Page 1 of 4 123 ... LastLast
Results 1 to 30 of 99

Thread: Density wave in spiral Arm

  1. #1
    Join Date
    Sep 2015
    Posts
    499

    Density wave in spiral Arm

    In the following article it is stated:

    http://www.astronomy.ohio-state.edu/...4/spirals.html

    "What are Spiral Arms?
    Spiral Arms are Density Waves that pass through the general disk of stars and gas.
    Density Waves are a kind of orbital traffic jam

    Orbits crowd together in the arms, stars pile up and make the regions look brighter.
    Gas clouds pile up, collide, fragment, and form new stars.
    O&B Stars ionize leftover gas (HII Regions), then die before moving far from the waves.
    Kalnaj Spiral Schematic
    Density Waves
    Density waves pass through the disk like water waves pass over the ocean.
    Stars move through the spiral arms.
    Gas clouds try to move through, but some are induced to form stars (collision or compression)
    We are not sure how the waves are excited:

    Tidal disturbance from a nearby companion?
    Excited by a stellar bar in the central regions?
    Both of these have been implicated, and there are strong (but not conclusive) arguments for each. Some computer simulations suggest that spiral structure may be transitory, but given how many disk-shaped galaxies show at least some spiral pattern, it must recur frequently."

    1. With regards to spiral arms:
    It is stated that: "O&B Stars ionize leftover gas (HII Regions), then die before moving far from the waves."

    A. Does it mean that if our Sun will move away from the spiral arm wave it should die?
    B. Could it be that our Sun was at some point of time (from its first day) outside the Arm?
    C. If it could be outside the arm, why other stars couldn't live outside the arm? Why do we kill them all as they move outside? Why do we claim: "then die before moving far from the waves"?
    D. Does it mean that there are no stars between the arms? If there is - would you kindly show just few examples of stars which are located between Orion Arm and the nearby arm? Not a group of stars - but only one star alone between the arms (in the disc).

    2. With regards to the density wave:
    A. It is stated: "Density Waves are a kind of orbital traffic jam".
    However, traffic jam is a random phenomenon. so why we couldn't get different variation of arm shape? Why not stright arm or even be a ZiG Zag arm?
    In about 70% of all the galaxies, we see a nice spiral arms. Therefore, don't you think that there must be a key idea in this structure? Could it be that our scientists have missed something?

    B. It is also stated that: "we are not sure how the waves are excited: Tidal disturbance from a nearby companion? or Excited by a stellar bar in the central regions?"
    If they don't know, could it be that we have a problem with this hypothesis? Why they are not willing to get the real answer for that?

  2. #2
    Join Date
    Mar 2010
    Location
    United Kingdom
    Posts
    6,829
    Quote Originally Posted by Dave Lee View Post
    1. With regards to spiral arms:
    It is stated that: "O&B Stars ionize leftover gas (HII Regions), then die before moving far from the waves."
    A. Does it mean that if our Sun will move away from the spiral arm wave it should die?
    No.

    Quote Originally Posted by Dave Lee View Post
    B. Could it be that our Sun was at some point of time (from its first day) outside the Arm?
    Yes

    Quote Originally Posted by Dave Lee View Post
    C. If it could be outside the arm, why other stars couldn't live outside the arm? Why do we kill them all as they move outside? Why do we claim: "then die before moving far from the waves"?
    There are stars outside the arms. No one has made any of the claims you are making.

    Quote Originally Posted by Dave Lee View Post
    D. Does it mean that there are no stars between the arms? If there is - would you kindly show just few examples of stars which are located between Orion Arm and the nearby arm? Not a group of stars - but only one star alone between the arms (in the disc).
    There are stars outside the arms.

    Quote Originally Posted by Dave Lee View Post
    A. It is stated: "Density Waves are a kind of orbital traffic jam".
    However, traffic jam is a random phenomenon. so why we couldn't get different variation of arm shape? Why not stright arm or even be a ZiG Zag arm?
    In about 70% of all the galaxies, we see a nice spiral arms. Therefore, don't you think that there must be a key idea in this structure? Could it be that our scientists have missed something?
    It also shows you how they form, which is not random. You are over extending the metaphor.

    Quote Originally Posted by Dave Lee View Post
    B. It is also stated that: "we are not sure how the waves are excited: Tidal disturbance from a nearby companion? or Excited by a stellar bar in the central regions?"
    If they don't know, could it be that we have a problem with this hypothesis? Why they are not willing to get the real answer for that?
    It gives two competing hypotheses that fit the data and states that they have not been able to test between them. That is not the same as 'dunno'.

    Some comments - the article explicitly states which objects act as tracers of the spiral arm. And the Sun is not among them, nor are most stars. It also states why the large stars are not found between arms. Reread the article, but switch off your "This must be wrong" filter and read what they are saying. It is very different to what you are presenting here.

  3. #3
    Join Date
    Mar 2003
    Posts
    2,972
    Quote Originally Posted by Dave Lee View Post
    1. With regards to spiral arms:
    It is stated that: "O&B Stars ionize leftover gas (HII Regions), then die before moving far from the waves."

    A. Does it mean that if our Sun will move away from the spiral arm wave it should die?
    Is our Sun an O or B class star? No, so your question is meaningless.
    Importantly for your misunderstanding, neither are the very great majority of stars. You might want to look up the relative numbers of the various classes of stars and you will see how uncommon OB Stars actually are.

    You also have it back to front. You seem to be suggesting that the act of leaving a spiral arm causes a star to die - it does no such thing. The reason OB stars don't leave the arm is that they are (comparitively) short-lived. They simply don't have TIME to leave.

    B. Could it be that our Sun was at some point of time (from its first day) outside the Arm?
    C. If it could be outside the arm, why other stars couldn't live outside the arm? Why do we kill them all as they move outside? Why do we claim: "then die before moving far from the waves"?
    D. Does it mean that there are no stars between the arms? If there is - would you kindly show just few examples of stars which are located between Orion Arm and the nearby arm? Not a group of stars - but only one star alone between the arms (in the disc).
    I'm not sure why you bother to ask these question when you completely ignore the responses. You have been told on multiple occasions across multiple threads that there ARE stars between the spiral arms.

    The active star-forming regions in the spiral arms and the short-lived but very bright OB Stars highlight the arms, but that does not mean the spaces between the arms are empty as you seem determined to believe. A lovely analogy I have used (and previously described to you) is Christmas lights on a tree seen from a distance. The lights will trace the shape of the tree, even though you might not be able to see the tree itself. In the same way, the OB stars & gas clouds trace out the shape of the arms.


    2. With regards to the density wave:
    A. It is stated: "Density Waves are a kind of orbital traffic jam".
    However, traffic jam is a random phenomenon. so why we couldn't get different variation of arm shape? Why not stright arm or even be a ZiG Zag arm?
    Waves in a specific confining environment are not random. They must comply to the laws of physics, which dictate how they will act.

    B. It is also stated that: "we are not sure how the waves are excited: Tidal disturbance from a nearby companion? or Excited by a stellar bar in the central regions?"
    If they don't know, could it be that we have a problem with this hypothesis? Why they are not willing to get the real answer for that?
    Who said they aren't? It is a very active area of study.

  4. #4
    Join Date
    Sep 2015
    Posts
    499
    Quote Originally Posted by AGN Fuel View Post
    Is our Sun an O or B class star? No, so your question is meaningless.
    Importantly for your misunderstanding, neither are the very great majority of stars. You might want to look up the relative numbers of the various classes of stars and you will see how uncommon OB Stars actually are.
    Based on your advice, I have tried to get better understanding on O or B class stars and density wave:

    https://sites.ualberta.ca/~pogosyan/...lecture24.html

    "Density-Wave Theory of Spiral Arms
    A spiral arm is just a region where the density of gas is little bit higher than average.
    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.
    The newly born stars continue to move at fast speeds and eventually leave the spiral arm.
    O and B type stars don't live for long, so they never move far from their birth arm.
    Lower mass stars live longer and have plenty of time to move to regions between the arms.
    Density wave move slower than stars, so stars, dust and HI catches up from behind, but new stars are born at the front of the wave."


    If I understand it correctly:

    "When the gas enters the density wave region, it is compressed and O and B type stars begin to form.
    The newly born stars continue to move at fast speeds and eventually leave the spiral arm.
    O and B type stars don't live for long, so they never move far from their birth arm."

    So, is it correct that all is needed to create new born O and B type stars is just a gas compressed?
    How could it be that a compressed gas of about 10–20% could set those new born stars? Is it feasible? Did we try to verify/simulate that process?

    If we look at our location in Orion Arm, do we see that gas compressed? Do we see the process of the new born stars in the Arm?

    In any case, I really don't understand the process which creates this compressed gas. And, if it is there, how could it stay forever (or at least during the whole life cycle of spiral galaxy)?

    Is it correct that we only see O and B type stars in the spiral arms but not outside?

    https://en.wikipedia.org/wiki/Density_wave_theory

    "Density wave theory or the Lin-Shu density wave theory is a theory proposed by C.C. Lin and Frank Shu in the mid-1960s to explain the spiral arm structure of spiral galaxies. Its primary competitor is the self-propagating star-formation model. The Lin-Shu theory introduces the idea of long-lived quasistatic density waves (also called heavy sound),[1] which are sections of the galactic disk that have greater mass density (about 10–20% greater).[2] The theory has also been successfully applied to Saturn's rings."


    Quote Originally Posted by AGN Fuel View Post
    Waves in a specific confining environment are not random. They must comply to the laws of physics, which dictate how they will act.
    On which law of physics the density wave in based on? Is it Newton law?
    I still don't understand how this law can create that unique spiral shape of compressed gas.


    We assume that the spiral arm has no impact on the orbital path of the stars in the arm. Therefore, stars are crossing the arms without any difficulties.
    However, as we claim that the gas density in the arm is greater than the gas density outside the arm, so by definition the stars orbits at different environments.

    How could it be that this high gas density can create new O&B stars as lower gas density gets in, kill them all as they get out ("O&B Stars ionize leftover gas (HII Regions), then die before moving far from the waves"), but it has no impact on the orbital path of all the other stars that get in and out while keeping their orbital path around the galaxy?

    Why do we claim that in the spiral arms the mass density is greater by about 10–20% than the mass outside the arms? Is it based on real measurements or just an estimation?
    Please look at the following spiral structure:
    http://www.sliderbase.com/spitem-1187-6.html

    Did we try to monitor the gas/mass density between the arms?
    If so, is there any valid data which highlight that information including the nearby stars that are located between the arms?
    Last edited by Dave Lee; 2018-May-12 at 09:40 AM.

  5. #5
    Join Date
    Mar 2003
    Posts
    2,972
    Quote Originally Posted by Dave Lee View Post
    "When the gas enters the density wave region, it is compressed and O and B type stars begin to form.
    All star types begin to form, including O and B. The difference is that O and B type stars are very short-lived. They don't have time to move far beyond the area where they are formed. The lower mass (but vastly more common) stars classes such as M & G (red dwarves, stars like our sun, etc) have time to migrate out of the spiral arms. It is these stars that make up the great majority of the population of stars between the arms.

    So, is it correct that all is needed to create new born O and B type stars is just a gas compressed?
    That's all you need to create any new star, including O & B. You just need enough of it.

    How could it be that a compressed gas of about 10–20% could set those new born stars? Is it feasible? Did we try to verify/simulate that process?
    Yes, per the Jean's Instability. Once the density of the gas cloud exceeds a limit, collapse of the cloud is inevitable.

    If we look at our location in Orion Arm, do we see that gas compressed? Do we see the process of the new born stars in the Arm?
    Ah, sweet irony... as example 1, how about the Orion Nebula?

    On which law of physics the density wave in based on? Is it Newton law? I still don't understand how this law can create that unique spiral shape of compressed gas.
    Wave propagation is an entire sub-genre of physics. Google search wave equations. Have Tylenol on standby.


    We assume that the spiral arm has no impact on the orbital path of the stars in the arm. Therefore, stars are crossing the arms without any difficulties.
    However, as we claim that the gas density in the arm is greater than the gas density outside the arm, so by definition the stars orbits at different environments.
    The gas is not stationary. It is co-rotating with the stars.

    How could it be that this high gas density can create new O&B stars as lower gas density gets in, kill them all as they get out ("O&B Stars ionize leftover gas (HII Regions), then die before moving far from the waves"), but it has no impact on the orbital path of all the other stars that get in and out while keeping their orbital path around the galaxy?
    The O & B stars do not die BECAUSE they leave the spiral arm. They die by virtue of their mass - large mass stars use their fuel much MUCH faster than low mass stars. They die because they use up their available fuel before they have the chance to move out of the spiral arm.
    Last edited by AGN Fuel; 2018-May-12 at 11:58 PM.

  6. #6
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    8,088
    Quote Originally Posted by AGN Fuel View Post
    All star types begin to form, including O and B. The difference is that O and B type stars are very short-lived. They don't have time to move far beyond the area where they are formed. The lower mass (but vastly more common) stars classes such as M & G (red dwarves, stars like our sun, etc) have time to migrate out of the spiral arms. It is these stars that make up the great majority of the population of stars between the arms.



    That's all you need to create any new star, including O & B. You just need enough of it.



    Yes, per the Jean's Instability. Once the density of the gas cloud exceeds a limit, collapse of the cloud is inevitable.



    Ah, sweet irony... as example 1, how about the Orion Nebula?



    Wave propagation is an entire sub-genre of physics. Google search wave equations. Have Tylenol on standby.
    My bold. Good point about a figurative headache when studying advanced topics like this.




    The gas is not stationary. It is co-rotating with the stars.
    Another good point. Let me add that when the stars move into the arm, they merely get closer together without any effect on one another beyond the usual ripple of gravitational perturbations. They are many light years apart. However, the gas consists of molecules a fraction of a centimeter apart, so on a scale of hundreds or thousands of light years it behaves as a fluid and is subject to fluid dynamics that do not apply to the stars. In this case a modest compression triggers episodes of collapse and star formation.



    The O & B stars do not die BECAUSE they leave the spiral arm. They die by virtue of their mass - large mass stars use their fuel much MUCH faster than low mass stars. They die because they use up their available fuel before they have the chance to move out of the spiral arm.
    I will readily acknowledge that I do not have a working understanding of all the details the experts in the field are studying. I would have needed a lot of advanced college and professional study to get up to speed. Nevertheless, the gist of what the experts are saying makes sense to me, and I see no reason in principle to second-guess them.

  7. #7
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    8,088
    Quote Originally Posted by Hornblower View Post
    I will readily acknowledge that I do not have a working understanding of all the details the experts in the field are studying. I would have needed a lot of advanced college and professional study to get up to speed. Nevertheless, the gist of what the experts are saying makes sense to me, and I see no reason in principle to second-guess them.
    Let me clarify that a bit. It is the initial occurrence and subsequent evolution of the spiral density wave pattern about which I would need a lot of advanced study to fully grasp. I have no difficulty in understanding why the short-lived superluminous O and "early" B stars are seldom found far from the arms.

  8. #8
    Join Date
    Oct 2009
    Location
    a long way away
    Posts
    10,258
    Quote Originally Posted by Dave Lee View Post
    In any case, I really don't understand the process which creates this compressed gas. And, if it is there, how could it stay forever (or at least during the whole life cycle of spiral galaxy)?
    This is quite a common phenomenon. Have you every had the experience of driving down a busy highway (motorway/autostrada/autobahn) and seeing everyone ahead of you put on their brakes at a certain point?

    When you get to that location, you have to put on your brakes as well because everyone has slowed down. You pass that point and everyone speeds up again. The only reason people are putting their brakes on and slowing down is because the people in front are slowing down and putting their brakes on. That is a compression wave. The only reason it disappears is because there is a change in the amount or speed of the approaching traffic. If you had a constant flow of traffic (or gas in the galaxy) then that "brake-light compression wave" could be completely stable. Depending on the speed and about of traffic it could move forwards or backwards slowly.

    (There is also a higher risk of accidents in these "compression zones" which one could say are analogous to star formation!)

  9. #9
    Join Date
    Sep 2015
    Posts
    499
    Quote Originally Posted by Strange View Post
    This is quite a common phenomenon. Have you every had the experience of driving down a busy highway (motorway/autostrada/autobahn) and seeing everyone ahead of you put on their brakes at a certain point?

    When you get to that location, you have to put on your brakes as well because everyone has slowed down. You pass that point and everyone speeds up again. The only reason people are putting their brakes on and slowing down is because the people in front are slowing down and putting their brakes on. That is a compression wave. The only reason it disappears is because there is a change in the amount or speed of the approaching traffic. If you had a constant flow of traffic (or gas in the galaxy) then that "brake-light compression wave" could be completely stable. Depending on the speed and about of traffic it could move forwards or backwards slowly.

    (There is also a higher risk of accidents in these "compression zones" which one could say are analogous to star formation!)

    Thanks

    I'm not a scientist.
    However, based on my limited knowledge I had the impression that the only force which drives the galaxy must be - Gravity force, which is based on NEWTON LAW.
    I know that Cars and tracks comes with integrated brakes, however, I have never heard about a star with integrated brakes.

    In gravity - there are no integrated brakes.
    On the contrary, as you get closer, you get higher gravity force & higher Velocity.
    This is the whole idea of Newton law.

    If you think that there is a braking system (which decrease the velocity as you get closer) - then please let me know the source for that system.
    If I understand you correctly - the "compression wave" is the source for that braking system.
    So, what is the source for the "compression wave" which works so hard against Newton Law?

    It was stated that:
    Quote Originally Posted by AGN Fuel View Post
    Wave propagation is an entire sub-genre of physics. Google search wave equations. Have Tylenol on standby.
    .
    So, is it sub genre of Newton law?
    If no, would you kindly explain on which law the wave is based on?
    Can you please offer a mathematical proof for this idea and how it can be there and work according to our expectation?
    Last edited by Dave Lee; 2018-May-16 at 05:22 AM.

  10. #10
    Join Date
    Aug 2002
    Posts
    8,929
    Quote Originally Posted by Dave Lee View Post

    So, is it sub genre of Newton law?
    If no, would you kindly explain on which law the wave is based on?
    Can you please offer a mathematical proof for this idea and how it can be there and work according to our expectation?
    In this case the stars can be seen as a gas, so the waves are based on hydrodynamics.
    All comments made in red are moderator comments. Please, read the rules of the forum here and read the additional rules for ATM, and for conspiracy theories. If you think a post is inappropriate, don't comment on it in thread but report it using the /!\ button in the lower left corner of each message. But most of all, have fun!

    Catch me on twitter: @tusenfem
    Catch Rosetta Plasma Consortium on twitter: @Rosetta_RPC

  11. #11
    Join Date
    Mar 2003
    Posts
    2,972
    Quote Originally Posted by Dave Lee View Post
    If no, would you kindly explain on which law the wave is based on?
    Can you please offer a mathematical proof for this idea and how it can be there and work according to our expectation?
    Sorry buddy, but there is no way on this green Earth that I am going to subject myself to the Heraclean task of trying to describe wave mechanics to you via an internet forum. Frankly, that way madness lies.

    With respect, if you are genuinely interested in this, you really should enroll in some basic physics classes.

  12. #12
    Join Date
    Mar 2010
    Location
    United Kingdom
    Posts
    6,829
    Quote Originally Posted by Dave Lee View Post
    However, based on my limited knowledge I had the impression that the only force which drives the galaxy must be - Gravity force, which is based on NEWTON LAW.
    Gravity is the dominant force. Plenty of other effects exist and make a contribution to the system.

    Quote Originally Posted by Dave Lee View Post
    I know that Cars and tracks comes with integrated brakes, however, I have never heard about a star with integrated brakes.
    It is an analogy, a different system used to illustrate a concept. You need to stop taking things like this to illogical extremes. The point was to show what a compression wave was, not to trigger a debate about whether stars have airbags and cup holders.

    Try searching for videos on "spiral galaxy density wave". There are several simulations showing how spiral patterns can come about by including correlations in elliptical orbits. And some showing how the alignments evolve to create the moving density wave.

  13. #13
    Join Date
    Sep 2015
    Posts
    499
    Quote Originally Posted by tusenfem View Post
    In this case the stars can be seen as a gas...
    Why is it?
    Do you mean like Friedmann equations which start with the simplifying assumption that the universe is spatially homogeneous and isotropic?

    https://en.wikipedia.org/wiki/Friedmann_equations

    The Friedmann equations start with the simplifying assumption that the universe is spatially homogeneous and isotropic, i.e. the cosmological principle; empirically, this is justified on scales larger than ~100 Mpc.

    However, in this case we discuss on a large scales of 100Mpc. The width of spiral arm (at our location) is estimated as 1LY. So how can we estimate a star as a gas in that short range?



    Quote Originally Posted by Shaula View Post
    Gravity is the dominant force. Plenty of other effects exist and make a contribution to the system.
    This is a key message.
    In one hand you claim that there are other forces which affect the star orbit.
    Let's assume that one of them is hydrodynamics.
    Therefore, the star orbital path is based on Newton law (dominant force), but also on other forces including hydrodynamics.
    On the other hand, when we try to calculate the orbital path of a star, all the other forces are totally neglected.
    Why don't we add their contribution to the orbital system?


    In any case, as the star enters the spiral arm (or density wave) - it is stated that it should slow down (due to the impact of the density wave).
    Therefore, as the star gets out from the spiral arm, don't you agree that it should move faster?
    If that was correct, then a star should change its velocities based on its location (in and out from the density wave).
    As the Sun is currently located in the spiral arm (density wave), it is now moving slowly.
    Once it gets out from the arm it must increase its velocity.
    However, if I understand it correctly, that contradicts our observation.
    We see that all the stars at a distance of 28,000 light years from the center, are moving at almost the same speed regardless from their location - in the arm or outside the arm.
    How can we explain that phenomenon?
    How can we claim that there is a traffic jam in the density wave, while there is no change in the orbital speed of all the stars in that radius - in the density wave or outside that wave?
    Last edited by Dave Lee; 2018-May-16 at 03:38 PM.

  14. #14
    Join Date
    Mar 2010
    Location
    United Kingdom
    Posts
    6,829
    Quote Originally Posted by Dave Lee View Post
    Why don't we add their contribution to the orbital system?
    We do for higher precision models of the galaxy. We leave them out for simpler models. Depends on what we are using the model for.

    How can we explain that phenomenon?
    If you followed my suggestion and watched a few videos you'd know.
    P.S., part of the answer is to ignore the reasoning you have given because I'm pretty sure no one actually said much of what you are arguing from.

  15. #15
    Join Date
    Aug 2002
    Posts
    8,929
    Quote Originally Posted by Dave Lee View Post
    Why is it?
    Do you mean like Friedmann equations which start with the simplifying assumption that the universe is spatially homogeneous and isotropic?
    where does that come from? don't go overboard, with friedman or whatever
    over all the stars, just like traffic, can be very well described as a collionless gas yhat happens to rotate around a central point
    so gasdynamics, or hydrodynamcs, can be used, and one of the phenomena is density waves, like sound waves
    it is all very basic, if you don't jump into over-the-top physics
    All comments made in red are moderator comments. Please, read the rules of the forum here and read the additional rules for ATM, and for conspiracy theories. If you think a post is inappropriate, don't comment on it in thread but report it using the /!\ button in the lower left corner of each message. But most of all, have fun!

    Catch me on twitter: @tusenfem
    Catch Rosetta Plasma Consortium on twitter: @Rosetta_RPC

  16. #16
    Join Date
    Mar 2003
    Posts
    2,972
    Quote Originally Posted by Dave Lee View Post
    The width of spiral arm (at our location) is estimated as 1LY. So how can we estimate a star as a gas in that short range?
    (My bolding) At first I thought this must have been a typo, given you are quoting the width of an entire spiral arm of a galaxy as being less than a quarter of the distance to the nearest star to Sol... but when you have for some reason introduced Friedmann equations into a discussion about intragalactic dynamics, I'm not so sure any more...

    Down the rabbit hole again we go.

  17. #17
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    8,088
    Some of Dave Lee's responses to attempts at answering his questions illustrate the pitfalls that can happen with analogies. All the highway traffic jam is intended to show is that individual cars can enter the clump, pass through, and then disperse again upon leaving. The clumping pattern stays in place for whatever reason, but is not the same set of cars as the day wears on. Dave appears to have jumped to the conclusion that we are asserting that an individual star will slow down upon entering the arm and speed up upon leaving, when we said nothing of the sort.

    The physical cause of the clumping in the galaxy is different from that on the highway. As a given star approaches the clump, nothing is forcing it to slow down. It is closing in on other stars that are already moving more slowly because they are in different orbits. It can fly right by them with a vanishingly small chance of a collision, whereas on the highway a driver closing in on a clump must slow down to avoid a collision, and then voluntarily speed up again when past the choke point. He his adjusting his speed with the gas and the brakes, while the star's motion is purely ballistic in accordance with Newtonian mechanics.

    For now let's ignore the numerous Sun-like and fainter stars that last for many orbits around the galaxy. As I see it they do not figure in the brightening of the arms. It is giant molecular clouds that get to pushing and shoving as they converge in different orbits and get into the clump. As has been noted before these clouds behave as fluids while a swarm of stars does not do so. The experts have concluded that the moderate amount of compression they undergo in these clumping encounters is enough to set off episodes of star formation, which gives us a sprinkling of disproportionately bright O and B stars that light up the arm. The ultraluminous stars in Orion and Canis Major are examples of this. On a galactic scale they are still close to where they were formed, and they will burn out before dispersing much. A rare pair of exceptions are Mu Columbae and AE Aurigae, which have been flung far and wide by an extremely close encounter in the vicinity of Orion's sword.

  18. #18
    Join Date
    Sep 2015
    Posts
    499
    Quote Originally Posted by AGN Fuel View Post
    (My bolding) At first I thought this must have been a typo, given you are quoting the width of an entire spiral arm of a galaxy as being less than a quarter of the distance to the nearest star to Sol... but when you have for some reason introduced Friedmann equations into a discussion about intragalactic dynamics, I'm not so sure any more...

    Down the rabbit hole again we go.
    Thanks
    Yes, it is a typo error.
    It should be 1KLY.

  19. #19
    Join Date
    Aug 2002
    Posts
    8,929
    I think that the wiki page on density waves explains a lot, and was already cited by Dave Lee himself.
    All comments made in red are moderator comments. Please, read the rules of the forum here and read the additional rules for ATM, and for conspiracy theories. If you think a post is inappropriate, don't comment on it in thread but report it using the /!\ button in the lower left corner of each message. But most of all, have fun!

    Catch me on twitter: @tusenfem
    Catch Rosetta Plasma Consortium on twitter: @Rosetta_RPC

  20. #20
    Join Date
    Oct 2009
    Location
    a long way away
    Posts
    10,258
    Quote Originally Posted by Hornblower View Post
    Some of Dave Lee's responses to attempts at answering his questions illustrate the pitfalls that can happen with analogies. All the highway traffic jam is intended to show is that individual cars can enter the clump, pass through, and then disperse again upon leaving. The clumping pattern stays in place for whatever reason, but is not the same set of cars as the day wears on. Dave appears to have jumped to the conclusion that we are asserting that an individual star will slow down upon entering the arm and speed up upon leaving, when we said nothing of the sort.
    To be fair, I did say that. But you are right, the same sort of clumping can occur without significant speed changes.

    In fact, the average speed in these highway traffic things is not much less than outside of them; it is just enough to make people touch their brakes (and the brake lights make the person behind touch theirs ...).

  21. #21
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    8,088
    Upon looking closely at the Wiki diagram below the image of M81, it appears to me that the convergence at the spiral wave is more a matter of differences in direction of motion than of differences in speed.

    Let me start with a preemptive remark about the forms of those elongated orbits. My initial reaction was that something had to be wrong there, because an orbit would have one focus rather than its own center at the galactic center. Then I remembered that with this distribution of mass the strength of the gravitational action toward the galactic center would not follow the inverse square formula, in fact, not even close. As the star rounds its apoapsis point and heads inward, the gravity will strengthen less than if all of the galactic mass was concentrated at the center. I can see it leveling off well short of 180 degrees around from apoapsis and heading back out. I have seen that in a sample orbit in a globular cluster simulation. I can imagine that if the distribution is just right the orbit could level off at 90 degrees and reach the next apoapsis at 180, giving the pattern shown in the chart.

    I am envisioning clockwise orbital motion in this example, in keeping with the finding that in typical spiral galaxies the arms are trailing with respect to the orbital motion. Thus the stars would be inbound as they approached the maximum density region, meaning they would speed up through the encounter. It also appears that a given star would drift toward the density maximum and then drift out on the same side, rather than cross over to the other side. Giant molecular clouds following the same orbits would undergo compression at this stage and set off episodes of star formation.

  22. #22
    Join Date
    Sep 2012
    Posts
    1,397
    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.
    While I appreciate the similar complexity, aren't the forces completely different? I.e Gravity is an attractive force while gas pressure repels.
    Depending on whom you ask, everything is relative.

  23. #23
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    8,088
    Quote Originally Posted by mkline55 View Post
    While I appreciate the similar complexity, aren't the forces completely different? I.e Gravity is an attractive force while gas pressure repels.
    If we had nothing but stars, I would say that fluid dynamics would not be a factor. A gas behaves as a fluid when the molecules collide frequently, and the collisions are elastic in nature. With nothing but stars, collisions will be few and far between, and they will not rebound in an elastic manner. At least that is my understanding of what we mean by fluid behavior.

  24. #24
    Join Date
    May 2003
    Posts
    5,861
    Quote Originally Posted by Hornblower View Post
    If we had nothing but stars, I would say that fluid dynamics would not be a factor. A gas behaves as a fluid when the molecules collide frequently, and the collisions are elastic in nature. With nothing but stars, collisions will be few and far between, and they will not rebound in an elastic manner. At least that is my understanding of what we mean by fluid behavior.
    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).
    Conserve energy. Commute with the Hamiltonian.

  25. #25
    Join Date
    Sep 2015
    Posts
    499
    Quote Originally Posted by Hornblower View Post
    Some of Dave Lee's responses to attempts at answering his questions illustrate the pitfalls that can happen with analogies. All the highway traffic jam is intended to show is that individual cars can enter the clump, pass through, and then disperse again upon leaving. The clumping pattern stays in place for whatever reason, but is not the same set of cars as the day wears on. Dave appears to have jumped to the conclusion that we are asserting that an individual star will slow down upon entering the arm and speed up upon leaving, when we said nothing of the sort.

    The physical cause of the clumping in the galaxy is different from that on the highway. As a given star approaches the clump, nothing is forcing it to slow down. It is closing in on other stars that are already moving more slowly because they are in different orbits. It can fly right by them with a vanishingly small chance of a collision, whereas on the highway a driver closing in on a clump must slow down to avoid a collision, and then voluntarily speed up again when past the choke point. He his adjusting his speed with the gas and the brakes, while the star's motion is purely ballistic in accordance with Newtonian mechanics.

    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%.

    As this isn't the case, I would like to use the following example:
    Let's assume that there are 1000 stars orbiting the center in a row (all of them has the same radius, the same velocity and the same distance from each other).
    Now, lets try to understand what is the outcome as they enter the density wave (spiral arm) section:

    Please advice if you agree with the following:

    1. Radius is constant, Velocity isn't constant (decreases at the density wave):
    In this case, it will take longer time to cross the wave. Therefore, more time means more stars in the wave. So - if the velocity decreases by 20%, I would assume that the density in the wave should be increased by also 20%.

    2. Radius is constant, Velocity is constant:
    In this case I really don't see why there will be higher density in the wave.

    3. Radius isn't constant, Velocity is constant:
    It seems to me that this isn't realistic case as in elliptical orbit the velocity must change based on the current location of the star.

    4. Radius isn't constant, Velocity isn't constant:
    It is feasible as it can be represented by elliptical orbit.
    I assume that Hornblower tries to explain this case:

    Quote Originally Posted by Hornblower View Post
    Upon looking closely at the Wiki diagram below the image of M81, it appears to me that the convergence at the spiral wave is more a matter of differences in direction of motion than of differences in speed.

    Let me start with a preemptive remark about the forms of those elongated orbits. My initial reaction was that something had to be wrong there, because an orbit would have one focus rather than its own center at the galactic center. Then I remembered that with this distribution of mass the strength of the gravitational action toward the galactic center would not follow the inverse square formula, in fact, not even close. As the star rounds its apoapsis point and heads inward, the gravity will strengthen less than if all of the galactic mass was concentrated at the center. I can see it leveling off well short of 180 degrees around from apoapsis and heading back out. I have seen that in a sample orbit in a globular cluster simulation. I can imagine that if the distribution is just right the orbit could level off at 90 degrees and reach the next apoapsis at 180, giving the pattern shown in the chart.

    I am envisioning clockwise orbital motion in this example, in keeping with the finding that in typical spiral galaxies the arms are trailing with respect to the orbital motion. Thus the stars would be inbound as they approached the maximum density region, meaning they would speed up through the encounter. It also appears that a given star would drift toward the density maximum and then drift out on the same side, rather than cross over to the other side. Giant molecular clouds following the same orbits would undergo compression at this stage and set off episodes of star formation.
    If I understand it correctly, in this case a star should get to the feasible density zone only at one point of its whole elliptical orbit cycle:
    "Thus the stars would be inbound as they approached the maximum density region, meaning they would speed up through the encounter. "
    Never the less, in each full cycle it has only one point of least distance (perihelion).
    In any case, in real spiral galaxies, stars should penetrate several times to density wave zones in one orbital cycle.
    So, I'm not sure what is the correct answer.
    Last edited by Dave Lee; 2018-May-17 at 09:24 PM.

  26. #26
    Join Date
    Oct 2009
    Location
    a long way away
    Posts
    10,258
    Quote Originally Posted by Dave Lee View Post
    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%.
    As the posts immediately above explain, it is the gas that is compressed by the density waves. The stars are unaffected. It is the increased density (and associated heating) that means there are more stars created. It is the greater number of young, bright stars that make the arms visible.

    1. Radius is constant, Velocity isn't constant (decreases at the density wave):
    No.

    2. Radius is constant, Velocity is constant:
    In this case I really don't see why there will be higher density in the wave.
    Because you are thinking about stars and not the gas.

  27. #27
    Join Date
    Sep 2015
    Posts
    499
    Quote Originally Posted by Strange View Post
    Because you are thinking about stars and not the gas.
    But I had been informed that:

    Quote Originally Posted by tusenfem View Post
    In this case the stars can be seen as a gas...

  28. #28
    Join Date
    Jun 2007
    Posts
    5,603
    Quote Originally Posted by Hornblower View Post
    If we had nothing but stars, I would say that fluid dynamics would not be a factor. A gas behaves as a fluid when the molecules collide frequently, and the collisions are elastic in nature. With nothing but stars, collisions will be few and far between, and they will not rebound in an elastic manner. At least that is my understanding of what we mean by fluid behavior.
    Stars that approach close enough to each other do scatter significantly, and the scattering is elastic except in the extraordinarily rare cases where they actually make contact. With enough stars, you can approximate the overall behavior as a gas. It doesn't matter that gravity is purely attractive, the pressure of a gas comes from the inertia of the particles, not the interactions by which they scatter. And due to the vast difference in scales, the behavior of the star-gas won't be anything like the behavior of the atom-gas occupying the same volume.

    But of course this is just an approximation for describing the overall behavior of a large number of bodies. It doesn't mean that something other than Newtonian mechanics is in play, as Dave Lee is predictably misinterpreting it to mean.

  29. #29
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    8,088
    Quote Originally Posted by cjameshuff View Post
    Stars that approach close enough to each other do scatter significantly, and the scattering is elastic except in the extraordinarily rare cases where they actually make contact. With enough stars, you can approximate the overall behavior as a gas. It doesn't matter that gravity is purely attractive, the pressure of a gas comes from the inertia of the particles, not the interactions by which they scatter. And due to the vast difference in scales, the behavior of the star-gas won't be anything like the behavior of the atom-gas occupying the same volume.

    But of course this is just an approximation for describing the overall behavior of a large number of bodies. It doesn't mean that something other than Newtonian mechanics is in play, as Dave Lee is predictably misinterpreting it to mean.
    I stand corrected about the interactions. I think I am on the right track in seeing this gas-like behavior of the stars as negligible over the relatively short range in which we are concerned in analyzing the behavior of the interstellar gas in the spiral pattern. Yes indeed, it becomes significant at larger scales over longer times.

  30. #30
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    8,088
    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%.

    As this isn't the case, I would like to use the following example:
    Let's assume that there are 1000 stars orbiting the center in a row (all of them has the same radius, the same velocity and the same distance from each other).
    Now, lets try to understand what is the outcome as they enter the density wave (spiral arm) section:

    Please advice if you agree with the following:

    1. Radius is constant, Velocity isn't constant (decreases at the density wave):
    In this case, it will take longer time to cross the wave. Therefore, more time means more stars in the wave. So - if the velocity decreases by 20%, I would assume that the density in the wave should be increased by also 20%.

    2. Radius is constant, Velocity is constant:
    In this case I really don't see why there will be higher density in the wave.

    3. Radius isn't constant, Velocity is constant:
    It seems to me that this isn't realistic case as in elliptical orbit the velocity must change based on the current location of the star.

    4. Radius isn't constant, Velocity isn't constant:
    It is feasible as it can be represented by elliptical orbit.
    I assume that Hornblower tries to explain this case:



    If I understand it correctly, in this case a star should get to the feasible density zone only at one point of its whole elliptical orbit cycle:
    "Thus the stars would be inbound as they approached the maximum density region, meaning they would speed up through the encounter. "
    Never the less, in each full cycle it has only one point of least distance (perihelion).
    In any case, in real spiral galaxies, stars should penetrate several times to density wave zones in one orbital cycle.
    So, I'm not sure what is the correct answer.
    In the model shown in the Wiki article each orbit has two periapsides and two apoapsides in each cycle. These are not Keplerian ellipses because the centrally directed gravitational force is not inverse square. In this model each star will encounter the spiral wave pattern twice in each orbital circuit. That can be seen simply by tracing the path around the elongated orbit.

    If I am not mistaken it is still something of a mystery as to why the orbits get into such an orientation distribution in the first place. Also, in a real galaxy things are not as neat and clean, and there are complications that result in a central bar, as in the case of our own Milky Way. My guess is that the experts have at least partial possible explanations on the basis of the fluid dynamics of the gas from which the stars formed. I would welcome some enlightenment in this area.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •