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Thread: Density wave in spiral Arm

  1. #31
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    Quote Originally Posted by Dave Lee View Post
    Now I'm really confused.

    I had the impression that:
    The density wave set some sort of trafic jam that decreases the velocity of stars in the arm. Therefore, this traffic jam increases the Mass/Gas density in the arm by about 10% - 20%.
    My bolding. Perhaps it might be useful to go back to the links you posted in Posts #1 and #4 of this thread and re-read the extracts you actually quote from those:

    Stars move through the spiral arms.
    Gas clouds try to move through, but some are induced to form stars (collision or compression)


    This spiral arm is called a density wave and travels slowly in a circular motion around the galactic centre.
    Interstellar dust and gas move in circles at a much larger speed and collide with the density wave.
    When the gas enters the density wave region, it is compressed and stars begin to form.


    You are focusing on the stars. Don't - focus on the gas instead. It's the gas that gets compressed as it enters the density wave and forms stars. It's newly formed OB stars that in turn excite the surrounding HII gas clouds that in turn outline the spiral arms.

    The stars that enter the spiral arm are not really relevant to what you are trying to understand. They are largely unaffected by their transition through the spiral arm (it's just that the "traffic" is getting slightly heavier due to the formation of new stars in the arm).

  2. #32
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    Quote Originally Posted by Grey View Post
    This is correct. We can see this especially well in galactic collisions. The stars move past each other without collisions or significant interaction for the most part, affected only by gravity. The gas, however, is compressed, slows down, and heats up. (In studies of collisions where we can trace the dark matter, we see that it moves with the stars, not the gas, suggesting that it, too, is largely collisionless).
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  3. 2018-May-18, 09:24 AM

  4. #33
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    Quote Originally Posted by AGN Fuel View Post
    You are focusing on the stars. Don't - focus on the gas instead. It's the gas that gets compressed as it enters the density wave and forms stars. It's newly formed OB stars that in turn excite the surrounding HII gas clouds that in turn outline the spiral arms.
    The stars that enter the spiral arm are not really relevant to what you are trying to understand. They are largely unaffected by their transition through the spiral arm (it's just that the "traffic" is getting slightly heavier due to the formation of new stars in the arm).
    Thanks for the great explanation.
    So, with your permission, let's see if I understand it correctly:

    1. Density wave region - means a region with higher density gas of about 10% - 20% with regards to the density outside this region (outside the spiral arm).
    2. Velocity - Stars move through the spiral arms. All the stars (like our Sun) orbit the galaxy at a constant velocity. The value of the velocity is a product of location.
    3. New born OB stars - Interstellar dust and gas move in circles at a much larger speed (comparing to the stars velocities) and collide with the density wave. When the gas enters the density wave region, it is compressed and new OB stars begin to form.
    4. Spiral Arms - Those new born OB stars must die as they move out from the density wave region. Therefore as they only exist in the density wave region, they technically increase the star density in that region (spiral arms). Hence, we see higher star density at the spiral arms.
    Is it correct?

    If so, what is the star density in the spiral arm comparing to the star density outside the arm? Is it also 10% - 20%?
    Actually, as the only contribution to star density comes from OB stars, I should ask: What is the ratio between the OB stars to all the other stars in the density wave region?

    As an example - let's look at Stars within 50 light years:
    http://www.atlasoftheuniverse.com/50lys.html
    There are 133 stars marked on this map. How many are OB stars?

  5. #34
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    You're nearly there but not quite.

    The formation of all star sizes is enhanced not just the OB stars.

    When you say the "OB stars must die" this is not quite the right way of looking at it. Simply moving out of a spiral arm is not the direct cause of death.

    It's just that their lifetimes are so much shorter than smaller stars it looks that way.

  6. #35
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    Quote Originally Posted by Dave Lee View Post
    Thanks for the great explanation.
    So, with your permission, let's see if I understand it correctly:

    1. Density wave region - means a region with higher density gas of about 10% - 20% with regards to the density outside this region (outside the spiral arm).
    2. Velocity - Stars move through the spiral arms. All the stars (like our Sun) orbit the galaxy at a constant velocity. The value of the velocity is a product of location.
    Not perfectly constant. In any elongated orbit the star will be speeding up along the inbound portion and slowing along the outbound portion. The average velocity is nearly constant over a large range of radius from the galactic center.
    3. New born OB stars - Interstellar dust and gas move in circles at a much larger speed (comparing to the stars velocities)
    My bold. I don't know how you inferred that. I see no basis for it in any source that has been presented here. It is my understanding that the stars and the gas clouds orbit pretty much in unison collectively. Of course there is some scatter about the mean among the stars, giving us the local proper motions we observe at close range.
    and collide with the density wave. When the gas enters the density wave region, it is compressed and new OB stars begin to form.
    4. Spiral Arms - Those new born OB stars must die as they move out from the density wave region. Therefore as they only exist in the density wave region, they technically increase the star density in that region (spiral arms). Hence, we see higher star density at the spiral arms.
    Is it correct?
    No. I will second what kzb said for emphasis. The short lifetime of these stars is a consequence of the prodigious rate at which they exhaust their core hydrogen, and not a consequence of merely drifting out of the arm.

    If so, what is the star density in the spiral arm comparing to the star density outside the arm? Is it also 10% - 20%?
    Actually, as the only contribution to star density comes from OB stars, I should ask: What is the ratio between the OB stars to all the other stars in the density wave region?
    My bold. No, no, a thousand times no. The ordinary long-lived stars that have drifted into the spiral pattern are present in a similar percentage of density increase. The newly formed OB stars are only a tiny fraction of all the stars in the arms. They light up the arm with their disproportionately high brightness.

    As an example - let's look at Stars within 50 light years:
    http://www.atlasoftheuniverse.com/50lys.html
    There are 133 stars marked on this map. How many are OB stars?
    None, provided we are referring to type O and "early" B that are thousands or tens of thousands times the Sun's luminosity. Even the "late" B stars such as Regulus are outside this range. The nearest OBs are hundreds of light years away, mostly in Orion and Canis Major.

  7. #36
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    Quote Originally Posted by Hornblower View Post
    The ordinary long-lived stars that have drifted into the spiral pattern are present in a similar percentage of density increase. The newly formed OB stars are only a tiny fraction of all the stars in the arms.
    Long-lived stars (which includes stars of class A through class M and brown dwarfs) are also newly formed along with the O and B stars. The O and B stars are only a tiny fraction of all the stars formed when the gas clouds are compressed.
    Selden

  8. #37
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    You started out reasonably well, but trailed off badly toward the end... just to support Hornblower's response which covers it all:

    Quote Originally Posted by Dave Lee View Post
    1. Density wave region - means a region with higher density gas of about 10% - 20% with regards to the density outside this region (outside the spiral arm).
    Strictly it's the mass density in the wave, but in general terms yes.

    2. Velocity - Stars move through the spiral arms. All the stars (like our Sun) orbit the galaxy at a constant velocity. The value of the velocity is a product of location.
    Well, you are well aware that instantaneous orbital velocities vary during any given orbit as the object moves from periapsis to apoapsis and back. However, yes, the mean orbital speed will typically be relatively constant for a star at a given radius.

    3. New born OB stars - Interstellar dust and gas move in circles at a much larger speed (comparing to the stars velocities) and collide with the density wave.
    Where did this idea come from? Why would the gas clouds be moving at a much larger speed than anything else orbiting at a given radius? In fact, measuring the velocity of gas clouds is a common method of measuring galactic rotation rates as a whole, because they orbit at roughly the same rate as everything else and are usually easier to measure at large distances.

    When the gas enters the density wave region, it is compressed and new OB Stars begin to form.
    ALL stars begin to form, including OB stars. This has been pointed out before.

    4. Spiral Arms - Those new born OB stars must die as they move out from the density wave region.
    For the umpteenth time, OB Stars do not die BECAUSE of the simple act of moving out of the spiral arm. I honestly don't understand why this concept baffles you. They do not leave the spiral arm because they do not live long enough to do so.

    As an analogy, a Mayfly lives for about 24 hours. So if a particular mayfly hatched in a field in California, you would not expect to find it had flown to Nebraska. Your argument is that the act of leaving the Californian field is what kills the mayfly, whereas it's simply the fact that it doesn't live long enough to get very far from where it hatched. Leaving the field has nothing to do with why it dies. Leaving the spiral arm has nothing to do with why an OB star dies.

    Therefore as they only exist in the density wave region, they technically increase the star density in that region (spiral arms). Hence, we see higher star density at the spiral arms.
    Is it correct?
    No, this is not correct. In one of the first posts in this thread, I suggested you look up the relative populations of the classes of stars. Had you done so, you would know that this is wrong. OB stars make up much less than 1% of the population of stars, even in the spiral arms.

    Actually, as the only contribution to star density comes from OB stars, I should ask: What is the ratio between the OB stars to all the other stars in the density wave region?
    And this is completely wrong. As above, OB Stars make up much less than 1% of the stars in the arm.

    As an example - let's look at Stars within 50 light years:
    http://www.atlasoftheuniverse.com/50lys.html
    There are 133 stars marked on this map. How many are OB stars?
    None. Not particularly unexpected given the exceptionally low frequency of OB stars and the fact that our sun has moved far beyond the region where it was born.

  9. #38
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    Thanks

    With regards to OB stars:
    Quote Originally Posted by AGN Fuel View Post
    OB stars make up much less than 1% of the population of stars, even in the spiral arms.
    Now it's clear to me that the density contribution of those stars is absolutely neglected (less than 1%). Therefore, I have no more questions about those stars.

    However, I still don't understand how we get the higher star density in the spiral arm.

    I had the impression that the density wave increases the gas density and the gas density increases the star density.
    If we see 10% to 20% higher density in the arm, there must be higher STAR density in the arm.
    We don't see gas in the ARM (although it is there at higher density).

    After reading your answers I'm not sure about it.

    Quote Originally Posted by AGN Fuel View Post
    Strictly it's the mass density in the wave, but in general terms yes.
    ALL stars begin to form, including OB stars. This has been pointed out before.
    What does it mean "mass density". Is it Gas + stars density?
    So, when we say: "Density wave" does it mean Gas + star density?

    But stars have a constant velocity at "average" so how do we get higher star density in the density wave?

    Sorry - I really can't understand this simple issue.(I have read several articles and see many video about this subject)
    Last edited by Dave Lee; 2018-May-19 at 03:56 PM.

  10. #39
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    Dave, I suggested this some time ago. Watch some of the simulation videos:

    https://www.youtube.com/watch?v=lMReQ6hVw5s

    And some words to go with it:
    https://www.youtube.com/watch?v=yiv6a8BVzPE

  11. #40
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    Yay movies!

    But I did notice right away that in the first sim the stars closer to the center move, (orbit), faster than the stars further out.
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  12. #41
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    Quote Originally Posted by Dave Lee View Post
    Thanks

    With regards to OB stars:


    Now it's clear to me that the density contribution of those stars is absolutely neglected (less than 1%). Therefore, I have no more questions about those stars.

    However, I still don't understand how we get the higher star density in the spiral arm.
    Let's go back to this sketch of a simplified model.
    https://en.wikipedia.org/wiki/Densit...ms_diagram.svg
    Take any two adjacent loops that are clearly separated at their closest approach. (I think this elongation is exaggerated to make a point, to the point of jamming up in the outer parts.) Let the stars move at the same velocity around both loops and watch how they converge to a minimum separation and then diverge. With billions of stars doing this we get the increased density in the spiral pattern.
    I had the impression that the density wave increases the gas density and the gas density increases the star density.
    You appear to be envisioning a flawed chain of causality. Instead of thinking of the density wave as some entity in itself that causes stars and gas to move in a certain way, try looking at it as a description of the convergence of orbiting stars and gas that are moving at similar speeds but in different directions. Most of the increase in star density is happening concurrently with the increase in gas density and is not a consequence of it. The stars would be doing the same thing without the gas. The episodes of new star formation add a few stars to the many existing ones that have converged here.

    If we see 10% to 20% higher density in the arm, there must be higher STAR density in the arm.
    Non sequitur. The stars and the gas are orbiting and converging roughly in unison,
    and therefore increase their respective densities by about the same percentage.

    We don't see gas in the ARM (although it is there at higher density).
    We "see" it if we observe in the 21cm radio frequency band.

    After reading your answers I'm not sure about it.
    Your initial impression was in part mistaken.

    What does it mean "mass density". Is it Gas + stars density?
    Yes.

    So, when we say: "Density wave" does it mean Gas + star density?
    Yes.


    But stars have a constant velocity at "average" so how do we get higher star density in the density wave?
    How many times to I need to repeat this? They are moving in different directions at different distances from the galactic center. Take another look at that orbital sketch above.


    Sorry - I really can't understand this simple issue.(I have read several articles and see many video about this subject)
    You appear to have a very sparse and fragmented grasp of physics. That's OK for starters. We all have to start somewhere. You need to study these things in an orderly manner. You need to avoid the temptation to run off in all directions and latch onto out-of-context snippets of these topics.

  13. #42
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    Quote Originally Posted by Dave Lee View Post
    Sorry - I really can't understand this simple issue.(I have read several articles and see many video about this subject)
    I very strongly recommend you watch the links provided by Shaula - in particular the second one. It is an excellent explanation with some very good supporting graphics.

    It shows very neatly how the material (stars and gas) can orbit at a consistent speed but still bunch up and separate again regularly. I think it may be this bit of the process that is flummoxing you, but the video describes the process very well.

  14. #43
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    Quote Originally Posted by Shaula View Post
    Dave, I suggested this some time ago. Watch some of the simulation videos:

    https://www.youtube.com/watch?v=lMReQ6hVw5s

    And some words to go with it:
    https://www.youtube.com/watch?v=yiv6a8BVzPE
    Thanks Shaula

    I have looked at those videos very carefully:

    1. Starting point - We see a galaxy with any ideally symmetrical star dispersion.
    2. Orbital movement - All the stars are moving in full synchronization with each other. Each star follows the one in front with the same direction and the same velocity.
    3. Shift - Over time, there is a minor shift in the star orbital cycle. After several cycles we get the expected spiral shape.
    4. Stop - The video stops at that nice point. No more.

    Is it real? Do you see any sort of physics in that video?
    Let's try to understand what we really see:

    1. Starting point - Our scientists claim that the galaxy increases its mass by "eating" and colliding with dwarf galaxies. So after all those violent activities, how can we claim that this starting point of ideally symmetrical star dispersion is feasible?
    2. Orbital movement - Please look at the nearby star movement. (If needed, I can send the link). We can see clearly that each star is moving at different direction and at a different speed. So, there is absolutely no synchronization between the stars. You can't find even one star which follows the one in front. So, how can we dream about that full orbital synchronization?
    3. Shift - Let's agree on this issue. Let's also agree that somehow, one of the billion galaxies had started from this none realistic starting point and all of its stars are moving in full synchronization. So, at one point of time it might set the desired spiral shape. However, how long it might keep it? Can we stop that shift and for how long?
    4. Stop - Sorry, in reality I can't see any option to stop the real life at that point. The shift must go on. So even if we get the spiral image, it must be a temporary image. The momentum of the shift should destroy the spiral shape.

    Do you agree with that? Do you agree that by random movement of stars in the galaxy, it is virtually impossible mission to get the requested spiral shape?
    Is there any video which gives real physics basis (Or physics law) for the creation of the spiral structure?
    Last edited by Dave Lee; 2018-May-20 at 04:39 AM.

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    1. No one is making that claim. The purpose of the video is not to simulate precisely galaxy formation but to demonstrate the basic principles of a density wave, precisely those principles you were struggling with. Do you now understand what a density wave is and how correlations in orbits can produce overdensities? If so the purpose of the video is achieved. If you now want to go on to the more realistic and complex simulations we can build off this. For example https://youtu.be/MncUDWhPB_E shows the formation of a spiral galaxy. Now that hopefully you understand density waves to a degree you can see, in the simulation, transient spiral features consistent with density waves forming during mergers and then a more stable and complex spiral structure forming (possibly driven by the bar)

    2. As I said. The video is an idealisation designed to introduce the concepts behind density waves, that level of uniformity is not needed. But your characterisation of stellar motion is also flawed. Residual proper motion of local stars is not a great guide to bulk galactic motion. Our galaxy has a disk, for one thing. If your characterisation of stellar motion was accurate it could not.

    3. Not required, see point one.

    4. Nope. The shift can be a one off, especially if it is part if a merger. Or a fly by. Plus on some level spiral structure is temporary, disks can be disrupted, galaxies evolve. But it is stable enough for us to see it.

    Dave, you keep trying to jump ahead of your understanding. We were not trying, with the examples given before, to accurately simulate a galaxy. We were trying to get you to understand the fairly basic physics concepts underlying the explanation for what we see. If you were getting confused by the idea of a density wave how did you think you would be able to understand its application to non-idealised systems? Now you have an example of a simulation (an n body one of course) that show spiral structure formation, plus the underlying basic physics that explains the structures you see.

    You may be shocked to know I don't agree with your conclusions. Again you are misapplying analogies or simplified models, making bad characterisations of observations and reasoning based on this.
    Last edited by Shaula; 2018-May-20 at 05:10 AM.

  16. #45
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    Thanks Shaula

    I do appreciate your effort to highlight my way for better understanding.
    I really wish to get the fairly basic physics concepts underlying the explanation for the spiral structure, as stated:

    Quote Originally Posted by Shaula View Post
    We were trying to get you to understand the fairly basic physics concepts underlying the explanation for what we see.
    However, as I look at all the videos including the following one:

    Quote Originally Posted by Shaula View Post
    If you now want to go on to the more realistic and complex simulations we can build off this. For example https://youtu.be/MncUDWhPB_E shows the formation of a spiral galaxy. Now that hopefully you understand density waves to a degree you can see, in the simulation, transient spiral features consistent with density waves forming during mergers and then a more stable and complex spiral structure forming (possibly driven by the bar).
    I only see a nice story/video.
    They do not give any physics information. They do not speak about Newton law or hydrodynamics:

    Quote Originally Posted by tusenfem View Post
    In this case the stars can be seen as a gas, so the waves are based on hydrodynamics.
    It is just a nice video clip without any real connection to physics.

    Let me ask you the following:

    You are master in science. You know all the physics laws - all of them.
    However, let's assume that you have never heard about the "Density wave".
    If I have to convince you that it is real, would you understand the basic physics elements for this idea based on those videos which I have set for you?
    Do you see any physics explanations in those videos?
    The question is not how to convince me, but how can you convince yourself based on those videos that have no real physics proof?

    If I have to convince you in something, won't you ask to see the mathematical proof?
    You have stated that:
    Quote Originally Posted by Shaula View Post
    Gravity is the dominant force. Plenty of other effects exist and make a contribution to the system.
    So, can you please show how Gravity is the dominant force for the Spiral stracture?
    Can you please introduce the mathematical proof for the density wave idea?
    Do it in Newton law, do it in hydrodynamic.
    Please, real physics proof for - Why there is high star concentration/density in the spiral arms.
    Last edited by Dave Lee; 2018-May-20 at 07:17 AM.

  17. #46
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    Quote Originally Posted by Dave Lee View Post
    They do not give any physics information. They do not speak about Newton law or hydrodynamics:

    It is just a nice video clip without any real connection to physics.
    The video is the output of a simulation built on physical laws including gravity. If you note - the video summarises a simulated data set that took 8 months of supercomputer time to generate. Do you really believe it takes a supercomputer 8 months to render a 360p video of a galaxy? N body systems, as has been explained to you before, are complex systems that require an iterative model to simulate. They don't have a simple analytical solution which is why you have to take approaches like this.

    Quote Originally Posted by Dave Lee View Post
    However, let's assume that you have never heard about the "Density wave".
    If I have to convince you that it is real, would you understand the basic physics elements for this idea based on those videos which I have set for you?
    Do you see any physics explanations in those videos?
    You already had the basic physics - it is gravitation and orbitology. Since I know about this then those videos would explain density waves quite neatly and simply. And the Zurich simulation would demonstrate that the principles worked in non-idealised systems. Plus the Whirlpool galaxy video has a basic, verbal explanation of the physics.

    Quote Originally Posted by Dave Lee View Post
    The question is not how to convince me, but how can you convince yourself based on those videos that have no real physics proof?
    Videos and examples like this were precisely how I was taught, so yes - I would be able to convince myself because it followed a tried and trusted method.
    1) Understand the basic physical laws (gravity in this case)
    2) Develop an idealised system to study more complex configurations of systems (the first video I linked)
    3) Develop a more complex simulation and test that the behaviour holds in realistic systems (the Zurich video)

    Quote Originally Posted by Dave Lee View Post
    If I have to convince you in something, won't you ask to see the mathematical proof?
    As I have told you before. N body systems do NOT have a simple analytical solution. You cannot have a simple 'proof' in the sense that you are asking about for real world systems. Instead we used idealised systems and more complex simulations to step towards understanding.

    Quote Originally Posted by Dave Lee View Post
    So, can you please show how Gravity is the dominant force for the Spiral stracture?
    Can you please introduce the mathematical proof for the density wave idea?
    Do it in Newton law, do it in hydrodynamic.
    Please, real physics proof for - Why there is high star concentration/density in the spiral arms.
    The 1969 paper (from the link in Wikipedia)
    A review of 60 years of density wave theory

    I'm guessing you are now going to quote little bits from each of these and claim they don't make sense so the idea must be wrong. Honestly, this is getting beyond a joke. You don't understand the explanations given. You are given visualisations to help you understand but apparently they are not 'real physics'. Now you are demanding a 'mathematical proof' that is going to be more complex than the explanations and visualisations you already don't understand. How is this going to help?

  18. #47
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    Thinking about this lets try something.

    Forget galaxies. Scrub them from your mind entirely. Please go through the steps below. Stop when you disagree.

    1) Do you accept that gravity is primarily what you need to describe simple orbits?
    (Wiki: https://en.wikipedia.org/wiki/Circular_orbit)
    2) Do you accept that orbits can be elliptical? Looking at the solar system do you agree that simple gravitation is all you need to describe elliptical orbits?
    (Wiki: https://en.wikipedia.org/wiki/Elliptic_orbit)
    3) Do you accept that gravitational tugs or other influences can cause orbits to precess? Looking at the solar system we see it happening and can measure it, including that for Earth)
    (Wiki: https://en.wikipedia.org/wiki/Apsidal_precession Maths: http://farside.ph.utexas.edu/teachin...l/node115.html)

    If you have got to here then you have all the basic physics you need to carry on. Forget anything else, just think about the orbits. And still do not think about galaxies.

    4) Do you agree that in this video what you can see at the start is a simple, idealised system of orbiting test particles?
    5) Do you agree that the two previously discussed pieces of physics (elliptical orbits, apsidal precession of orbits) result in the orbital changes seen in the video?
    6) Do you agree that the orbital changes result in two denser parts of the system (the spiral shapes)?

    If you have got to here then you understand how density waves (the spiral shapes) can produce areas of increased density.

    So now we try to see if the stumbling block is applying it to galaxies.

    7) Do you agree that stars, approximately / to first order, orbit the centre of mass of the galaxy?
    8) Do you agree that the orbits of stars, while they are slightly more complex due to the fact that the two body approximation is only an approximation, are roughly ellipses?
    9) Do you agree that these ellipses can precess like any other orbit?

    If you have got to here you should be happy that using just gravity and ignoring some of the complexity of the system we can produce a two arm spiral of denser areas in a galaxy provided we allow something to induce a simple orbital precession. Because you are cramming more diffuse matter into a smaller area you get shock fronts, matter pile-ups and some of these pile-ups get dense enough to collapse, leading to star formation. Some of these stars are very bright, large, short lived ones which is why we see bright blue spiral arms. They don't live for long so we don't see them far from their point of birth. Other stars and matter that did not get co-opted into star formation carries on its merry way.

    So, next.

    10) Do you accept that real galaxies are more complex systems than have so far been described?
    11) Do you accept that complex simulations are required to study real galaxies?
    12) Do you accept that computational hydrodynamics and n-body simulations are the right way to do this?
    (e.g. GRUPEGAS http://iopscience.iop.org/article/10...6/719/1/012006 or GADGET https://wwwmpa.mpa-garching.mpg.de/gadget/)

    If you have got to here, and accept the simulation code as accurate then you should agree that the Zurich video is an acceptable way to see if the basic physics we have described already captures the essence of the more complex systems. And because we see spiral galaxies form it does.

  19. #48
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    So:
    Are density waves of stars alone possible, or are they impossible because of not enough elasticity?
    Look at lenticular galaxies. Disc but no gas, and it is smooth disc with no spiral density waves.

    Also: gas is affected by gas pressure.
    Stars are not. Neither is dark matter.
    Does the gravity of gas concentrated in gas density waves also attract an additional amount of old stars into spiral arms? Does it attract an additional amount of dark matter?
    Of course, gas can be converted into new stars, but not to dark matter. You need to keep track whether you are talking about the total concentration of stars (old plus new) or the concentration of old stars alone.

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    Quote Originally Posted by chornedsnorkack View Post
    So:
    Are density waves of stars alone possible, or are they impossible because of not enough elasticity?
    Elasticity is not required. As the previous examples show stars and other orbiting bodies can easily form density waves due to correlations in their orbits. These are not acoustic waves propagating in the ISM.

    Quote Originally Posted by chornedsnorkack View Post
    Look at lenticular galaxies. Disc but no gas, and it is smooth disc with no spiral density waves.
    Lenticulars are practically defined by their lack of material to form new stars - they could easily have density waves ongoing but because they are not starforming you won't then get the bright stars that act as tracers. So you simply won't see them.

    Quote Originally Posted by chornedsnorkack View Post
    Does the gravity of gas concentrated in gas density waves also attract an additional amount of old stars into spiral arms? Does it attract an additional amount of dark matter?
    It is just matter. It behaves like any other overdensity would. So it will have a gravitational effect. But as you can see from the simulations above it is not a large enough effect to dominate over other effects.

    Quote Originally Posted by chornedsnorkack View Post
    Of course, gas can be converted into new stars, but not to dark matter. You need to keep track whether you are talking about the total concentration of stars (old plus new) or the concentration of old stars alone.
    Why? For most purposes you just have to keep track of the total amount of matter. The gas/star distinction is important for star formation. The OB/other ratio is important to understand contrast and ionisation effects. What does the time since formation of a star matter if it is present?

  21. #50
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    Thanks Shaula.
    I really appreciate your high effort.

    Quote Originally Posted by Shaula View Post
    Thinking about this lets try something.

    Forget galaxies. Scrub them from your mind entirely. Please go through the steps below. Stop when you disagree.

    1) Do you accept that gravity is primarily what you need to describe simple orbits?
    (Wiki: https://en.wikipedia.org/wiki/Circular_orbit)
    2) Do you accept that orbits can be elliptical? Looking at the solar system do you agree that simple gravitation is all you need to describe elliptical orbits?
    (Wiki: https://en.wikipedia.org/wiki/Elliptic_orbit)
    3) Do you accept that gravitational tugs or other influences can cause orbits to precess? Looking at the solar system we see it happening and can measure it, including that for Earth)
    (Wiki: https://en.wikipedia.org/wiki/Apsidal_precession Maths: http://farside.ph.utexas.edu/teachin...l/node115.html)

    If you have got to here then you have all the basic physics you need to carry on. Forget anything else, just think about the orbits. And still do not think about galaxies.

    4) Do you agree that in this video what you can see at the start is a simple, idealised system of orbiting test particles?
    5) Do you agree that the two previously discussed pieces of physics (elliptical orbits, apsidal precession of orbits) result in the orbital changes seen in the video?
    6) Do you agree that the orbital changes result in two denser parts of the system (the spiral shapes)?

    If you have got to here then you understand how density waves (the spiral shapes) can produce areas of increased density.

    So now we try to see if the stumbling block is applying it to galaxies.

    7) Do you agree that stars, approximately / to first order, orbit the centre of mass of the galaxy?
    8) Do you agree that the orbits of stars, while they are slightly more complex due to the fact that the two body approximation is only an approximation, are roughly ellipses?
    9) Do you agree that these ellipses can precess like any other orbit?
    Yes, Fully understand and fully agree.

    Quote Originally Posted by Shaula View Post
    10) Do you accept that real galaxies are more complex systems than have so far been described?
    11) Do you accept that complex simulations are required to study real galaxies?
    12) Do you accept that computational hydrodynamics and n-body simulations are the right way to do this?
    (e.g. GRUPEGAS http://iopscience.iop.org/article/10...6/719/1/012006 or GADGET https://wwwmpa.mpa-garching.mpg.de/gadget/)

    If you have got to here, and accept the simulation code as accurate then you should agree that the Zurich video is an acceptable way to see if the basic physics we have described already captures the essence of the more complex systems. And because we see spiral galaxies form it does.
    10. Yes, fully agree.
    11. 12.
    Yes and no.
    Yes - If we ignore Newton law than complex simulations are required for our study.
    No - Could it be that by Newton law we might solve the enigma in a very simple way?

    You have stated that "Gravity is the dominant force", so where is the calculation for that dominant force?
    I don't see any gravity calculation in Zurich Video.
    How can we totally ignore Newton law in our calculations/simulations.
    Don't you think that as our scientists believe that the density wave is real, they should find a way to prove it by Isaac Newton (or kepler)?

    So, the only issue which is missing is - Gravity force.
    Can we prove the density wave by Newton law? Is it feasible?

  22. #51
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    Quote Originally Posted by Dave Lee View Post
    10. Yes, fully agree.
    11. 12.
    Yes and no.
    Yes - If we ignore Newton law than complex simulations are required for our study.
    No - Could it be that by Newton law we might solve the enigma in a very simple way?
    Right. So we are back at the same issue we've been over before. You apparently do not believe that many body simulations are required because you have an unproven belief that all systems can be turned into a quasi-two-body problem and solved analytically.

    From past experience there is nothing anyone can do to change your mind about this so I won't even bother to try. You are wrong. It is as simple as that. I know you won't accept it, I know you will continue to try to find some justification for your belief system. I'm not interested in this part of the conversation. I've been round it far too many times with you before. If this whole thread is just yet another "Dave believes everything is a two body problem" thread then this will be my last contribution to it. You allow your personal beliefs to trump actual physics and refuse to follow the scientific method. There is no point arguing with people who do that in areas their beliefs cover.

    Quote Originally Posted by Dave Lee View Post
    You have stated that "Gravity is the dominant force", so where is the calculation for that dominant force?
    In the code, the source for which is available in the links I gave you, and papers about these pieces of code. If you are not willing to read the background on the code or read the source yourself then I can't help you.

    Quote Originally Posted by Dave Lee View Post
    I don't see any gravity calculation in Zurich Video.
    OK. How do you think the video was produced? What forces do you think they simulated?

    Quote Originally Posted by Dave Lee View Post
    How can we totally ignore Newton law in our calculations/simulations.
    We don't ignore gravity and it is utterly ridiculous that you think we do. I've provided links to the code, links to the descriptions of the software. How on Earth you came to this conclusion baffles me. You must have systematically ignored every piece of information provided. I'm genuinely baffled as to how anyone could honestly look at the information provided and say this.

    Quote Originally Posted by Dave Lee View Post
    Don't you think that as our scientists believe that the density wave is real, they should find a way to prove it by Isaac Newton (or kepler)?
    You mean by simulating idealised Keplerian orbits (the first video) or by using simulations based on gravity (Zurich video) and studying the formation of spiral structure? Or by comparing simulations to observation (review paper)? Oh ... Wait, they did.

    Quote Originally Posted by Dave Lee View Post
    So, the only issue which is missing is - Gravity force.
    Can we prove the density wave by Newton law? Is it feasible?
    It is what the videos provided and the papers linked did. Turn this around. You've been given papers with the mathematical detail in spanning 60 years, reviews of the observational evidence, idealised toy systems showing the relevant phenomenology and detailed simulations proving the phenomenology is valid in complex systems. You been given the code that produces these simulations, descriptions of the physics they account for and visualisations of the output that match the real world. What more proof, in detail, do you think you need?

  23. #52
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    Quote Originally Posted by Dave Lee View Post
    Thanks Shaula.
    I really appreciate your high effort.
    It would be nice to show your appreciation by taking time to fully understand his answers. You have asked essentially the same questions over and over again, have been given the same answers over and over again, and yet you ignore them at the same time you express gratitude.

    Yes - If we ignore Newton law than complex simulations are required for our study.
    No - Could it be that by Newton law we might solve the enigma in a very simple way?
    No one said that Newton is ignored. You keep making this up -- please stop.

    The simulations are based on Newton's laws. The laws themselves are simple, but that does not mean that the calculations will be. That's why we need numerical simulations. The laws are used to set up the calculations, and computers -- really powerful, very fast ones -- do the math for us. So stop pretending that we are ignoring Newton. You have been told this many times, but in your zeal to prove that scientists are lying, you keep ignoring this answer. Please stop. It is disingenuous of you to pretend that you have not been told this.

    You have stated that "Gravity is the dominant force", so where is the calculation for that dominant force?
    Again, the calculations are what the simulations carry out. You are not going to replicate those by hand, Dave. Your physics ability is simply inadequate. So quit asking.

    I don't see any gravity calculation in Zurich Video.
    And again, you won't. You are seeing the results of the calculations. You need to study real science to understand that not all calculations can be done by paper and pencil, especially not by someone with your background. There's no point to continue demanding to see such calculations.

    How can we totally ignore Newton law in our calculations/simulations.
    As above, no one is ignoring Newton. You are ignoring this answer.

    Don't you think that as our scientists believe that the density wave is real, they should find a way to prove it by Isaac Newton (or kepler)?
    As above, Newton is not being ignored. It's not a conspiracy, Dave. Scientists are not lying to you.

    Given your demonstrated ignorance of science, you need to adopt a different attitude to the one you consistently evince. When you see something you don't understand, assume it's due to your ignorance. Stop assuming that it's the result of a deliberate attempt by scientists to deceive.

    So, the only issue which is missing is - Gravity force.
    Can we prove the density wave by Newton law? Is it feasible?
    As above, it's in the simulations. It's not missing. Stop saying that it is.

  24. #53
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    Thanks

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    Quote Originally Posted by chornedsnorkack View Post
    Look at lenticular galaxies. Disc but no gas, and it is smooth disc with no spiral density waves..
    I agree with Shaula - I am not sure how there could NOT be density waves in S0 galaxies. It's just that the lack of gas in such galaxies means that the star forming regions (and the H II regions caused by the hot stars formed therein) means that you would not see the pattern traced out.

    i.e. the wave is there, but you can't see it.

  26. #55
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    Please look at the following image of late-type spiral galaxies

    https://en.wikipedia.org/wiki/Lentic...ubble-Grid.jpg

    We see that ALL of them are symmetrical.
    How the density wave can explain that symmetrical spiral shape of the galaxies?

  27. #56
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    Quote Originally Posted by Dave Lee View Post
    Please look at the following image of late-type spiral galaxies

    https://en.wikipedia.org/wiki/Lentic...ubble-Grid.jpg

    We see that ALL of them are symmetrical.
    How the density wave can explain that symmetrical spiral shape of the galaxies?
    Did you actually READ the wikipedia article, or did you just obsessively focus on one figure without context?

    Your scattershot approach to science is a very inefficient way to go about learning, as has been pointed out to you before. Are you truly interested in educating yourself, or are you dogmatically wedded to a prosecutorial approach to science? It would be helpful to know the answer, as it would help inform appropriate responses.

  28. #57
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    I really want to understand how the density wave works.
    In this article there is not even one word about the symmetrical shape of all the galaxies.
    https://en.wikipedia.org/wiki/Lenticular_galaxy
    Why is it?
    How can we explain that symmetrical shape?

  29. #58
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    Quote Originally Posted by Dave Lee View Post
    Please look at the following image of late-type spiral galaxies

    https://en.wikipedia.org/wiki/Lentic...ubble-Grid.jpg

    We see that ALL of them are symmetrical.
    How the density wave can explain that symmetrical spiral shape of the galaxies?
    Is this supposed to be funny?

    I admit I'm not the quickest to realise when my chain is being yanked, but the penny has finally dropped. Enough now.

  30. #59
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    Quote Originally Posted by Dave Lee View Post
    I really want to understand how the density wave works.
    In this article there is not even one word about the symmetrical shape of all the galaxies.
    https://en.wikipedia.org/wiki/Lenticular_galaxy
    Why is it?
    How can we explain that symmetrical shape?
    Lenticular galaxies don't have spiral arms. What features are you talking about when you talk about symmetry? The disk?

    Or are you talking about the grid you linked to? The grid that clearly doesn't show actual images of spiral galaxies but instead sketches of idealised shapes that are used as part of a galactic classification guide?

  31. #60
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    The symmetrical shape is very critical.
    Let me explain:
    We know that the stars orbit in elliptical cycle.
    Therefore, in one side of the cycle it is located at the most inward to the center while in the other side is located at the most outwards from the center.
    So, did they consider that elliptical cycle shape in Zurich?

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