jay@npois.UUCP (Anton Winteroak) (07/02/85)
Hydrogen, and some helium have been around since the epoch when energy condensed into matter. Helium, Lithium, etc up to iron are spewed out by novas and supernovas. Note that it is only the core that is converted to silicon and iron and whatnot. The outer layers are simply blown off. Elements heavier than iron are almost exclusively created in the shockwaves in supernovas (though I suppose that some of this stuff could get created by processes near the central black hole of many galaxies) by the process of neutron capture, and beta decay. A good textbook for explaining this ingreater detail is Donald Clayton's book Stellar evolution and Nuclear Synthisis. You should also notice that there are prefered isotopes for the fusion process to create, but we have lots of non-prefered isotopes in our planet. These are also created in the shockwave of the supernova. The temperature there is very high.