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baskerbosse
2010-Jul-28, 11:36 PM
Hi everyone,

I recently (a couple of months ago) found the Astronomy cast podcast, and I'm working my way through this excellent series of poadcasts.
While listening to episode 128 about dust the other day, there was something I did not understand:

If the dust generated by supernovae is like cigarette ash, -where do the big rocks come from?
And why do they seem to be so 'sorted'. -Some are iron, some are silicates, some are ice.
If they are formed by combining small particles, wouldn't they be all mixed up?
Don't the bigger rocks form in supernovae as well?
(How else would you end up with a lump of iron)

/Peter

Hornblower
2010-Jul-29, 12:32 AM
Hi everyone,

I recently (a couple of months ago) found the Astronomy cast podcast, and I'm working my way through this excellent series of poadcasts.
While listening to episode 128 about dust the other day, there was something I did not understand:

If the dust generated by supernovae is like cigarette ash, -where do the big rocks come from?
And why do they seem to be so 'sorted'. -Some are iron, some are silicates, some are ice.
If they are formed by combining small particles, wouldn't they be all mixed up?
Don't the bigger rocks form in supernovae as well?
(How else would you end up with a lump of iron)

/Peter
When a cloud of gas and dust undergoes gravitational collapse and starts clumping into large bodies, tremendous amounts of heat and pressure build up in those bodies. The interior pressure is from the weight of the overlying material, and the heat is the result of transforming the kinetic energy of the incoming stuff as it grinds to a halt. This action melts the dust, forming a molten protoplanet. Once the clumping has run its course and everything stabilizes, the protoplanet cools down and the surface material solidifies. It is here that the big rocks form, not in the original supernova blast.

If the protoplanet is big enough, I would expect the heavy stuff such as iron to settle toward the core, while the lighter silicates float above it. Water is less dense yet, and as such would form an icy crust under favorable conditions.

I would expect that iron meteoroids are fragments from collisions of differentiated protoplanets.

baskerbosse
2010-Jul-30, 02:57 AM
Sounds reasonable, but how large would that body have to be for the heat to build up sufficiently?
That would have to be a considerable weight in order to melt the core.
(I can't see kinetic impact energy of cigarette ash causing much heat)

I just listened to the asteroid belt episode. It appers to consist of types: carbon objects, Silicate, iron and ice.
It was said that the asteroid belt was never being given the chance to form a planet.
If that is the case, I still don't get how this works, unless the rocks come from planets getting smashed to bits in the infant solar system?

(still confused, but on a higher level)

/Peter

loglo
2010-Jul-30, 03:40 AM
It doesn't matter what the material is made of, if you ram a few billion tons of it together you are going to get a lot o heat! :)

Ken G
2010-Jul-30, 04:34 AM
Sounds reasonable, but how large would that body have to be for the heat to build up sufficiently?I believe the limit is a large asteroid, and it might be similar to the limit needed to be spherical. Basically, if the gravity is strong enough to shape it, it's strong enough to heat it. But don't quote me.

I just listened to the asteroid belt episode. It appers to consist of types: carbon objects, Silicate, iron and ice.I don't think they could be very pure though, unless they came from parent bodies that differentiated (that's the term for smelting the metals and sinking them the core). At higher temperatures closer to the Sun, metals stick together better than silicates, because it is above the silicate melting point. So that tends to select for more metallic objects closer to the Sun. Maybe they then get stirred into the asteroid belt later on, I really don't know. Maybe there's also a tendency for like compounds to stick together better, though I haven't heard that. I think the key is that molecular and ionic forces are responsible for the initial buildup, and only after the object gets pretty large does gravity kick in. Of course once gravity kicks in, you get a kind of runaway effect, and it can lead all the way to a gas giant.

If that is the case, I still don't get how this works, unless the rocks come from planets getting smashed to bits in the infant solar system?

(still confused, but on a higher level)I believe your question is at the research frontier, but it's not an area I can speak to with any particular experience.

Nereid
2010-Jul-31, 12:24 AM
Hi everyone,

I recently (a couple of months ago) found the Astronomy cast podcast, and I'm working my way through this excellent series of poadcasts.
While listening to episode 128 about dust the other day, there was something I did not understand:

If the dust generated by supernovae is like cigarette ash, -where do the big rocks come from?
And why do they seem to be so 'sorted'. -Some are iron, some are silicates, some are ice.
If they are formed by combining small particles, wouldn't they be all mixed up?
Don't the bigger rocks form in supernovae as well?
(How else would you end up with a lump of iron)

/Peter(bold added)

Icy vs the rest (stony, metallic, whatever) is easy to understand: objects which end up inside the 'snow line' lose their ices very quickly (on astronomical timescales), just as comets create such a spectacular show.

The snow line may be understood, at a high level, as the radius - from the parent star - where a solid object in thermal equilibrium is hot enough for ices (water ice, methane ice, ammonia ice, carbon dioxide ice, mixtures of these, ...) to turn into vapour. As all the ices (except CO2) are rich in H, and as H is dominant element*, undifferentiated matter from a molecular cloud will have far, far more of these than any other element or compound. So, while icy bodies certainly have some silicates and iron (actually, iron-nickel) - as well as very minor amounts of other stuff - they are 'icy' to a first approximation.

CO2? After H and He, the next most common elements include C and O! So CO2 (and CO) is common.

An undifferentiated blob of stuff inside the snow line does, in fact, contain lots of carbon compounds (hence carbonaceous asteroids, meteorites, etc); there'd also be a fair amount of silicates and iron-nickel, basically unprocessed from the molecular cloud (other than chemical reactions that happen at temperatures objects in thermal equilibrium allow, inside the snow line).

As has already been noted, the iron, stony-iron, and rocky/stony asteroids/meteorites come from the break up of objects that got hot enough to differentiate.

* helium is #2, and of course it never forms a solid, or even a liquid, in the ISM

Nereid
2010-Jul-31, 12:30 AM
Sounds reasonable, but how large would that body have to be for the heat to build up sufficiently?
That would have to be a considerable weight in order to melt the core.
(I can't see kinetic impact energy of cigarette ash causing much heat)

I just listened to the asteroid belt episode. It appers to consist of types: carbon objects, Silicate, iron and ice.
It was said that the asteroid belt was never being given the chance to form a planet.
If that is the case, I still don't get how this works, unless the rocks come from planets getting smashed to bits in the infant solar system?

(still confused, but on a higher level)

/Peter(bold added)

Not planets, but planetesimals. Think of them as asteroids large enough to have been hot enough to allow at least some differentiation (do you know what this is?), up to Mars-sized objects. They certainly formed, and they certainly collided! The surfaces of Mercury and the Moon contain some vivid examples of just such collisions, and the Moon itself is thought to have formed as a result of a collision between particularly large planetesimals (do you know why there are essentially no examples of these collisions/planetesimals on the surfaces of the Earth and Venus, and why only parts of Mars have such evidence?).

HypersonicMan
2010-Aug-01, 09:28 PM
Let's take a step back here and go through how we think the process involved in planet formation works.

1) Dying stars make lots of "dust" during their death throes. Supernovae produce some stuff, but red giant stars also make quite a bit, too. This dust gets blown out into interstellar space, but occasionally clumps together with gas in star-forming regions called giant molecular clouds (like the Orion nebula). This dust is fine grained stuff, like cigarette ash. There is a certain "cosmic abundance" of various elements that make up the dust particles that reflects the formation process in dying stars. Silicon and oxygen are quite common, as well as magnesium and iron, so iron/magnesium silicate is one of the most common components of dust. Carbon is also relatively common. Less common are things like gold and strontium and whatnot.

2) Newly created stars are born out of the gravitational collapse of clumps of mostly gas (hydrogen and helium) plus a bit of dust. Our sun formed in this way, and during the formation process it sported a gas and dust accretion disk.

3) The accretion disk is where things get interesting. Star formation is a very energetic process. You are basically taking material from interstellar space (the giant molecular cloud) and squeezing it down into the gravity well of a newly-formed star. It tends to get very very hot in places. Down near the forming sun (say 1 AU, which is the current Sun-Earth distance) the disk would have been so hot early on that all the dust would have basically vaporized.

4) At some point the new sun would have stopped accreting new material, and the disk would start to cool down. As it cooled, all the vaporized rock-forming material would start to condense as a new generation of dust. The first stuff to condense in the inner part of the disk are the high-temperature things like calcium and aluminum-rich minerals. It also turns out that the sun looks like it formed near a supernova, which seeded the disk full of highly radioactive material like the aluminum isotope 26Al. This will become important shortly. Also, the disk was a highly dynamic place, so some new dust in the hot inner solar system got mixed in with the old dust in the outer solar system. Some bits of dust went through heating events of some kind and melted and stuck together into little droplets of material that, when cooled, are called chondrules.

5) Next, the bits of newly formed dust (mostly in the inner solar system) and old dust (mostly in the outer solar system) globbed together into "planetesimals" (tiny planets). This process is very poorly understood, but my favorite model right now involves turbulent eddys that concentrated of dust, chondrules, and other assorted bits of material into big gravitationally bound aggregates that collapsed to form roughly 100 km sized planetesimals. The asteroids belt (the source of most of our meteorites) likely formed in this way.

6) Whether or not a particular planetesimal melted and differentiated depended on *when* it formed. Remember 26Al? It has a very short radioactive decay half-life. Planetesimals that formed early would have incorporated lots of 26Al and then subsequently melted. Planetesimals that formed later would have incorporated less 26Al, because 26Al was constantly decaying into stable 26Mg. Our meteorite record shows us that the asteroids in the asteroid belt underwent a huge variety of thermal histories. Some asteroids differentiated completely (Vesta, the source of other V-type asteroids, the parent bodies of iron meteorites). Some asteroids partially differentiated (Ceres), or didn't differentiate at all but got hot enough to mobilize water in their interiors, and some asteroid were hardly heated at all. Most asteroids in the main asteroid belt did not differentiate, but remain in a "primitive" state (if somewhat thermally altered), and therefore contain all the original bits of chondrules and dust that globbed together when they formed. Some bits of cometary dust that originate from the outer solar system where the disk never got hot enough to melt the dust actually contains some material older than the solar system, like nanodiamonds that were formed in dying stars.

7) In the inner solar system, lots of planetesimals collided and mushed together to form bigger objects called planetary embryos. The embryos then collided with each other to make planets. For some reason (likely due to the influence of Jupiter) the asteroid belt could not retain enough planetesimals to make planets, so planet formation there got "stalled" (which is convenient, as we can use it to help piece together the above story). The asteroids collided with each other, and over the age of the solar system have produced lots of fragments, some of which end up on Earth as meteorites.

astromark
2010-Aug-01, 10:24 PM
:eh: back in post 5 Ken said, " Don't quote me." ... well sorry Ken. I will. Balanced well reasoned SCIENCE. I find your understanding inspiring.

In this Q and A... you are valued and therefor will get quoted.

I do not see concrete rules for planet or planetoid formation... Flexible understanding. The mass of the objects in a accretion disk is the governing factor. I think we understand it well...

Ken G
2010-Aug-02, 01:57 AM
6) Whether or not a particular planetesimal melted and differentiated depended on *when* it formed. Remember 26Al? It has a very short radioactive decay half-life. Planetesimals that formed early would have incorporated lots of 26Al and then subsequently melted. Planetesimals that formed later would have incorporated less 26Al, because 26Al was constantly decaying into stable 26Mg. I warned not to quote me-- here we have a much more detailed accounting (thanks, that was a tour de force), with a stress on a very interesting process: differentiation due to a very "special" seeding of our solar system, early on, by radioactive 26Al, which apparently played a key role in increasing the amount of differentiation over and above what simple gravitational heating (of the larger bodies) could do.

I had not appreciated how "special" this might make our solar system. Note that any time we find something quite unusual about our own planet or solar system, we immediately wonder if it played a key role in the appearance of life, because it would be unexpected for any particularly unusual thing to have happened otherwise. If it does turn out to be important for life, and it does turn out to be quite unusual, its discovery reduces the potential for life elsewhere. I wonder if that is also the case here, with 26Al and the timing of a nearby supernova? Or maybe, it's not so unusual after all, because star-forming regions where you are making low-mass solar systems are generally dotted with high-mass stars that go supernova. That might mean that many low-mass systems are bathed early on in radioactive elements that get incorporated during the planetesmal forming phase, espeically if that phase is not much shorter than the period of supernovas (a few million years).

HypersonicMan
2010-Aug-02, 04:07 AM
Or maybe, it's not so unusual after all, because star-forming regions where you are making low-mass solar systems are generally dotted with high-mass stars that go supernova. That might mean that many low-mass systems are bathed early on in radioactive elements that get incorporated during the planetesmal forming phase, espeically if that phase is not much shorter than the period of supernovas (a few million years).

There are basically two kinds of star forming environments: Massive clouds that make massive stars that can go supernova (the "Orion-type" clouds) and smaller clouds that don't have enough mass to make massive stars (the "Taurus-type" clouds). Orion-type clouds are rare, but they are huge star-making factories and crank out stars by the millions. Taurus-type clouds are more numerous, but individually don't make nearly as many stars. From what I understand, the way the numbers work out that Orion-type clouds end up making most of the solar type stars, but maybe not by a very large margin. Because of the evidence of supernova products in the makeup of our solar system (supernova products that had to have been made very close in time to when our solar system formed), we think that our sun was made in an Orion-type cloud. Lots of solar type stars were probably made in Taurus-type clouds, however, and therefore could never have incorporated highly radioactive supernova products like 26Al and 60Fe. Even in Orion-type clouds, there is enough room in them that probably lots of solar type stars can form far enough away in space or time from supernovae that they don't see these products.

So in some ways our solar system is "special" because of the presence of radioactive supernova products during its formation, but maybe not *that* special.

It's interesting to speculate what a solar system is like without all the radioactive heating agents that supernovae can deliver, as there are probably lots of them out there. They probably wouldn't have iron meteorites, for instance.

Ken G
2010-Aug-02, 06:06 AM
From what I understand, the way the numbers work out that Orion-type clouds end up making most of the solar type stars, but maybe not by a very large margin. Now that's interesting, so our solar system may be a little special, but not a lot special, if indeed it was made in an Orion-type cloud.

Even in Orion-type clouds, there is enough room in them that probably lots of solar type stars can form far enough away in space or time from supernovae that they don't see these products.Then we are nudging toward more special rather than less special, which raises the possibility that the 26Al, causing increased differentiation, might be important for advanced life.


It's interesting to speculate what a solar system is like without all the radioactive heating agents that supernovae can deliver, as there are probably lots of them out there. They probably wouldn't have iron meteorites, for instance.Yes, and the extra prize goes to the discovery of how those would have hindered advanced life. Of course, nearby supernovae don't have to aid advanced life, but depending on how special it is, one suspects that it probably does-- given that it happened for us. I can't think of any reason that differentiating the planetesimals would promote the creation of planets like the Earth, and once you get Earth you'll get the same differentiation ultimately, you'd think. Does smelting a bunch of iron change something about how planets form?

Nereid
2010-Aug-02, 06:19 AM
Add to that the fact that the specifics of star/planet formation may well have changed, in our neck of the Milky Way woods, over 5+ Ga, and may well differ somewhat depending on the metallicity of the Orion and/or Taurus clouds.

AFAIK, star formation is a very active area of research, with a range of interesting conclusions regarding things like initial mass function, rate, and how these vary by environment.

Ken G
2010-Aug-02, 06:36 AM
That raises the point that one need not look for reasons that 26Al could have spurred advanced life if there are other aspects that come hand-in-hand with 26Al, so are just as unusual, that are actually the responsible agents. Perhaps it is the other stuff the supernova gave us that was critical, and the 26Al was just a spandrel that came along for the ride, giving us iron dust on our rooftops once we'd gotten smart enough to build houses by using whatever else the supernova gave us.

George
2010-Aug-03, 12:06 AM
I also want to say thanks for the formation summary, HypersonicMan.

I am curious if any fair guesses exist regarding the timing of the supernova. Was it early and, perhaps, a candidate for triggering our regional collapse (followed by likely fragmentation of the GMC)? Or, alternatively and less likely, could the 26Al have delayed the collapse due to the increase in cloud temperature it would have caused, along with other decay elements?

It does seem likely that we had many bright neighbors back in our nursery days. :)

George
2010-Aug-03, 12:20 AM
That raises the point that one need not look for reasons that 26Al could have spurred advanced life if there are other aspects that come hand-in-hand with 26Al, so are just as unusual, that are actually the responsible agents. Perhaps it is the other stuff the supernova gave us that was critical, and the 26Al was just a spandrel that came along for the ride, giving us iron dust on our rooftops once we'd gotten smart enough to build houses by using whatever else the supernova gave us. Interesting point, and iron seems like a good candidate, though Fe60 may not be so nice. :)

HypersonicMan
2010-Aug-03, 04:37 PM
I am curious if any fair guesses exist regarding the timing of the supernova. Was it early and, perhaps, a candidate for triggering our regional collapse (followed by likely fragmentation of the GMC)? Or, alternatively and less likely, could the 26Al have delayed the collapse due to the increase in cloud temperature it would have caused, along with other decay elements?


There are some radioactive supernova products with even shorter half-lives than 26Al that are thought to have been incorporated into the early solar system (mainly by finding their stable daughter isotopes). My copy of Meteorites and the Early Solar System II (a fantastic resource if you're interested in reading in depth on this subject) mentions that 41Ca was present in early solar system solids, with a half life of only 100,000 years. This implies that the first solids in the solar system condensed within about 1 million years of a supernova.

One thing to keep in mind is that most of the interior heat of the Earth (and other rocky bodies) is currently mostly generated from the decay of uranium and thorium, which have very long half-lives (billions of years). So while these elements are made in supernovae, they can persist long enough in time that there is no requirement that they be made locally. Therefore important processes like volcanism and plate tectonics that are driven by internal planetary heat sources don't require that the planetary system formed close to a supernova.

HypersonicMan
2010-Aug-03, 04:43 PM
I also want to say thanks for the formation summary, HypersonicMan.
Or, alternatively and less likely, could the 26Al have delayed the collapse due to the increase in cloud temperature it would have caused, along with other decay elements?


Not likely. The heat due to the collapse of the cloud into the protoplanetary nebula would have dwarfed the amount generated due to trace amounts of radiaoactive isotopes. The nebula is made almost entirely out of hydrogen and helium gas. These heat sources only become important for smallish rocky things.



It does seem likely that we had many bright neighbors back in our nursery days. :)

There's no doubt. It would have been a busy and violent place.

forrest noble
2010-Aug-03, 04:47 PM
baskerbosse,


Question about dust

Hi everyone,

I recently (a couple of months ago) found the Astronomy cast podcast, and I'm working my way through this excellent series of poadcasts.
While listening to episode 128 about dust the other day, there was something I did not understand:

If the dust generated by supernovae is like cigarette ash, -where do the big rocks come from?
And why do they seem to be so 'sorted'. -Some are iron, some are silicates, some are ice.
If they are formed by combining small particles, wouldn't they be all mixed up?
Don't the bigger rocks form in supernovae as well?
(How else would you end up with a lump of iron)

/Peter

Sorry I'm late to the party. I think your question is a good one. I also think a lot of good answers have been given on this thread and as you can probably see there are differing theories on this matter. But I will add my too cents.

When stellar nebula are forming from a dense cloud of this dust that you discussed, these proto-stellar nebula are initially very cold enabling gravity to push them/ pull them together. As this condensation process continues, there is much contact and frictions between these particles which initially have differing relative motions. The frictional heat produced radiates within the cloud as it continues to condense. As gravity takes over the larger bodies "attract" the smaller ones which gain momentum toward the larger, and frictional heat upon contact with additional heat from gravitational compression. There would come a time when the inner atmosphere of this proto-star would be denser than the Earth's atmosphere and much hotter. Liquids of all types would begin to agglomerate via gravity. Solids could be encapsulated by these liquids which would hasten agglomeration via gravity. As the heat centralizes its radiation would accordingly push gases outward from the central area where they could condense in the colder environments of the outer regions of the proto-star. Liquids of all types help the gravitational process since it assists in the "capture" of both solids and liquids by reducing the carom effect following collisions.

In the inner, hotter regions metals would begin to melt and agglomerate as liquids. Solids with higher melting points could be captured by these molten conglomerations via gravity. The heaviest elements would be more likely to remain in the center of this proto-stellar nebula, and the lighter, less dense elements and gasses would accordingly be pushed by radiation farther out.

This brings us to the asteroid belt and meteorites. Metals of like melting points should accordingly be found together more often than metal of differing melting points. Examples are Iron and Chromium which have similar melting points, and gold and uranium which have much lower but similar melting points. Because of their density, the latter would more likely condense near the centers of larger asteroids and planets and more likely be alloyed together in meteorites than elements with widely differing melting points.

Such theories might come in handy and verifiable when it's time for mankind to start mining the asteroid belt.

HypersonicMan
2010-Aug-03, 05:40 PM
As this condensation process continues, there is much contact and frictions between these particles which initially have differing relative motions.


It's not friction that heats up the nebula, it's compression. The diffuse gas of the giant molecular cloud is being compressed as it falls in to the protostar.



The heat radiates within the cloud as it continues to condense. As gravity takes over the larger bodies "attract" the smaller which gain energy and momentum, and frictional heat upon contact and gravitational compression.


I mentioned above that the process of planetesimal formation is poorly understood. We don't know exactly how the first planetesimals formed. The nebular condensates are ashy, dusty things. The classic pictures of planetesimal formation has the little dust grains sticking together into bigger grains due to van deer Waals forces or some other type of "stickiness" inherent in the grains. The problem is, once they get above a certain size (say a few millimeters) it's really hard to get little bits of rock to stick together, and you need to grow them really fast. If you don't grow planetesimals to at least about a kilometer in diameter in a few thousand years, gas drag will push them down into the Sun.

That's why I like some of the new models being developed now that have the first planetesimals being formed from dense clumps inside turbulent eddys within the disk. In these models, you go straight from the dust and mm stuff to 100 km planetesimals. Also, there is some evidence that both the primordial asteroid belt and the primordial Kuiper belt were made primarily of 100 km objects. Even more intriguing, in the Kuiper belt this collapse process could have also led to binary formation, analogous to the way binary stars are thought to form. This could explain why there are so many binary KBOs (upwards of 30% of the cold classical Kuiper belt are thought to be binaries).



There would come a time when the atmosphere of this proto-star is denser than our atmosphere and much hotter. Liquids of all types would begin to agglomerate via gravity. Solids could be encapsulated by these liquids which would hasten agglomeration via gravity. As the heat centralizes its radiation would accordingly push gases outward from the central area where they could condense in the colder environments of the outer regions of the proto-star.


I'm not even sure what any of this means. At the sizes of the first condensates, gravity would not have been very important. And what do you mean by "heat centralizes its radiation?" The protosolar nebula was a big disk that was very hot near the sun and very cold at its outer edges. There was substantial mass transport between the hot and cold regions, but it was probably driven by diffusion and turbulence. It was a big spinning turbulent cloud.



In the inner, hotter regions metals would begin to melt and agglomerate as liquids. Solids with higher melting points could be captured by these molten conglomerations via gravity. The heaviest elements would remain in the center of this proto-stellar nebula, and the lighter less dense elements could be pushed by radiation farther out.


That's not at all what happened. There's not evidence of large-scale density sorting in the solar system. The reason rocky planets are so dense is that the lighter volatile things couldn't condense out of the nebula at the temperatures in the terrestrial planet zone. In the outer part of the disk (say beyond about 3 AU...again an AU is the Earth-Sun distance, so 3 AU is near the outer part of the current main asteroid belt) the temperatures would have been cold enough that water ice could condense (the snow line). Due to the large abundance of oxygen (and, of course, the almost unlimited supply of hydrogen), huge amounts of water ice could condense, meaning that beyond the snow line there was more solid mass to work with, so giant planet cores could form, and subsequently capture lots of the nebular gas.

Outer solar system objects contain plenty of rocky and metallic material. They just contain lots more water. A typical icy satellite, like Callisto or Titan, contains about half rocky stuff and half icy stuff. The rocky half is the same stuff that you'd find in a rocky planet in roughly the same proportions. Magnesium/iron silicates, iron and nickel, and so on.



This brings us to the asteroid belt and meteorites. Metals of like melting points should be found together more often than metal of differing melting points. Examples are Iron and Chromium which have similar melting points, and gold and uranium which have much lower melting points. The latter would more likely condense near the centers of larger asteroids and planets and more likely be alloyed together in meteorites than elements with widely differing melting points.

Melting point has nothing to do with it. The reason certain elements end up in the cores of planets (like nickel and the platinum group elements like iridium) has to do with chemical affinity, not melting point. Some elements dissolve in liquid iron, and some do not. The iron-dissolving elements are called siderophiles, and rock-dissolving elements are called lithophiles. Iron cores do not "condense" at the center of a body (like a planets or asteroid), they separate out of the body if the body heats up enough to melt. Iron, being very dense compared to rock, tends to sink to the center of the body, dragging lots of dissolved siderophiles with it. That's why iridium is a good tracer for extraterrestrial materials, because non-differentiated objects like comets and most asteroids contain far more iridium in them than the surface of the Earth, which keeps its iridium in its core.

George
2010-Aug-03, 05:48 PM
There's no doubt. It would have been a busy and violent place. Yes, I like to think of the formation period as more of a mud wrestling match compared to a symphony. I look forward to even more grand vistas of the dynamic nurseries.

In reading Bally & Reipurth's book, "Birth of Stars and Planets", I see that stellar collisions are deemed likely in the larger nurseries since spacing is bumper-to-bumper. Is there evidence of this; Flashing stars, perhaps?

More to the OP (opening post), it is my understanding that the early disk composition is about the same as the resulting host star's composition (=98% H & He). Also, much of the dust is very small -- smaller than visible light's wavelength. [Time changes that, no doubt, as chondrules form.] Are these views correct?

forrest noble
2010-Aug-03, 06:02 PM
HypersonicMan,

Interesting info


And what do you mean by "heat centralizes its radiation?" In the central region of proto-stars radiated heat would be more intense.


Some elements dissolve in liquid iron, and some do not. The iron-dissolving elements are called siderophiles, and rock-dissolving elements are called lithophiles. Iron cores do not "condense" at the center of a body (like a planets or asteroid), they separate out of the body if the body heats up enough to melt.

Of course this is true but for them to dissolve in iron they would need to have an equivalent or lower melting point if the iron was barely molten and not hot enough to melt the other elements, otherwise they would be encapsulated. Heavier molten/ liquid elements react to gravity by sinking more toward the center of massive liquids than less dense material which would include molten metals. Chemical affinity as you suggest, is important to the extent that elements that could form compounds together would be more likely to be found together if conditions would allow these compounds to form or remain in tact.

baskerbosse
2010-Aug-06, 04:51 AM
Thanks everyone,

This is a very interesting field, I think I will show considerably more interest in meteorite compositions after all this!
I would be very interested in seeing some examples of compositions. And to think that I had just assumed they were all formed in supernovae.. ;-)
(A very complicated process, solar system formation..!)

thanks,
Peter