How does gravity work to make a star ?

[dougettinger]

Chris Peterson quoted just recently, "Still the area around a protostar - even the acretion disk around it - is very low density. The only zone that actually ends up with a density greater than a hard vacuum is the center of a proto-star itself - a tiny volume compared to the entire system."

I calculated my own estimate of a proto-star disk by estimating its original volume being thin and original mass being five times a solar mass and the figures are not much greater than the determination for the density of an intermolecular cloud (IMC). I never fully realized before how very tenuous the gas and dust field really is that makes up a proto-star disk.

How does a central dense core appear from this tenuous cloud ? How does the initial gravity of this core of dust and gas even begin to affect the other gas and dust particles that are ten or even a fraction of an AU away ? How is there enough inertia created to form a disk from a irregular shaped cloud ? I am starting to become a non-believer of the present solar system formation hypothesis.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

I calculated my own estimate of a proto-star disk by estimating its original volume being thin and original mass being five times a solar mass and the figures are not much greater than the determination for the density of an intermolecular cloud (IMC). I never fully realized before how very tenuous the gas and dust field really is that makes up a proto-star disk.

How does a central dense core appear from this tenuous cloud ? How does the initial gravity of this core of dust and gas even begin to affect the other gas and dust particles that are ten or even a fraction of an AU away ? How is there enough inertia created to form a disk from a irregular shaped cloud ? I am starting to become a non-believer of the present solar system formation hypothesis.

It doesn't take a high density, just a regionally high mass and a density gradient. The gas and dust in such a region all has the same velocity relative to distant objects- or, maybe a better way of saying it, it all has near zero velocity relative to itself. So all you need is a small density fluctuation, as from some sort of shock wave, and with time the material will come together. It's a positive feedback situation; as the material gets closer, the gravitational gradient increases, and the material comes together faster.

This can actually be simulated with a completely physical model, so there's little reason to doubt the physicality of the theory (not hypothesis) describing stellar system formation.

[dougettinger]

This referred physical computer model has to be a very critical part of any stellar system formation theory. I suspect the shock wave front is only a small portion of the width of the entire affected cloud (?) For these models what do you suppose the velocity vector differential is between the shock wave and the IMC ? Does the shock wave itself introduce materials that may be clumpy ? Does the material get pushed into a pile like a bulldozer can push sand into a pile ?

Perhaps these questions are too detailed. Maybe I could be referred to a site that discusses these physical models. I still have a very difficult time sensing how a sizable core is created to have enough gravity, the weakest known force field, reach outward in AU units to tug on particle size matter to create a spiraling solar system like our own. In other words, I am questioning the assumed initial conditions and any skipping of time spans to achieve a pre-conceived result.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

This referred physical computer model has to be a very critical part of any stellar system formation theory. I suspect the shock wave front is only a small portion of the width of the entire affected cloud (?) For these models what do you suppose the velocity vector differential is between the shock wave and the IMC ? Does the shock wave itself introduce materials that may be clumpy ? Does the material get pushed into a pile like a bulldozer can push sand into a pile ?

Shock waves are typically slow. They are produced by material that is moving rapidly through the IMC. It is because the shock fronts are slow that there is time for gravitational clumping to occur.

I still have a very difficult time sensing how a sizable core is created to have enough gravity, the weakest known force field, reach outward in AU units to tug on particle size matter to create a spiraling solar system like our own.

The absolute strength of the force isn't really important. What matters is that gravity is the dominant force in an interstellar cloud. You could put a marble in a uniform cloud, and eventually a star will form from the density gradient it produces. How could it be otherwise, unless you posit other forces that affect the cloud more strongly than gravity? But evidence of that is lacking.

[dougettinger]

I do not understand why a shock wave from probably a supernova would be slower that the velocity vector of a certain IMC (?) How does a random cloud obtain more velocity than a blast of material from a supernova ?

You used the example of a marble. How is a marble created from a vacuum that has a few tenuous particles ? And if the marble was the beginning of, say our Sun, then it has to be moving at about 250 km/s, the velocity of our current Sun. And the velocity vector of the marble should be approximately at a right angle to the moving cloud of matter. Does the physical computer model indicate a reasonable time for the marble to collect particle by particle to create a solar mass? By reasonable time, I mean less than 13 billion years.

One thing is being made clear by the present theory. The only working tool is gravity. Obviously, there is not enough heat energy and/or plasma remaining in either in the IMC or the shock wave to create electromagnetic forces ?

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

I do not understand why a shock wave from probably a supernova would be slower that the velocity vector of a certain IMC (?) How does a random cloud obtain more velocity than a blast of material from a supernova ?

You've misunderstood me. The material from the supernova moves very fast with respect to the IMC. The shock wave is what moves slowly. The shock front is an interaction between the material in the IMC and the material from the supernova- it is not either material by itself.

You used the example of a marble. How is a marble created from a vacuum that has a few tenuous particles ?

The marble was just an example to illuminate the idea that any density gradient will result in a positive feedback situation, leading to clumping and larger gradients with time. And the reality is that no IMC is 100% uniform. It contains density gradients simply because of the statistics of diffusion, and probably contains material much larger than dust, as well- a full range of material from dust-sized to planet-sized, according to some sort of power-law.

And if the marble was the beginning of, say our Sun, then it has to be moving at about 250 km/s, the velocity of our current Sun. And the velocity vector of the marble should be approximately at a right angle to the moving cloud of matter.

I don't follow your logic here. The IMC that our system formed in was moving in about the same orbit as the Sun, but the clumps that seeded star formation were moving at the same speed as the entire cloud. Why do you assume the marble is moving within the cloud?

Does the physical computer model indicate a reasonable time for the marble to collect particle by particle to create a solar mass? By reasonable time, I mean less than 13 billion years.

My understanding is that these models show stellar systems forming in perturbed gas and dust clouds with time scales of a few million years. That is consistent with observation.

One thing is being made clear by the present theory. The only working tool is gravity. Obviously, there is not enough heat energy and/or plasma remaining in either in the IMC or the shock wave to create electromagnetic forces ?

I'm not sure what you mean by "heat energy" in this case. One important source of perturbations in IMCs is the radiation from surrounding stars. This certainly creates density gradients that can either seed or disrupt star formation. There is no evidence of electrical field effects inside nubulas. Magnetic fields are sometimes present, and probably produce some structure. So these may play a role in creating density gradients. But for the most part, the fields are weak and the vast majority of the material is non-magnetic and uncharged, so gravity is probably the dominant force.

[dougettinger]

I do not understand why a shock wave from probably a supernova would be slower that the velocity vector of a certain IMC (?) How does a random cloud obtain more velocity than a blast of material from a supernova ?

You've misunderstood me. The material from the supernova moves very fast with respect to the IMC. The shock wave is what moves slowly. The shock front is an interaction between the material in the IMC and the material from the supernova- it is not either material by itself.

The shock wave does slow down as it moves away from the supernova and it does change densities in the interacting gases. Do the materials from the supernova slow down, too, due to some drag function of the shock front ? I am curious to know what amount of density change occurs and what affect different velocity differentials have on the overall outcome ?

You used the example of a marble. How is a marble created from a vacuum that has a few tenuous particles ?

The marble was just an example to illuminate the idea that any density gradient will result in a positive feedback situation, leading to clumping and larger gradients with time. And the reality is that no IMC is 100% uniform. It contains density gradients simply because of the statistics of diffusion, and probably contains material much larger than dust, as well- a full range of material from dust-sized to planet-sized, according to some sort of power-law.

I knew the marble was an example and a good one at that. You are now admitting there will be a full range of sizes of material agglomerations that can include planet-sized. Of course, starting with planet-sized agglomerations can make computer modeling a whole lot easier. You have introduced something called the "power-law". Is this power law still dealing with Newton's laws of motion and gravity? I still am questioning how these tenusous gases and dust can generate a marble size or planet-size agglomeration of matter.

And if the marble was the beginning of, say our Sun, then it has to be moving at about 250 km/s, the velocity of our current Sun. And the velocity vector of the marble should be approximately at a right angle to the moving cloud of matter.

I don't follow your logic here. The IMC that our system formed in was moving in about the same orbit as the Sun, but the clumps that seeded star formation were moving at the same speed as the entire cloud. Why do you assume the marble is moving within the cloud?

I was thinking about a shock front or interacting gas cloud moving faster in order to pile up material in the center to create some core or seed for star growth.

Does the physical computer model indicate a reasonable time for the marble to collect particle by particle to create a solar mass? By reasonable time, I mean less than 13 billion years.

My understanding is that these models show stellar systems forming in perturbed gas and dust clouds with time scales of a few million years. That is consistent with observation.

For these time scales what was the initial and final masses of the agglomerations ?

One thing is being made clear by the present theory. The only working tool is gravity. Obviously, there is not enough heat energy and/or plasma remaining in either in the IMC or the shock wave to create electromagnetic forces ?

I'm not sure what you mean by "heat energy" in this case. One important source of perturbations in IMCs is the radiation from surrounding stars. This certainly creates density gradients that can either seed or disrupt star formation. There is no evidence of electrical field effects inside nubulas. Magnetic fields are sometimes present, and probably produce some structure. So these may play a role in creating density gradients. But for the most part, the fields are weak and the vast majority of the material is non-magnetic and uncharged, so gravity is probably the dominant force.

Materials in the IMC come from the dispersal of nova and supernova remnants. Once these remnants become part of a IMC they have lost there plasma characteristics, cooled , condensed, and formed atomic particles. Then the IMC has no electromagnetic properties. This is my assumption.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

The shock wave does slow down as it moves away from the supernova and it does change densities in the interacting gases. Do the materials from the supernova slow down, too, due to some drag function of the shock front ?

You are still confusing the material moving away from the supernova (which may itself contain shock fronts, but does not have to) with the shock fronts generated in the IMC when fast moving material strikes it. Shock fronts can move either faster or slower than the individual components that make it up. For instance, spiral arms of galaxies are a type of density front, where the stars that make them up are moving faster than the overall spiral structure.

I am curious to know what amount of density change occurs and what affect different velocity differentials have on the overall outcome ?

I have no idea. I would assume only a very small density gradient is required to start a regional collapse of material. And I doubt that velocity plays much role.

I knew the marble was an example and a good one at that. You are now admitting there will be a full range of sizes of material agglomerations that can include planet-sized. Of course, starting with planet-sized agglomerations can make computer modeling a whole lot easier.

Not at all. As far as I know, the models ignore anything larger than gas and dust, because it represents such a small percentage of the total mass that it is insignificant to the process. Even a tiny density variation in a region of dust extending a light year creates a gravitational field gradient orders of magnitude larger than a planet.

You have introduced something called the "power-law". Is this power law still dealing with Newton's laws of motion and gravity?

A power-law describes some sort of scaling relationship. In this case, it would describe the percentage of mass as a function of the particle size. For example, 90% of the mass is made up of gas; 90% of the remaining mass is made up of dust grains smaller than 1 micrometer; 90% of the remaining mass is made up of dust grains smaller than 1mm, and so forth (the numbers are just examples). A power-law like this might tell you that only one ten-billionth of the total mass is made up of particles larger than 1km.

I work with this type of law all the time in my meteor research. A key component of the work is counting the number of meteors of different sizes in order to define the specific coefficients of the applicable power-law. This allows organizations like NASA and ESA to accurately estimate the probability that a particle of a certain size will impact a given space asset, and therefore to rationally determine how much to shield different spacecraft.

I still am questioning how these tenusous gases and dust can generate a marble size or planet-size agglomeration of matter.

Again, I'd ask you how they can fail to do so, unless you assume there is a repulsive force that can overcome gravitational attraction.

For these time scales what was the initial and final masses of the agglomerations ?

I don't know. There are lots of papers about this stuff, and with a bit of research you could track some down. I think the status of the work at this point is that the models demonstrate the physicality of stellar formation by accretion in IMCs, but there are still key elements missing, since the models fail to explain (or poorly explain) a large percentage of observed star systems. Most researchers would probably say that the theories are on the right track and roughly describe what is happening, but need a lot of work still. The important point for the discussion we're having, at this level, is that there appears nothing in the slightest bit unphysical about density fluctuations in IMCs seeding regions of accretion that can become stars. It's not the complete picture, but it's a good first approximation of reality.

Materials in the IMC come from the dispersal of nova and supernova remnants. Once these remnants become part of a IMC they have lost there plasma characteristics, cooled , condensed, and formed atomic particles. Then the IMC has no electromagnetic properties. This is my assumption.

I wouldn't say it has none, but I don't think there's much evidence to suggest that electromagnetic properties play a significant role in the dynamics of cool molecular clouds.

[dougettinger]

Thanks for explaining the "power law". So the power law could determine the possible number of larger agglomerations of matter. Then accretion would begin around these larger groupings of matter. How does a disk form from groupings of matter randomly located in three-dimensional space? Why don't different accretion disks form with various alignments in the same region of several square AUs? These accretion disks could develop substantial angular momentum that should resist being pulled into a common accretion disk that could form a solar system like ours or a typical binary star system.

Do clouds in Earth's own atmosphere gather especially in horizontal directions to the surface because of gravity? I am trying hard to understand a cloud of material in outer space being affected by its own generated gravity field.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

Thanks for explaining the "power law". So the power law could determine the possible number of larger agglomerations of matter. Then accretion would begin around these larger groupings of matter.

I don't think so, at least not until the accretion process is well underway. That process starts because of density gradients in gas, not even dust, let alone the rare larger body.

How does a disk form from groupings of matter randomly located in three-dimensional space? Why don't different accretion disks form with various alignments in the same region of several square AUs? These accretion disks could develop substantial angular momentum that should resist being pulled into a common accretion disk that could form a solar system like ours or a typical binary star system.

A disc is the only possible solution when you have material that interacts, as normal matter does. As material comes together, it becomes fluid-like, subject to the laws of fluid dynamics. Angular momentum is transferred between particles, and is conserved. Material is either brought into the dominant plane, or is ejected from the system.

Do clouds in Earth's own atmosphere gather especially in horizontal directions to the surface because of gravity? I am trying hard to understand a cloud of material in outer space being affected by its own generated gravity field.

Only indirectly. Gravity produces pressure and temperature gradients in the atmosphere, which affect cloud formation and structure.

A cloud in space will be non-uniform, due to external influences and simple statistics. Within that cloud, locally dense regions will preferentially attract material from nearby. The result is that the dense regions get denser and the sparse regions get sparser. Eventually, the dense areas become hot enough to start fusion, stars are born, and they blow away the remaining dust and gas in the region. You're left with a bunch of new stars in a relatively dust and gas free region of space.

[dougettinger]

Does the dense region eventually create a vortex in the direction of its travel that then turns into an accretion disk ?

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

Does the dense region eventually create a vortex in the direction of its travel that then turns into an accretion disk?

You get an accretion disc as material comes together, and the plane and direction of the accretion disc are determined by the net angular velocity of the components. The direction the material is moving, and its velocity, is largely irrelevant.

[dougettinger]

However, if a clump of material or a planetisimal approaches an accretion disk closely within the plane and near the edge of the disk while having the same translational vector as the disk and similar tangential vector, then the clump should be captured. Is this correct ? Any subsequent capture of materials, especially clumps, after the accretion disk has formed is heavily dependent on the alignment of velocities and vectors of the disk and the clump.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

However, if a clump of material or a planetisimal approaches an accretion disk closely within the plane and near the edge of the disk while having the same translational vector as the disk and similar tangential vector, then the clump should be captured. Is this correct ? Any subsequent capture of materials, especially clumps, after the accretion disk has formed is heavily dependent on the alignment of velocities and vectors of the disk and the clump.

Yes, sort of. Any material will be captured (meaning it will end up in a closed orbit) if its velocity at any point becomes less than the escape velocity of the forming system at that point. With gas and dust, collisions and fluid dynamics play an important role in reducing velocity. With larger material, there may be drag or minor collisions, but most likely larger material would have less interaction and therefore would be less likely to end up in orbit, unless it happened to pass very close to the center, once that center had become very dense.

[dougettinger]

One important condition I placed on this clump (let's call it a planetisimal) is that it has similar velocity vectors for the translation and spin of the accretion disk. I believe you concur that capture is very possible if fluid dynamics of collisions and drag also play their part.

Now it is time to spring the next question which you have heard before. But I present it in a similar fashion as this previous discussion. Using the same parameters of similar velocity vectors for the Sun's velocity (translation) and say the orbital velocity of Uranus or Neptune (tangential), is it possible for dust, small or large planetisimals to be captured from interstellar space by the solar system ? Assume that there are similar fluid dynamic properties of minor collisions, drag of the solar wind, gravitational pull of the outer planets including the newly discovered minor planets beyond Pluto. Of course, it is understood that the range of parameters for a capture are very narrow.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

Now it is time to spring the next question which you have heard before. But I present it in a similar fashion as this previous discussion. Using the same parameters of similar velocity vectors for the Sun's velocity (translation) and say the orbital velocity of Uranus or Neptune (tangential), is it possible for dust, small or large planetisimals to be captured from interstellar space by the solar system ? Assume that there are similar fluid dynamic properties of minor collisions, drag of the solar wind, gravitational pull of the outer planets including the newly discovered minor planets beyond Pluto. Of course, it is understood that the range of parameters for a capture are very narrow.

Again, thinking this way confuses the issue. All bodies in the observable Universe are in orbit around the Sun. What you are interested in, I think, are those in closed (elliptical) orbits. Any such body will have a velocity less than the escape velocity at its distance. Meet that requirement, and you are in a closed orbit.

If a body is in an open (parabolic or hyperbolic) orbit to begin with, it cannot be captured by the Sun alone. That basically includes all dust, planetesimals, or anything else coming from "outside" the Solar System. In order to be captured, they have to shed some velocity. In a protostar system, they can easily do that via drag mechansisms. But our system is now very sparse, so the mean distance between collisions is large. Incoming material will not be able to lose much velocity this way. That leaves the transfer of momentum through a three-body interaction. For anything of macroscopic size, that is the only realistic way for the Sun to capture it. In almost all cases, that would mean a body in a hyperbolic path with respect to the Sun would be converted to a body in an elliptical path around the Sun by transferring momentum to Jupiter, in a near pass. That's the same thing that happens when long period comets are converted to short period. The dynamics are very similar to how you analyze extrasolar material (which long period comets very nearly are, in terms of orbital dynamics).

[dougettinger]

I am absolutely enthralled with your explanation: "_ _ _ that would mean a body in a hyperbolic path with respect to the Sun would be converted to a body in an elliptical path around the Sun by transferring momentum to Jupiter, in a near pass." You have a beautiful way of expressing the very difficult three(or more)-body interaction problem.

I now confess that I have a different, perhaps radical, idea for strange events and anomalies that have occurred in our pristine solar system. I have respect for the "Nice" Theory and the Oort Cloud but I favor my own. The common supernova events of spiral galaxies not only produce dust clouds, higher metals, but also produce an almost infinite number of condensed planetisimals of all sizes and in various groupings that infrequently and randomly are captured by our solar system ever since its conception. Probably during the 3.9 billion year Later Bombardment Period an unusual number and larger sizes of these planetisimals were captured. You see - I need this hypothesis to satisfy a reason for the Earth-Moon system. The "Nice" Theory has its problems.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

I now confess that I have a different, perhaps radical, idea for strange events and anomalies that have occurred in our pristine solar system. I have respect for the "Nice" Theory and the Oort Cloud but I favor my own. The common supernova events of spiral galaxies not only produce dust clouds, higher metals, but also produce an almost infinite number of condensed planetisimals of all sizes and in various groupings that infrequently and randomly are captured by our solar system ever since its conception. Probably during the 3.9 billion year Later Bombardment Period an unusual number and larger sizes of these planetisimals were captured. You see - I need this hypothesis to satisfy a reason for the Earth-Moon system. The "Nice" Theory has its problems.

I think it is doubtful that a supernova produces much in the way of planetesimals. It's that power-law thing I discussed earlier. Nobody knows the actual coefficients, but I think most people do think that the curve is very steep- that only a tiny, tiny fraction of the total mass ejected in a SN is larger than dust grains.

From an observational standpoint, you need to concern yourself with meteorites, lunar samples, micrometeorites, and recovered material from space missions. All of these can be reliably dated, and everything ever examined dates to the beginning of the Solar System. Older material has been found inside meteorites, so we know something about the material that the Solar System formed from. Nothing has been found that could have realistically come from outside the Solar System. So there is a solid statistical argument that if anything is ending up captured by the Sun or colliding with Solar System bodies, its total mass must be a very small fraction of what is already here in the form of comets and asteroids. So small that out of thousands of kilograms of samples, we haven't yet found anything extrasolar.

I'm not sure what your issue is with the theory of Moon formation, but there is no evidence that any material making up either the Moon or Earth originated at a different time than the formation of the Solar System. If an extrasolar body were involved, you would certainly expect such evidence in the form of material with very peculiar dating.

[dougettinger]

I now confess that I have a different, perhaps radical, idea for strange events and anomalies that have occurred in our pristine solar system. I have respect for the "Nice" Theory and the Oort Cloud but I favor my own. The common supernova events of spiral galaxies not only produce dust clouds, higher metals, but also produce an almost infinite number of condensed planetisimals of all sizes and in various groupings that infrequently and randomly are captured by our solar system ever since its conception. Probably during the 3.9 billion year Later Bombardment Period an unusual number and larger sizes of these planetisimals were captured. You see - I need this hypothesis to satisfy a reason for the Earth-Moon system. The "Nice" Theory has its problems.

I think it is doubtful that a supernova produces much in the way of planetesimals. It's that power-law thing I discussed earlier. Nobody knows the actual coefficients, but I think most people do think that the curve is very steep- that only a tiny, tiny fraction of the total mass ejected in a SN is larger than dust grains.

Thanks for discussing my personal hypothesis. One part of my hypothesis explains how the environment after a supernova is conducive to creating aggolomerations of matter into ready-made differentiated cores that also assemble themselves into systems. Both gravity and electromagnetic forces operate in this environment.

From an observational standpoint, you need to concern yourself with meteorites, lunar samples, micrometeorites, and recovered material from space missions. All of these can be reliably dated, and everything ever examined dates to the beginning of the Solar System. Older material has been found inside meteorites, so we know something about the material that the Solar System formed from. Nothing has been found that could have realistically come from outside the Solar System. So there is a solid statistical argument that if anything is ending up captured by the Sun or colliding with Solar System bodies, its total mass must be a very small fraction of what is already here in the form of comets and asteroids. So small that out of thousands of kilograms of samples, we haven't yet found anything extrasolar.

By your power law most of the exosolar materials have ceased to exist or are extremely rare. Rocky collisional items such as astreroids and meteorites are from existing solar system bodies. The exosolar bodies creating the collisions were mostly volatile materials that have dissipated or been combined with solar materials. It would be very interesting to obtain dating for the rings of the outer planets and for Kuiper Belt planetoids.

I'm not sure what your issue is with the theory of Moon formation, but there is no evidence that any material making up either the Moon or Earth originated at a different time than the formation of the Solar System. If an extrasolar body were involved, you would certainly expect such evidence in the form of material with very peculiar dating.

Definte dating was obtained for the Later Bombardment Period. Very young isotopes were discovered on one or more comets that reveal an exosolar nature. This data was blamed on the solar system passing through a supernova remnant. Perhaps these were captured planetisimals.

Doug Ettinger
Pittsburgh, PA

[bystander]

Making stars: Studies show how cosmic dust and gas shape galaxy evolution
University of Chicago | 22 Nov 2010

[dougettinger]

Thank you for this very interesting reference. It explains the importance of cosmic dust aiding in making 2nd and 3rd generation stars. I am not sure it helps to explain how first generation stars were created when no dust existed.

Doug Ettinger
Pittsburgh, Pa

[Chris Peterson]

Thank you for this very interesting reference. It explains the importance of cosmic dust aiding in making 2nd and 3rd generation stars. I am not sure it helps to explain how first generation stars were created when no dust existed.

Dust isn't a necessary requirement for star formation. It is more like a catalyst, which makes the process more efficient by shielding gas molecules. But that doesn't mean that gas by itself can't come together to form stars.

[dougettinger]

I will be attempting a simple calculation to show gravity's work in producing a small agglomeration of matter from an IMC. I will use two different starting points, one with just hydrogen particles and one that includes dust particles.

Regarding the previous galaxy evolution article, I get the picture that spiral or flat galaxies came first and eventually collided to form in many cases spherical or elliptical shaped galaxies. My original understanding was that elliptical galaxies collided to form spiral galaxies. So which is the current thinking on this subject or is it still in a quasi-state of resolution ?

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

I will be attempting a simple calculation to show gravity's work in producing a small agglomeration of matter from an IMC. I will use two different starting points, one with just hydrogen particles and one that includes dust particles.

I think models generally ignore dust so far as gravity is concerned. It only represents a fraction of a percent of the total mass of an interstellar cloud, and contributes nothing more than trace contaminants to a star (or protostar).

For reference, I believe models of star formation in the early Universe consider density gradients of about one part in a million in an otherwise uniform region of gas.

[dougettinger]

I am more concerned with 2nd and 3rd generation stars where dust particles could provide a differential for the size of particle masses. What would you use for a starting density for hydrogen molecules in a typical IMC ?

Doug Ettinger
Pittsburgh, PA