You start with a protoplanetary disk (a "Bok" globule or something like it). It eventually contracts to form a star in the center, and planets form in "resonant" orbits around that star. This much I understand just fine; that's just gravity at work. My question is, how is that most of the hydrogen in the system "falls" into the central star, while a good portion of the "metallic" substances (in stellar terminology) forms the rocky and icy planets, comets and asteroids?
It would seem to me that ALL of the material would want to fall toward the center, but if that was the case, the star would have its beginning as a huge, rocky/icy planet that finally collapses and starts "burning." The next alternative would seem to be that the *heaviest* stuff would want to collect in the center, leaving most of the lighter elements in the surrounding disk.
That obviously isn't correct, I know, but this is what I'm having trouble understanding. All of the available materials online about stellar and planetary evolution just say, "The stuff contracts to form planets and star(s)." There's no discussion of how the different elements sort themselves out as they obviously do, from studying nature itself. For example, Earth's crust is largely silicon, the core is largely iron and nickel, and the atmosphere is primarily nitrogen. The sun, on the other hand, is primarily hydrogen. What causes these elements to "sort?" Am I even using the right term?
Thanks a bunch in advance.
