Accretion of the Solar System?

When planets form through a process called 'accretion' it is generally believed that denser elements with a high atomic weight gradually form a core, with lighter elements ending up as the outer layers including the surface crust and atmosphere.

But in the case of solar systems that accretion process seems to be reserved, with the lightest elements hydrogen and helium collecting at the center and condensing into stars.

Why?

Why are heavy metals absent from the core of the sun?

 

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Maybe this thread would be better served by the Astronomy section...if it can be moved by a mod.

 

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The Sun is a main sequence POP 1 star designated GV.
Around 74% H, 24% He and 2% metal rich.

Simply put our Sun and the planets formed from a relatively low metallic accretion disk.
With your second paragraph about the lighter elements gathering towards the center of the disk, perhaps you are referring to extra stellar systems, and the fact that we have seen many Jupiter size gaseous planets and larger in short orbital periods around their star.
This is explained by Planetary migration, earlier in a stellar systems life and occurs due to gravitational interactions between the many thousands of smaller planetary bodies that were present in that epoch.

The accepted accretion disk formation methodology goes like this....
A huge cloud of gas and stellar debris gravitationally bound, is disturbed by a nearby supernova. This commences the cloud of gas to start collapsing and start to spin faster and faster.
Eventually the pressure at the core of this flattening disk undergoes fusion and ignites into a star. The stellar wind from the newly formed star then sweeps out ridding the inner system of the lighter gas and dust, leaving the heavier stuff behind, to form terrestrial planets like Mercury Venus, Earth and Mars, with the gaseous and icy giants further out.

 

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I think the answer to your question "why is it different with the Sun?" is that it isn't different. I would imagine that the Sun does have a core where its heavier elements settle.

As a percentage however the heavier elements are very low in the Sun.

 

carcano, your view of accretion is oversimplified. Several points have been made by other posters. Here are some more.

The segregation of elements within planetary bodies is governed by chemistry as well as physics. Goldschmidt raised three classes: siderophile (concentrated in iron); lithophile (concentrated in rock); chalcophile (concentrated in ores). Thus. although uranium is dense it is preferentially located in rock forming minerals. Carbon, though quite light, is abundant in the mantle, and so forth.

The accretion disc is hot as a consequence of its collapse and radiation from the proto star at its heart. Close to the proto star only the most refractory minerals can condense. An important boundary is the ice line, beyond which temperatures are low enough for water and other ices to form.

paddoboy, gravitational instabilities are sufficient to cause collapse of GMCs. However, there is solid evidence that in the case of the solar system this collapse was initiated, or hastened by a nearby supernova.

 

Agreed.
Can't argue with that.

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What I'm asking about is the events taking place in a time period PRIOR to fusion.

Within a swirling mass of elements that has not yet even formed a disc shape.

One clump of these random elements will eventually form the center of a system....but why is it always a clump of hydrogen and not something with a bit more ooomph?

 

Because at this epoch, only 13.83 billion years after the BB, and taking into account that Sun like stars, have a 10 billion year lifetime, we are only up to Gen 2 or 3, stellar production.
In other words, not enough heavier elements.
And of course hydrogen is the easiest of our elements to fuse, requiring the lowest temperatures.
Again the procedure seems to be......
A huge cloud of gas and stellar debris gravitationally bound, is disturbed by a nearby supernova. This commences the cloud of gas to start collapsing and start to spin faster and faster.
Eventually the pressure at the core of this flattening disk undergoes fusion and ignites into a star. The stellar wind from the newly formed star then sweeps out ridding the inner system of the lighter gas and dust, leaving the heavier stuff behind, to form terrestrial planets like Mercury Venus, Earth and Mars, with the gaseous and icy giants further out.

 

paddoboy: just aiming for a little more precision. By the time the stellar wind has blown out the gas and light dust the the planets have already formed. One of the challenges for planetologists seeking to model the mechanisms of planet formation was to explain how cores for the gas and ice giants could form rapidly enough to attract their gaseous and icy envelopes before the sun reached its T-Tauri stage and blew all the gas away. There is a relatively short window for this to happen, but it appears to be quite possible.

Also, for clarification, the powerful stellar wind arising during the T-Tauri stage is not initiated by energy from hydrogen fusion, but appears to be powered by lithium "burning". Generally we do not consider the star to have "switched on" and entered the main sequence until hydrogen fusion begins.

In the case of protoplanet formation, once these have reached a sufficient size (and temperature) they will differentiate by physics (as Carcano imagines) and by chemistry (as I noted in an earlier post). This tendency will be over-ridden in the proto-star by the turbulence that is present and by the low percentage of denser elements (as noted by paddoboy).

 

Because the overwhelming majority of the matter in molecular clouds, that collapse into stars, is hydrogen

 

No problem. Much appreciated in actual fact.

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Of all the elements, the hydrogen nucleus has the lowest gravitational pull...iron for example is approx 55 times stronger.

Considering the difficulty of creating enough pressure for fusion to occur under insanely expensive lab conditions I dont see how this happens in space at temperatures near absolute zero.

A popular idea is that a 'disturbance from a nearby supernova' creates enough pressure for this to happen...but what kind of pressure exists in a near vacuum???

 

The pressure [and temperatures] are created by gravitational collapse of inter-stellar dust and debris clouds, over many tens of millions and hundreds of millions of years.
This is not just a theoretical stab in the dark anymore, it has been observed in action with many extra-solar systems and nebula such as the Eagle Nebula.

 

Dust...you mean solid hydrogen particles with a temperature of less than -260 Celsius?

 

I mean what we have observed stellar remnant hydrogen clouds and dust.
http://hubblesite.org/gallery/tours/tour-m16/

http://hubblesite.org/reference_desk/faq/answer.php.id=32&cat=nebulae

 

To emphasise the point made by paddoboy: no one has said fusion occurs "in space at temperatures near absolute zero". The temperatures occur at the heart of stars that have formed following the collapse of giant molecular clouds (GMCs).

You are correct to believe that very high temperatures are required for this. Prior to hydrogen fusion occurring the proto-star is already radiating significantly from the heat generated by collapse, and then by fusion of lithium. But much of the lithium has been consumed before the central temperatures and pressures reach the point that hydrogen "burning" commences.

Near vacuums are not absolute vacuums. Densities of clouds and of wave fronts from exploding supernovae all vary. More particles per cubic metre, equals more pressure. This is simple physics.

No. In case you had trouble accessing paddoboy's links, the dust is made up of very small particles of silicates and organic molecules, in some cases coated with ices.

Carcano, either we are doing an exceptionally poor job of explaining this to you, or you are resisting our explanations because they conflict with some fundamental belief you hold. It would be helpful to know which.

 

Perhaps the use of the term 'proto-star' should be replaced with 'brown dwarf'.

http://en.wikipedia.org/wiki/Brown_dwarf

"Brown dwarfs heavier than about 13 MJ are thought to fuse deuterium, and those above ~65 MJ fuse lithium as well."

 

Again, I am addressing an era of time prior to ignition so to speak...before there was a sun to gravitate to.

However, I still think you are making a good point of critical importance here...as it relates to element ratios.