3.1 Cosmic History of the Biogenic Elements and Compounds 



The principal matter of the early universe was hydrogen and helium (fig. 3-1 ). 

 Local density concentrations in the overall expansion of the young universe led 

 to the gravitational contraction of galactic-mass gas clouds within which further 

 fragmentation and further collapse led to the formation of the first generation 

 of Milky Way stars. Or, perhaps the stars began to collapse first and aggregations 

 of these protostellar clouds eventually formed galactic-mass assemblages of gas 

 and stars. Either way, in those collapsing stellar-mass fragments where the cen- 

 tral temperature rose to ~10 7 K, hydrogen atoms fused to form helium and 

 produced a stable, self-luminous, main-sequence star. When the hydrogen fuel 

 was exhausted, further contraction of the stellar core raised the temperature to 

 ~10 8 K, whereupon helium could fuse to form carbon. Eventual depletion of 

 helium resulted in further core contraction and an increased temperature until 

 oxygen could be fused from carbon, and so on and so on, until the peak of the 

 nuclear binding energy curve was encountered at 56 Fe. 



Further fusion reactions being endothermic, the fate of the now-evolved, 

 old giant star depends on its initial mass and how much of its outer layers it shed 

 with each phase of core burning and gravitational readjustment. Sufficiently 

 small stars enter a stable, nonnuclear-burning, white dwarf configuration in 

 which the pressure from a degenerate gas provides the needed support against 

 further gravitational collapse. Gradually the central temperature drops as the 

 white dwarf cools off, evolving into an increasingly unobservable black dwarf; 

 it eventually "goes out." From the point of view of the biogenic elements, 

 except for the mass that has been lost from the stellar surface along the way, 

 such stars represent a graveyard in which the elements are entombed and are 

 never again accessible for chemical evolution. However, if the star runs out of 

 nuclear fuel and is still sufficiently massive, no stable white dwarf configuration 

 is accessible. These massive stars end their life cycle not with a whimper, like the 

 white dwarfs, but with a bang. In a spectacular supernova explosion, much of 

 the outer mass of the star is hurled back into the interstellar medium while the 

 stellar core implodes to another stable configuration: a neutron star or a black 

 hole. During this violent stellar demise, enough energy is available to drive the 

 endothermic fusion reactions, thereby producing the full repertoire of stable 

 elements as well as many unstable isotopes. Thus, the first generation of stars 

 seeded the interstellar medium with heavy elements that became incorporated 

 into subsequent generations of stars. These then have the potential for somewhat 

 more complex nuclear reactions, particularly in the conversion of hydrogen to 

 helium. Like their predecessors, some of these later stars return enriched matter 

 to the interstellar medium and lock the rest of it away into stable, degenerate 

 configurations. Currently, something like a few solar masses of enriched material 

 are added to the Milky Way's interstellar medium each year, and the mean metal- 

 licity (i.e., the abundance of chemical elements heavier than helium) of the 

 galaxy does not appear to have changed much since the collapse of the protosun. 



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