (sometimes forming a planetary nebula), but this atmosphere would contain many of the heavier elements.  If the star was massive enough, it would undergo a catastrophic collapse and trigger a "supernova", releasing much of its interior heavy elements to deep space.  The supernova explosion could also fuse elements to form heavier elements than iron.  In space, the elements could join together chemically and form molecules, including hydrocarbons so essential to life.

We are therefore considered "second generation" star material.  All of our heavy elements must have originated deep in the interiors of stars long deceased, released into space through atmospheric expulsion or stellar explosion, and eventually formed a new molecular gas cloud which would gravitationally collapse and become our Sun and the newly formed planets.

I hope this helps.  If you are interested in how the molecular cloud actually collapses to form stars and planets, you can search for "Jean's Mass" or "Jean's Criteria", named after Sir James Jeans, who first postulated the conditions of molecular cloud collapse.

Prof. James Gort