Virtually all the elements heavier than hydrogen and helium - for example, all the material that makes planets like the earth - come from the evolution of stars. Elements up to the atomic weight of iron are a result of burning of material in the cores of massive stars, and they are released into interstellar space when these stars explode as supernovae. Elements above iron in atomic weight (for example, lead or uranium) are made during the supernova explosions themselves. All of these materials merge with the general interstellar gas and are then taken up into new stars as they form from this gas.