G'day. i'm a 2nd year Aero engineer student from Australia. i've had this question on my mind for the past few days now and can't seem to find much material on it. i was just wondering how the lightest elements (hydrogen and some helium) managed to come together in the center of the solar system and form the sun, whereas the heavy elements (those beyond Iron) condensed away from the center of the solar system to form the rocky inner planets. it just seems sort of wierd that lighter elements would "sink" to the middle to form the sun. or does the mass of the atoms have no effect on the placement of them in the solar system? Regards Zander.