Solar System Formation...why does the sun get all the hydrogen?

Factual Questions
1

The other thread in here on where heavy elements come from got me thinking (always a dangerous prospect):

A sun fuses hydrogen into heavier and heavier elements till it can fuse no longer. In some cases the star goes supernova and creates the remaining elements on the periodic table and scatters it all back into space.

  1. If previous stars ate all the hydrogen how does a new star coalesce out of the nebula (assuming the nebula is the remains of a previous star)? I mean, shouldn’t the nebula be seriously lacking in hydrogen?

  2. Once a solar system starts to form why does the star get mostly hydrogen? I’m sure it gets all sorts of stuff but I assume it is predominantly hydrogen. The planets seems to get most of the heavier elements and not much hydrogen (relative to a star that is…they still get plenty of hydrogen too but they get more of the heavy stuff instead).

Shouldn’t a proto-star’s gravitational pull yank on heavy stuff even more than the light stuf thus gathering more in?

My WAG is it all has something to do with the spinning of the solar system but I don’t know that the same dynamics apply out there as a spinning system would on earth (throwing heavier stuff to the outside…lighter stuff on the inside). Also, if this spinning is the reason shouldn’t gas giant planets be closer in to the sun and increasingly rocky planets on the way out to the edge of the system? If that were the case then you wouldn’t see something like our own solar system.

2

My WAGs.

  1. The stars don’t eat all the hydrogen, just as much as they can until they don’t have enough to keep the fusion reaction going. I don’t know what proportion must be consumed tho.

  2. Planets would find it difficult to hold much free hydrogen, because it’s so light; it can escape weaker gravitational fields when it gathers energy (floats to top of atmosphere, and bounces off into space).

However, if it’s in space, it gets pulled exactly the same as heavier stuff (doesn’t it? feather and ball dropped on the moon type thing…)

I’ve no idea how a solar system works itself out. :slight_smile:

3

dylan_73’s right. Allow me to elaborate.

Fusion only takes place where there is a huge amount of heat and pressure. In stars, you can’t have fusion in the outer parts, only deep down. A high-mass star only converts about ten percent of its mass to helium before it blows itself (including those un-fused, hydrogen-rich outer layers) to bits. You’re right, though, that as time goes on, more and more hydrogen is used up and more and more heavy elements are left behind. The universe is still young enough that there’s only tiny fraction of heavier elements around.

I’m afraid intuition has led you down the wrong path, my friend.

You need a larger object (like the Sun or Jupiter) in order to have the gravity to hold on to lighter gasses. What happens when you let go of a balloon filled with hydrogen on Earth? If you don’t have a lot of experience with hydrogen balloons (cough Hindenberg! cough) think of a helium balloon. Let go of the string, and it goes up, right? Eventually the balloon pops and the gas escapes to space.

It takes more gravity to hold on to a lighter gas because the lighter the gas molecule is, the faster it will move for a given temperature. At typical temperatures on Earth, the velocity of a hydrogen molecule is greater than the Earth’s escape speed. The escape speed of Jupiter or the Sun is much higher, so they can hold on to their hydrogen.

Actually, we see the opposite in our solar system because planets are built out of solid stuff. Near the Sun, only rock and metal are solid, so the planets are made of rock and metal. Further out ices are solid, so planets (and their satellites) are built out of metal, rock, and ice.

The gas giants were built out of ice, originally, too, but they got big enough that they could hold on to hydrogen and helium from the solar nebula, so they ended up with very thick hydrogen/helium atmospheres.

4

Not all. We’ve got a whole lot of hydrogen here on Earth, trapped in molecules like water, hydrocarbons, and organic compounds.

IIRC, the Sun has 99% of the mass of the solar system. With that much mass, it’s got to have just about the lion’s share of every element.

5

What everybody above has said re: gravity is correct. I just wanted to add that another reason stars are primarily hydrogen is that hydrogen is far and away the most common element in the universe. In fact, roughly speaking, hydrogen and maybe some helium were the only elements around in the early days of the universe. Everything heavier was created through the fusion that takes place in stars.

6

Could it just be that the original primordial disk of cosmic crap is spinning, so there is a centrifugal force as well as a gravitational one? This might encourage the lighter hydrogen to accrete to the middle, whilst the planets get the metals and such.

7

Spinning or not, only one significant force holds a forming solar system together, and that is gravity. If it is spinning, the centripetal force that keeps a given object moving in a circle is gravity.

8

That would only work if there was a bouyant force–if the metals were displacing the hydrogen. Even in the presolar nebuala, we’re talking about space; the pressures aren’t that high.

9

other points to remember…

  • Hydrogen is the most abundant element in the universe. By far. Earlier generations of stars have not used it all up, therefore, new stars can still form.

  • A nebula is not purely from a dead/dying star. Nebulae can get ‘used up’ for making stars, but there’s lots of still-viable nebulae out there. All of which are moving & reshaping, etc.

  • The early stages of the sun are more ‘violent’ (the ‘T Tauri’ stage I think it’s called). During this time, the atmospheres of the planets close to the sun are blown away, thus leaving the rocky core.

10

It’s also worth noting that we’re not sure how typical our solar system is, with rocky planets close in and gaseous planets further out. We now know that there’s plenty of stars which have gas giant planets very close to the star. They’re probably not the most common sort of solar system (we don’t currently have the technology to detect another solar system like ours), but a few decades ago, astronomers would have said that that was completely impossible.