An element with a greater atomic number than iron.
"We are stardust..." - Joni Mitchell
Elements heavier than
hydrogen are generally produced by
nuclear fusion during the formation and evolution of stars, a process which begins when clouds of gas become dense enough and energetic enough to begin
nuclear reaction. As these clouds undergo
gravitational collapse to form
stars, pressure and temperature rise, and the reactions intensify. Hydrogen
nuclei fuse to form
helium, which in turn undergoes fusion to become
beryllium. Further fusion may then take place to produce elements of higher atomic weight.
Unfortunately however, only elements up to the atomic number of iron are produced in this manner, as the energy required to start a fusion reaction in iron isn't generally available (the energy produced by the reaction is less than that taken to produce the reaction). Our own Sun is incapable of producing anything heavier than carbon,for instance.
So, where does the iron come from? Obviously, iron (and heavier elements) do exist, so more energetic reactions must be taking place somewhere else, other than in the normal, run of the mill stars. The answer lies in the cataclysmic explosions accompanying the birth of novae and supernovae. The enormous energies released in these stars is enough to begin fusion in iron and heavier elements, and the expanding cloud of gas in the nebula surrounding the dying star carries them out into the rest of the universe.
Without these elements, we'd have no copper, sodium, potassium and so on in our bodies. If life were possible, it would certainly be much different. We are indeed stardust.
More detail at http://www.ebi.calpoly.edu/BioSci/Courses/
BIO/BIO414/History/H01.html