ABSTRACT
Once the universe was created by the Big Bang, the only abundant elements present were hydrogen (H) and helium (He). These elements were not evenly distributed throughout space, and under the influence of gravity they began to "clump" to form more concentrated volumes. Evidence of this uneven distribution can be found in the anisotropies detected in the Cosmic Background Radiation (CMB) by the COBE satellite in the early 90's. These clumps would eventually form galaxies and stars, and through the internal processes by which a star "shines" higher mass elements were formed inside the stars. Upon the death of a star (in a nova or a supernova) these high mass elements, along with even more massive nuclei created during the nova or supernova, were thrown out into space to eventually become incorporated into another star or celestial body.
The conditions inside a star that allow the formation of the higher mass elements can be related to a pushing match between gravity and the energy released by the star. Gravity creates a force that would cause a star to shrink and collapse, but the energy released by nuclear reactions within the star flows outward, and produces thermal pressure that opposes gravity. When these two forces are balanced, the star maintains a particular size. But when there is some type of imbalance, the star (or some part of it) will expand or contract in response to the stronger of the two forces.
When the universe was first created, essentially all matter was in the form of two elements- hydrogen and helium. Their relative abundance (by weight) was 75% hydrogen and 25% helium. (This means that for every He nucleus there were 12 H nuclei/protons) They were not evenly distributed throughout space. This is critical, because this uneven distribution allowed gravity to act in the areas of higher concentration to initiate the "clumping" of matter. If everything were evenly spread out, nothing would have happened, for each atom would have been attracted evenly from all directions, and would have remained where it was relative to neighboring atoms. As a result of slight discrepancies in the distribution of matter, gravity was able to initiate the collapse of huge volumes of H and He into more concentrated areas of gas. These areas eventually would evolve to form galaxies. Within these areas, there was a second level of more concentrated clumping of H and He that would form stars, where the higher mass elements would be created.
In these more concentrated areas, as the clouds of H and He (called nebulas) collapsed, the atoms were speeding up as they were pulled toward the center by gravity. This caused two things to happen. First, the increase in the velocity of the atoms resulted in an increase in the temperature of the material. At some point, the temperature became high enough so that the material began to glow. Second, the atoms were becoming packed more tightly, increasing the density, and the frequency of collisions between atoms. As this happened, the mass of H and He became more spherical. At this stage the mass of H and He is called a protostar.
or copy and paist http://aether.lbl.gov/www/tour/elements/stellar/stellar_a.html
