In Cosmos, Sagan explains to us laymen that stars fuse hydrogen into helium, and then if a star is big enough it creates higher order elements in later stages, before going supernova. What isn't explained is why such a high proportion of heavier elements (heavy relative to hydrogen) collect to form planets. I can *assume* that the heavier elements come together first, in the earth, in the sun, in all of the planets, and that only the larger objects capture and hold the lighter elements. IF this were so, then I'd expect that the Sun contains far more iron and stone than the Earth in terms of mass, but less as a percentage of total mass. That is, I'd expect the sun to be built upon a foundation of heaver elements which had been ejected from supernovae. I'd *expect* this, but it wasn't stated or explained, so I just don't know. Now, if this is the case, does it suggest that the sun also contains a center consisting of Uranium, maybe even Plutonium or other elements? If traces of Uranium are a part of the Earth's constitution, I'd think there should be much more within the sun. Or, do these elements undergo fission in the early evolution of a small star?