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Solar system formation
dvandorn
post Dec 3 2008, 05:16 AM
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QUOTE (Juramike @ Dec 2 2008, 11:11 PM) *
Here is a recent article in space.com (I tried to link to this a few days ago, but it disappeared from the space.com archives...today it's back) about how Jupiter may have a much bigger core (14-16 Earth masses of rock!) than previously proposed. Previous predictions ranged from a core of 7 Earth masses of rock to no core at all. Juno should help nail down the absolute size of the core, and therefore, whether a rock core was required for the initial accretion.

Which came first: gas or rock?

And if rock is required to initiate accretion of gas giants... what about stars?

-the other Doug

(This discussion was originally in the Juno thread but was moved to a separate topic - moderator)


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Doc
post Dec 3 2008, 08:56 AM
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Stars from gas giants to form binary systems? Can Juno shed some on this?


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Juramike
post Dec 3 2008, 01:34 PM
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QUOTE (dvandorn @ Dec 3 2008, 12:16 AM) *
And if rock is required to initiate accretion of gas giants... what about stars?

-the other Doug


Stars are failed planets!?!? laugh.gif laugh.gif laugh.gif


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dvandorn
post Dec 3 2008, 05:25 PM
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Not failed planets -- spectacularly successful planets. wink.gif

Seriously, stars are formed, almost by definition, around the largest mass concentrations in a given stellar nursery nebula. What starts that process? For Population I stars, where there is almost nothing except gas in the birth nebula, obviously heavier elements play a very limited role, if any role at all. But for Population II stars, I've always wondered if the star begins with the largest collection of heavy elements (i.e., rocks) in the neighborhood, working from there to gather up enough gas to create such a super-gas-giant that the gas pressure in the interior becomes intense enough to support hydrogen fusion.

Looked at from the opposite side -- if a brown dwarf is a super-super Jupiter and is a failed star, and Jupiter-class planets require rocky cores to begin accretion, then doesn't it track that successful Pop II stars would start their accretion processes in the same manner as gas giants?

Something that has occurred to me more than once is that, if Pop II stars indeed accrete around rocky cores, what state do those cores achieve after several billion years of the temperatures and pressures at the core of their stars?

-the other Doug


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Fran Ontanaya
post Dec 3 2008, 06:04 PM
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Higher momentum and radioactive decay could be other paradigm-shifter factors. I supose big rocky cores would have reached higher temperatures 5Gy ago, and sported proportionally greater magnetic fields.

(I cannot imagine something cooler than an early planet/star working as a Bussard collector, ionizing gas and herding it with its magnetic field)


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charborob
post Dec 3 2008, 07:16 PM
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QUOTE (dvandorn @ Dec 3 2008, 12:25 PM) *
Something that has occurred to me more than once is that, if Pop II stars indeed accrete around rocky cores, what state do those cores achieve after several billion years of the temperatures and pressures at the core of their stars?

Once the nuclear fusion reactions start inside a star, wouldn't the rocky core, if one is present, be eventually vaporized and become mixed with the gas?
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ugordan
post Dec 3 2008, 07:31 PM
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QUOTE (charborob @ Dec 3 2008, 08:16 PM) *
Once the nuclear fusion reactions start inside a star, wouldn't the rocky core, if one is present, be eventually vaporized and become mixed with the gas?


Unless the core is smaller than the region where fusion occurs, I don't see how fusion would even start there in the first place.


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nprev
post Dec 4 2008, 02:41 AM
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I could see dust coalescing into a rocky core as a stellar seed...but it wouldn't be there long. Considering that the Sun's core has more than 10,000 times the volume of the entire Earth & is 150 times as dense as water, I suspect that any such core became nothing but plasma pretty early in the accretion process, probably long before fusion actually started. We're talking about a LOT of matter in one place, here.


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dvandorn
post Dec 4 2008, 08:19 AM
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I think some kind of plasma form for the rocky core is most likely, by the time a star ignites. But the natural process of gravity is such that the heaviest elements in the molten rocky core (prior to its plasma-ization) would have settled in the very center, the core of the core, so to speak. The lighter elements, the silica and iron and aluminum, et. al., would have formed shells around the presumed nickel-iron-uranium-etc. core.

As such a highly differentiated core is pummeled by the energy of its immediate "atmosphere" fusing around it, it would dissociate into atomic forms, I imagine, but at those incredible temperatures and pressures, and with that many photons trying to force their way into the heavy element mass, you'd think that generally symmetrical pressures all around would keep the heavy elements in the center.

Now, if there is *turbulence* in the fusing atmosphere around the heavy elements, uneven or patterned flow of fusing plasma around the core, it could possibly be eroded, great hunks of heavy elements being picked up and tossed higher into the gas plasma. But being heavier, you'd think that even in the seething energy environment, they'd still evenutally sink back down to the center.

Also, as a star ages, more and more of its mass is made up of helium, the fused by-product of hydrogen fusion. The helium is heavier than the hydrogen, and sinks down toward the core, where the very high temperatures and pressures cause the helium to fuse, making lithium. The lithium continues to sink, and at some point it fuses, on through a process that creates elements as heavy as iron. So the star is slowly forming even more heavy elements, which I would imagine would sink and collect around the original heavy-element core.

So, even though you might think that a rocky stellar core might just be vaporized, and you'd be sort of correct, in the conditions at a stellar core the vapor would have nowhere else to go but to be compressed and sink back into the center of the core. And as the star gets older, it creates more and more heavy elements until the original heavy element abundance has increased greatly.

Of course, when the star is big enough, its rapid expansion as the last of the hydrogen is used up and the associated sudden light pressure release blows the star apart. The explosion occurs *between* the core and the surface of the star, though, causing an equally forceful implosion that blasts the heavy-element core so hard as to make iron fuse, and heavier elements fuse, creating the true heavy elements, and another supernova seeds clouds of gas with even more raw materials for new stars, planets and moons.

The really interesting information we might be able to extrapolate from Juno data is the dynamics of the core. The temperatures and pressures at the core of Jupiter may well be enough to have caused any rocky core it once had to have settled into a differentiated heavy element plasma ball. Understanding those dynamics would be very enlightening in extrapolating to the later phases of stellar core evolution about which I have been speculating above.

-the other Doug


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AndyG
post Dec 4 2008, 09:25 AM
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Is see from this site that the contents of the sun are (% by mass):
  • 75% hydrogen
  • 23% helium
  • 0.9% oxygen
  • 0.1% iron
  • 0.09% silicon
  • 0.008% nickel
  • 0.902% everything else
The Earth from this site is:
  • 31% iron/nickel
  • 69% everything else (mainly silicates - SiO2)
Which suggests there's enough material for ~900 Earth mantles and ~1200 Earth cores in the sun. Say 1000 Earths? (About 0.3% of the sun's mass.)

Caveat - the abundances for the sun are from spectroscopic surface observation. While mixing does go on, is the photosphere a good reflection of the deeper solar core? Could the sun contain considerably more than 1000 Earthworths of building material?

Andy
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tanjent
post Dec 4 2008, 02:26 PM
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This modeling of stellar interiors calls to mind an article I read many years ago, I think in Scientific American, about what happens when two stars happen to collide. Assuming that they hit directly enough to avoid falling into some kind of a spinning dance with each other, I recall the assertion was that the denser one would often just pass through the lighter one. In particular white dwarves are apparently dense enough to blow through other stars in this manner. It boggles the mind to envision matter as dense as the stellar cores we are discussing but nonetheless offering so little resistance when the "irresistable force meets the immovable object". Of course those models may be obsolete now. I don't really recall much about how the results were obtained.
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Doc
post Dec 4 2008, 03:19 PM
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Yep, thats definitely an old model;-)
The interacting gravitational forces should tear up the less dense star. On the other hand, stars colliding head on...? :-/


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Fran Ontanaya
post Dec 4 2008, 04:40 PM
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Armchair Q. Wouldn't the atoms break apart under such temperature, pressure and radiation and, after a while, make an H-only star's core become undistinguishable from a dirty star's core? The fussion of that soup, up to Iron, would happen high enough that protons and neutrons can stick together faster than they are broken apart, beyond the 'surface' of the core. (That would be fission/fussion cells, analog to convective cells.)


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Juramike
post Dec 4 2008, 05:43 PM
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While the atomic electron clouds and probability shells will break apart and make a soup at stellar core pressures and temperatures, I think the nuclei themselves remain intact.

The thing I'm not sure of (and echo's Gordan's point above) is if a soup composed of larger nuclei would hinder the fusion of lighter elements.

Would a soup of silicon nuclei and deuterium nuclei fuse slower than just a soup of deuterium nuclei alone?

Or are there funky catalytic cycles that could make things easier (like a heavier element version of the CNO cycle)?

[I'll admit total ignorance here.]

-Mike


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stevesliva
post Dec 4 2008, 05:46 PM
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I can't speculate, but some people have:
http://adsabs.harvard.edu/abs/1988Ap&SS.144..519R
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